number of forty-four. This number has been retained throughout the
series, at least theoretically; but one tooth on either side of each jaw,
the anterior premolar, which in all the Eocene and Miocene species was
well developed, persisting through the lifetime of the animal, is in all
modern Horses rudimentary, functionless, and generally lost at an early
period of life, evidently passing through a stage which must soon lead
to its complete disappearance. The canines have also greatly diminished
in size, and are rarely present in the female sex, so that practically
a very large number of adult Horses of the present day have eight teeth
less than the number possessed by their predecessors. The diastema or
interval between the incisor and premolar teeth (of essential importance
in the domesticated Horse to his master, as without it there would be no
room for inserting the special instrument of subjugation to his commands,
the bit) already existed in the earliest known forms, but has gradually
increased in length. The incisors have undergone in comparatively recent
times that curious change producing the structure more fully described
hereafter, which distinguishes the Horse’s incisors from those of all
other known animals, with the exception of the extinct _Macrauchenia_.
Lastly, the molars have undergone a remarkable series of modifications,
much resembling in principle those that have taken place in several other
groups of herbivorous animals. Distinctions in form which existed between
the premolars, at least in the anterior part of the series, and the true
molars have gradually disappeared, the teeth becoming all very uniform
in the shape and structure of their grinding surface. The crowns of all
these teeth in the early forms were very short (see Fig. 158, _a_); there
was a distinct constriction, or neck, between the crown and roots; and
when the tooth was developing, as soon as the neck once rose fairly above
the alveolar margin, the tooth remained permanently in this position.
The term “brachydont” expresses this condition of teeth, the mode of
growth of which may be illustrated by those of man. The free surface had
two nearly transverse curved ridges, with valleys between (Fig. 157,
_a_); but the valleys were shallow and had no deposit of cement filling
them, the whole exposed surface of the unworn tooth being formed of
enamel. When the ridges became worn down the dentine of the interior was
exposed, forming islands surrounded by enamel. With the progress of time
the crowns of the teeth gradually became longer, the valleys deeper,
and the ridges not only more elevated but more curved and complex in
arrangement. To give support to these high ridges and save them from
breaking in use, the valleys or cavities between them became filled up to
the top with cement, and as the crown wore down an admirable grinding
surface consisting of patches and islands of the two softer substances,
dentine and cement, separated by variously reduplicated and contorted
lines of intensely hard enamel, resulted (Fig. 157, _c_). The crown
continued lengthening until in the modern Horses it has assumed the form
called “hypsodont” (Fig. 158, _b_). Instead of contracting into a neck,
and forming roots, its sides continue parallel for a considerable depth
in the socket, and as the surface wears away, the whole tooth slowly
pushes up, and maintains the grinding edge constantly at the same level
above the alveolus, much as in the perpetually growing Rodent’s teeth.
But in existing Horses there is still a limit to the growth of the
molar. After a length is attained which in normal conditions supplies
sufficient grinding surface for the lifetime of the animal, a neck and
roots are formed, and the tooth is reduced to the condition of that of
the brachydont ancestor. It is perfectly clear that this lengthening
of the crown adds greatly to the power of the teeth as organs of
mastication, and enables the animals in which it has taken place to find
their sustenance among the comparatively dry and harsh herbage of the
open plains, instead of being limited to the more succulent vegetable
productions of the marshes and forests in which their predecessors
probably dwelt.
[Illustration: FIG. 158.—_a_, Outer view of second upper molar teeth
of _Anchitherium_ (brachydont form); _b_, corresponding tooth of Horse
(hypsodont form).]
The modifications of the limbs which took place _pari passu_ with those
of the teeth must have been associated with increased speed, especially
over firm and unyielding ground. Short, stout legs, and broad feet, with
numerous toes, spreading apart from each other when the weight of the
creature is borne on them, are sufficiently well adapted for plodding
deliberately over marshy and yielding surfaces, and the Tapirs and the
Rhinoceroses, which in the structure of the limbs have altered but little
from the primitive Eocene forms, still haunt the borders of streams and
lakes and the shady depths of the forests, as was probably the habit
of their ancient representatives, while the Horses are all inhabitants
of the open plains, for life in which their whole organisation is in
the most eminent degree adapted. The length and mobility of the neck,
position of the eye and ear, and great development of the organ of smell,
give them ample means of becoming aware of the approach of enemies,
while the length of their limbs, the angles the different segments form
with each other, and especially the combination of firmness, stability,
and lightness in the reduction of all the toes to a single one, upon
which the whole weight of the body and all the muscular power are
concentrated, give them speed and endurance surpassing that of almost any
other animal. When surprised, however, they are by no means helpless,
both fore and hind feet becoming at need powerful weapons of defence.
If we were not so habituated to the sight of the Horse as hardly ever
to consider its structure, we should greatly marvel at being told of a
mammal so strangely constructed that it had but a single toe on each
extremity, on the end of the nail of which it walked or galloped. Such
a conformation is without a parallel in the vertebrate series, and is
one of the most remarkable instances of specialisation, or deviation
from the usual type, in accordance with particular conditions of life.
It is clear, both from the structure of the foot itself, and also by an
examination of the intermediate forms, that this toe corresponds to the
middle or third digit of the complete typical or pentadactyle foot; and
there is very strong evidence to show that by a gradual concentration of
all the power of the limb upon this toe, and the concurrent dwindling
away and final disappearance of all the others, the present condition of
the Horse’s foot has been produced.
_Protohippus._[258]—In this Lower Pliocene North American genus (also
described as _Merychippus_) the cheek-teeth resemble those of the
generalised species of _Equus_, but have shorter crowns; while the
milk-molars approximate to the permanent molars of _Anchitherium_. Each
foot has three digits.
[Illustration: FIG. 159.—Three right upper cheek-teeth of _Hipparion_.
_a_, Antero-external column; _b_, postero-external column; _c_,
postero-internal column, or posterior pillar; _d_, antero-internal
column, or anterior pillar; _f_, posterior intermediate column; _i_,
anterior intermediate column. (From the _Palæontologia Indica_.)]
_Hipparion._[259]—Upper cheek-teeth (Fig. 159), with the antero-internal
column, or anterior pillar as it may be conveniently termed in this
family, detached throughout the greater part of its height from the
adjacent column. Either a single or three digits in each foot. First
upper premolar large and persistent. This genus was very widely
distributed in the Pliocene, occurring in Europe, Asia, and North
America. In the typical European forms, and also in those of North
America, there were three digits in the feet (Fig. 156, _d_); but in
the Indian _H. antilopinum_ (separated by Cope as _Hippodactylus_) the
lateral digits seem to have disappeared. There is some doubt whether or
no _Hipparion_ should occupy a place in the direct ancestry of the Horse,
and Professor Cope suggests that while in America the intermediate place
between _Anchitherium_ and _Equus_ was held by _Protohippus_, in Europe
the same position was occupied by _Hipparion_—a view which involves the
dual origin of the Horses of the New and Old Worlds.
_Equus._[260]—Upper cheek-teeth with the anterior pillar (except in
a very early stage of wear) joined by a narrow neck to the adjacent
column (Fig. 157, _c_). Each foot with a single complete digit, but with
remnants of the proximal portions of the second and fourth metapodials
(Fig. 156, _e_); some extinct forms having claw-like rudiments of the
terminal phalangeals of the lateral digits. First upper premolar very
small or altogether absent in existing species, but in some fossil
species larger and persistent; first lower premolar only occasionally
developed in some fossil forms. Ears long. Tail long, with long hairs
either at the end or throughout. A callosity on the inner side of the
fore limb above the carpus.
_Fossil Species._—In the Pleistocene Horses of South America described
as _Hippidium_, as well as in the closely allied ones from North America
for which the name _Pliohippus_ has been proposed, the upper molars
are shorter and more curved than in the existing species, while their
anterior pillar is not longer antero-posteriorly than in _Hipparion_;
the lateral claw-like hoofs persisting. Some of the European Pliocene
species (like _E. stenonis_) agree with these species in the form of
the grinding surface of the anterior pillar of the upper molars. In one
of the species from the Lower Pliocene of India (_E. sivalensis_)—which
was a contemporary of _Hipparion_—and in all the existing species, the
grinding surface of the pillar in question is greatly elongated in the
antero-posterior direction, as in Fig. 157, _c_.
Fossil remains of Horses are found abundantly in deposits of the most
recent geological age in almost every part in America, from Eschscholtz
Bay in the north to Patagonia in the south. In that continent, however,
they became quite extinct, and no Horses, either wild or domesticated,
existed there at the time of the Spanish conquest, which is the more
remarkable as, when introduced from Europe, the Horses that ran wild
proved by their rapid multiplication in the plains of South America
and Texas that the climate, food, and other circumstances were highly
favourable for their existence. The former great abundance of _Equidæ_
in America, their complete extinction, and their perfect acclimatisation
when reintroduced by man, form curious but as yet unsolved problems in
geographical distribution.
_Existing Species._—The existing species of the genus are the following:—
The Horse, _Equus caballus_, is distinguished from the others by the long
hairs of the tail being more abundant and growing quite from the base as
well as the end and sides, and also by possessing a small bare callosity
on the inner side of the hind leg, just below the “hock” or heel joint,
in addition to the one on the inner side of the fore limb above the
carpus, common to all the genus. The mane is also longer and more
flowing, and the ears are shorter, the limbs longer, the hoofs broader,
and the head smaller.
Though the existing Horses are not usually marked in any definite manner,
or only irregularly dappled, or spotted with light surrounded by a darker
ring, many examples are met with showing a dark median dorsal streak
like that found in all the other members of the genus, and even with
dark stripes on the shoulders and legs indicating “the probability of
the descent of all the existing races from a single dun-coloured, more
or less striped, primitive stock, to which our horses still occasionally
revert.”[261]
In Europe wild Horses were extremely abundant in the Neolithic or
polished-stone period. Judging from the quantity of their remains
found associated with those of the men of that time, the chase of
these animals must have been among man’s chief occupations, and they
must have furnished him with one of his most important food supplies.
The characters of the bones preserved, and certain rude but graphic
representations carved on bones or reindeers’ antlers, enable us to know
that these Horses were rather small in size, and heavy in build, with
large heads and rough shaggy manes and tails, much like, in fact, the
present wild horses of the steppes of the south of Russia. They were
domesticated by the inhabitants of Europe before the dawn of history, but
it is doubtful whether the majority of the animals now existing on the
Continent are derived directly from them, as it is more probable that
they are descendants from Horses imported through Greece and Italy from
Asia, derived from a still earlier domestication, followed by gradual
improvement through long-continued attention bestowed on their breeding
and training. Horses are now diffused by the agency of man throughout
almost the whole of the inhabited parts of the globe, and the great
modifications they have undergone in consequence of domestication and
selective breeding are well exemplified by comparing such extreme forms
as the Shetland pony, dwarfed by uncongenial climate, the thoroughbred
racer, and the London dray-horse. In Australia, as in America, horses
imported by the European settlers have escaped into the unreclaimed
lands, and multiplied to a prodigious extent, roaming in vast herds over
the plains where no hoofed animal ever trod before.
A wild Horse from Central Asia, named _E. prezevalskii_,[262] is
described as having callosities on both limbs and broad hoofs like _E.
caballus_; but the long hairs of the tail do not begin until about half
way down its length. It also differs from _E. caballus_ in having a short
erect mane and no forelock; neither is there any dorsal stripe. The
ears are of moderate size; the whole body is of a whitish-gray, paler
beneath, and reddish on the head and upper parts of the limbs. If rightly
described this form would appear to be intermediate between the true
Horses and the Asses.
The second species is the domestic Ass (_E. asinus_), and the wild Asses
of Africa (_E. asinus_, var. _africanus_ and var. _somalicus_[263]). The
domestic Ass, which is now nearly as widely diffused and useful to man as
the Horse, was known in Egypt long before the latter, and is doubtless
of African origin. The ears are long, the mane erect, the tail without
long hairs at the base, and there are no callosities on the hind limbs.
There is a dark dorsal stripe, and another across the shoulders; while
the limbs are frequently banded. Of the wild forms the Nubian race (var.
_africanus_) has distinct dorsal and shoulder stripes, but the rings on
the limbs are often very indistinct; while in the Somali race the dorsal
stripe is indistinct, and the shoulder stripe wanting, but the rings on
the limbs are very boldly marked. Teeth and bones from a Pleistocene
cavern deposit in Madras have been referred to _E. asinus_.
The Asiatic wild Asses, which roam in small herds in the open plains
of Syria, of many parts of Persia, of the north-west of India, and
the highlands of Tartary and Tibet, from the shores of the Caspian
to the frontiers of China, differ from the last in being of a more
rufous or isabelline colour, instead of pure gray, in wanting the dark
streak across the shoulder, and having smaller ears. They have all a
dark-coloured median dorsal stripe. Though it is considered probable by
many zoologists that they form but a single species[264] (_E. hemionus_),
they present such marked variations in size and form that they have
commonly been divided into three—the Syrian Wild Ass (_E. hemippus_), the
Onager (_E. onager_) from Persia, Baluchistan, the Punjab, Sind, and the
desert of Kach, and the Kiang or Dzeggetai (_E. hemionus_) of the high
table-lands of Tibet, where it is usually met with at an elevation of
15,000 feet and upwards above the sea-level. The last is considerably
larger than either of the others, and differs from them in external
appearance, having more the aspect of the horse. They are all remarkably
swift, having been known to outstrip the fleetest Horse in speed.
[Illustration: FIG. 160.—The Quagga (_Equus quagga_).]
Lastly, there are four striped species, all inhabitants of Africa. These
constitute the genus _Hippotigris_ of Hamilton-Smith, but they are
not separable except by their coloration from the true Asses, and one
of them, the Quagga (_E. quagga_), may be considered as intermediate.
This animal was formerly met with in vast herds on the great plains of
South Africa, between the Cape Colony and the Vaal River, but now, in
common with most of the larger wild animals of that region, is becoming
extremely scarce, owing to the encroachments of European civilisation,
if, indeed, it is not already extinct. In length of ears and character of
tail it more resembles the Horse than it does the Ass, although it agrees
with the latter in wanting the callosity on the inner side of the hind
leg, just below the hock, characteristic of the Horse. The colour of the
head, neck, and upper parts of the body is reddish-brown, irregularly
banded and marked with dark brown stripes, stronger on the head and neck
and gradually becoming fainter until lost behind the shoulder. There
is a broad dark median dorsal stripe. The under surface of the body,
the legs, and tail are nearly white, without stripes. The crest is very
high, surmounted by a standing mane, banded alternately brown and white.
Though never really domesticated, Quaggas have occasionally been trained
to harness. The accompanying figure is reduced from a painting made from
one of a pair which were driven in Hyde Park in the early part of the
present century. The name is an imitation of the shrill barking neigh of
the animal—“ouag-ga, ouag-ga,” the last syllable very much prolonged. It
must be remembered, however, in reading books of African travel that the
same word is very commonly applied by hunters to Burchell’s Zebra.
Of the Zebras proper, the one which was first known to Europeans, and
was formerly considered the most common, is the True Zebra (_E. zebra_),
sometimes called the Mountain Zebra. It inhabits the mountainous regions
of the Cape Colony; but now, owing to the advances of civilised man into
its somewhat restricted range, it has become very scarce, and is even,
like the Quagga, threatened with extermination at no distant date. The
second species, Burchell’s Zebra (_E. burchelli_), still roams in large
herds over the plains to the north of the Orange River, but in yearly
diminishing numbers. Both species are subject to considerable individual
variations in marking, but the following are the principal characters by
which they can be distinguished.
[Illustration: FIG. 161.—True or Mountain Zebra (_Equus zebra_).]
_E. zebra_ (Fig. 161) is the smaller of the two (about 4 feet high at
the shoulders), and has longer ears, a tail more scantily clothed with
hair, and a shorter mane. The general ground colour is white, and the
stripes are black; the lower part of the face is bright brown. With the
exception of the abdomen and the inside of the thighs, the whole of the
surface is covered with stripes, the legs having narrow transverse
bars reaching quite to the hoofs, and the base of the tail being also
barred. The outsides of the ears have a white tip and a broad black mark
occupying the greater part of the surface, but are white at the base.
Perhaps the most constant and obvious distinction between this species
and the next is the arrangement of the stripes on the hinder part of the
back, where there are a number of short transverse bands passing from the
median longitudinal dorsal stripe towards, and sometimes joining with,
the uppermost of the broad stripes which run obliquely across the haunch
from the flanks towards the root of the tail. There is often a median
longitudinal stripe under the chest.
[Illustration: FIG. 162.—Burchell’s Zebra (_Equus burchelli_).]
_E. burchelli_ (Fig. 162) is a rather larger and more robust animal, with
smaller ears, a longer mane, and fuller tail. The general ground colour
of the body is pale yellowish-brown, the limbs nearly white, the stripes
dark brown or black. In the typical form they do not extend on to the
limbs or the tail; but there is a great variation in this respect, even
in animals of the same herd, some being striped quite down to the hoofs
(this form has been named _E. chapmani_). There is a strongly marked
median longitudinal ventral black stripe, to which the lower ends of the
transverse side stripes are usually united, but the dorsal stripe (also
strongly marked) is completely isolated in its posterior half, and the
uppermost of the broad haunch stripes runs nearly parallel to it. A much
larger proportion of the ears is white than in the other species. In
the middle of the wide intervals between the broad black stripes of the
flanks and haunches fainter stripes are generally seen.
_E. grevyi._—Under this name a Zebra has been described which was sent in
1882 to Paris from the Galla country, lying to the south of Abyssinia,
the most northern locality in which Zebras have previously been met
with. In many of its characters it resembles _E. zebra_, but the stripes
are much finer and more numerous than in the typical examples of that
species, and it has a strong, black, and isolated dorsal stripe. Even
allowing for the great variations that are met with in the markings of
animals of this group, the aberrant characters of this individual are
quite sufficient to separate it specifically from the true Zebra of South
Africa. Other similar specimens have been recently brought from the
Somali country.
The flesh of the Zebras is relished by the natives as food, and their
hides are very valuable for leather. Although the many attempts that
have been made to break in and train these animals for riding or driving
have sometimes been rewarded with partial success, they have never been
domesticated in the true sense of the word.
There are thus at least seven modifications of the Horse type at
present existing, sufficiently distinct to be reckoned as species by
all zoologists, and easily recognised by their external characters.
They are, however, all so closely allied that each will, at least in a
state of domestication or captivity, breed with perfect freedom with
any of the others. Cases of cross breeds are recorded between the Horse
and the Quagga, the Horse and Burchell’s Zebra, the Horse and the
Hemionus or Asiatic wild Ass, the common Ass and the Zebra, the common
Ass and Burchell’s Zebra, the common Ass and the Hemionus, the Hemionus
and the Zebra, and the Hemionus and Burchell’s Zebra. The two species
which are perhaps the farthest removed in general structure, the Horse
and the Ass, produce, as is well known, hybrids or Mules, which in
some qualities useful to man excel both their progenitors, and in some
countries, and for certain kinds of work, are in greater requisition
than either. Although occasional instances have been recorded of female
Mules breeding with the males of one or other of the pure species, it is
doubtful if any case has occurred of their breeding _inter se_, although
the opportunities of doing so must have been great, as Mules have been
reared in immense numbers for at least several thousands of years. We may
therefore consider it settled that the different species of the group are
now in that degree of physiological differentiation which enables them
to produce offspring with each other, but does not permit of the progeny
continuing the race, at all events unless reinforced by the aid of one of
the pure forms.
The several members of the group show mental differences quite as
striking as those exhibited by their external form, and more than
perhaps might be expected from the similarity of their cerebral
organisation. The patience of the Ass, the high spirit of the Horse, the
obstinacy of the Mule, have long been proverbial. It is very remarkable
that, out of so many species, two only should have shown any aptitude for
domestication, and that these two should have been from time immemorial
the universal and most useful companions and servants of man, while all
the others remain in their native freedom to this day. It is, however,
still a question whether this really arises from a different mental
constitution causing a natural capacity for entering into relations
with man, or whether it may not be owing to their having been brought
gradually into this condition by long-continued and persevering efforts
when the need of their services was keenly felt. It is quite possible
that one reason why most of the attempts to add new species to the list
of our domestic animals in modern times have ended in failure is that it
does not answer to do so in cases in which existing species supply all
the principal purposes to which the new ones might be put. It can hardly
be expected that Zebras and Quaggas fresh from their native mountains
and plains can be brought into competition as beasts of burden and
draught with Horses and Asses, whose naturally useful qualities have been
augmented by the training of thousands of generations of progenitors.
Not unfrequently instances occur of domestic Horses being produced with
a small additional toe with complete hoof, usually on the inside of the
principal toe, and, though far more rarely, three or more toes may be
present. These malformations are often cited as instances of reversion to
the condition of some of the earlier forms of equine animals previously
mentioned. Such explanations, however plausible they appear at first
sight, are nevertheless very doubtful. All the feet of polydactyle horses
which we have examined bear little resemblance to those of _Hipparion_
or _Anchitherium_, but look rather as if due to that tendency to
reduplication of parts which occurs so frequently as a teratological
condition, especially among domestic animals, and, whatever its origin,
certainly cannot in many instances, as the cases of entire limbs
superadded, or of six digits in man, be attributed to reversion.
_Anatomy._—The anatomical structure of the Horse has been described in
great detail in several works devoted to the subject, which will be
mentioned in the bibliography, though these have generally been written
from the point of view of the veterinarian rather than of the comparative
anatomist. The limits of the present work will only admit of the most
salient points being indicated, particularly those in which the Horse
differs from the other Ungulata. Unless otherwise specified, it must be
understood that all that is stated here, although mostly derived from
observation upon the Horse, applies equally well to the other existing
members of the group.
[Illustration: FIG. 163.—Side view of skull of Horse, with the bone
removed so as to expose the whole of the teeth. _PMx_, Premaxilla;
_Mx_, maxilla; _Na_, nasal; _Ma_, malar or jugal; _L_, lachrymal; _Fr_,
frontal; _Sq_, squamosal; _Pa_, parietal; _oc_, occipital condyle; _pp_,
paroccipital process; _i¹_, _i²_, and _i³_³, the three incisors; _c_, the
canine; _pm¹_, the situation of the rudimentary first premolar, which has
been lost in the lower, but is present in the upper jaw; _pm²_, _pm³_,
and _pm⁴_, the three fully developed premolars; _m¹_, _m²_, and _m³_, the
three true molars.]
_Skeleton._—The skull (Fig. 163) as a whole is greatly elongated, chiefly
in consequence of the immense size of the face as compared with the
hinder or true cranial portion. The basal line of the cranium from the
lower border of the foramen magnum to the incisor border of the palate is
very nearly straight. The orbit, of nearly circular form, though small in
proportion to the size of the whole skull, is distinctly marked, being
completely surrounded by a strong ring of bone with prominent edges.
Behind it, and freely communicating with it beneath the osseous bridge
(the postorbital process of the frontal) forming the boundary between
them, is the small temporal fossa occupying the whole of the side of
the cranium proper, and in front is the great flattened expanse of the
“cheek,” formed chiefly by the maxilla, giving support to the long row of
cheek-teeth, and having a prominent ridge running forward from below the
orbit for the attachment of the masseter muscle. The lachrymal occupies
a considerable space on the flat surface of the cheek in front of the
orbit, and below it the jugal or malar does the same. The latter sends
a horizontal or slightly ascending process backwards below the orbit to
join the under surface of the zygomatic process of the squamosal, which
is remarkably large, and, instead of ending as usual behind the orbit,
runs forwards to join the greatly developed postorbital process of the
frontal, and even forms part of the posterior and inferior boundary of
the orbit, an arrangement not met with in other mammals. The closure
of the orbit behind distinguishes the skull of the Horse from that of
the Rhinoceros and Tapir, and also from all of the Perissodactyles of
the Eocene period. In front of the cerebral cavity, the great tubular
nasal cavities are provided with well-developed turbinal bones, and are
roofed over by very large nasals, broad behind, and ending in front in
a narrow decurved point. The opening of the anterior nares is prolonged
backwards on each side of the face between the nasals and the elongated
slender premaxillæ. The latter expand in front, and are curved downwards
to form the semicircular alveolar border supporting the large incisor
teeth. The palate is narrow in the interval between the incisor and
cheek-teeth, in which are situated the large anterior palatine foramina.
Between the cheek-teeth it is broader, and it ends posteriorly in a
rounded excavated border opposite the hinder edge of the penultimate
molar. It is mainly formed by the maxillæ, as the palatines are very
narrow. The pterygoids are delicate slender slips of bone attached to
the hinder border of the palatines, and supported externally by, and
generally ankylosed to, the rough pterygoid plates of the alisphenoid,
with no pterygoid fossa between. They slope very obliquely forwards,
and end in curved, compressed, hamular processes. There is a distinct
alisphenoid canal for the passage of the internal maxillary or main
branch of the external carotid artery. The base of the cranium is long
and narrow; the alisphenoid is very obliquely perforated by the foramen
rotundum, but the foramen ovale is confluent with the large foramen
lacerum medium behind. The glenoid surface for the articulation of the
mandible is greatly extended transversely, concave from side to side,
convex from before backwards in front, and hollow behind, and is bounded
posteriorly at its inner part by a prominent post-glenoid process. The
squamosal enters considerably into the formation of the temporal fossa,
and, besides sending the zygomatic process forwards, it sends down
behind the meatus auditorius a post-tympanic process which aids to hold
in place the otherwise loose tympano-periotic bone. Behind this the
exoccipital gives off a very long paroccipital process. The periotic
and tympanic are ankylosed together, but not with the squamosal. The
former has a wide but shallow floccular fossa on its inner side, and
sends backwards a considerable “pars mastoidea,” which appears on the
outer surface of the skull between the post-tympanic process of the
squamosal and the exoccipital. The tympanic forms a tubular meatus
auditorius externus directed outwards and slightly backwards. It is not
dilated into a distinct bulla, but ends in front in a pointed styliform
process; and completely embraces the truncated cylindrical tympanohyal,
which is of great size, in correspondence with the large development of
the whole anterior arch of the hyoid. This consists mainly of a long
and compressed stylohyal, expanded at the upper end, where it sends
off a triangular posterior process. The basihyal is remarkable for the
long, median, pointed, compressed “glossohyal” process, which it sends
forward from its anterior border into the base of the tongue. A similar
but less developed process is found in the Rhinoceros. The mandible is
largely developed, especially the region of the angle, which is expanded
and flattened, giving great surface for the attachment of the masseter
muscle. The condyle is greatly elevated above the alveolar border; its
articular surface is very wide transversely, and narrow and convex
from before backwards. The coronoid process is slender, straight, and
inclined backwards. The horizontal ramus, long, straight, and compressed,
gradually narrows towards the symphysis, where it expands laterally to
form with the ankylosed opposite ramus the wide, semicircular, shallow
alveolar border for the incisor teeth.
The vertebral column consists of seven cervical, eighteen dorsal, six
lumbar, five sacral, and fifteen to eighteen caudal vertebræ. There
may be nineteen rib-bearing vertebræ, in which case five only will be
reckoned as belonging to the lumbar series. The odontoid process of the
atlas is wide, flat, and hollowed above, as in the Ruminants. The bodies
of the cervical vertebræ are elongated, strongly keeled, and markedly
opisthocœlous, or concave behind and convex in front. Their neural laminæ
are very broad, the spines almost obsolete, except in the seventh, and
the transverse processes not largely developed. In the trunk vertebræ the
opisthocœlous character of the centrum gradually diminishes. The spinous
processes of the anterior thoracic region are high and compressed. To
these is attached the powerful elastic ligament, _ligamentum nuchæ_, or
“paxwax,” which passing forwards in the middle line of the neck above the
neural arches of the cervical vertebræ, to which it is also connected,
is attached to the occiput and supports the weight of the head. The
transverse processes of the lumbar vertebræ are long, flattened, and
project horizontally outwards or slightly forwards from the arch. The
metapophyses are moderately developed, and there are no anapophyses.
The caudal vertebræ, except those quite at the base, are slender and
cylindrical, without processes and without chevron-bones beneath. The
ribs are eighteen or nineteen in number on each side, flattened, and
united to the sternum by short, stout, tolerably well ossified sternal
ribs. The sternum consists of six pieces; the anterior or presternum
being extremely compressed, and projecting forwards like the prow of a
boat. The segments which follow gradually widen, and the hinder part of
the sternum is broad and flat.
As in all other Ungulates, there are no clavicles. The scapula is
long and slender; the suprascapular border is rounded, and slowly and
imperfectly ossified. The spine is very slightly developed; rather above
the middle its edge is thickened and somewhat turned backwards, but it
gradually subsides at the lower extremity without forming any acromial
process. The coracoid process is a prominent rounded nodule. The humerus
is stout and rather short, and has a double bicipital groove. The ulna
is quite rudimentary, being only represented by little more than the
olecranon. The shaft gradually tapers below, and is firmly ankylosed to
the radius. The latter bone is of nearly equal width throughout. The
three bones of the first row of the carpus (the scaphoid, lunar, and
cuneiform) are subequal in size. The second row consists of a very broad
and flat magnum, supporting the great third metacarpal, having to its
radial side the trapezoid, and to its ulnar side the unciform, which
are both small, and articulate distally with the rudimentary second and
fourth metacarpals. The pisiform is large and prominent, flattened,
and curved; articulating partly with the cuneiform and partly with the
lower end of the radius. The large metacarpal is called in veterinary
anatomy “cannon-bone”; the small lateral metacarpals, which gradually
taper towards their lower extremities, and lie in close contact with
the large one, are called “splint-bones.” The single digit consists of
a moderate-sized proximal (_os suffraginis_, or large pastern), a very
short middle (_os coronæ_, or small pastern), and a wide, semilunar,
ungual phalanx (_os pedis_, or coffin-bone). There is a pair of large
nodular sesamoids behind the metacarpo-phalangeal articulation, and a
single large transversely extended sesamoid behind the joint between the
second and third phalanx, called the “navicular bone.”[265]
The carpal joint, corresponding to the wrist of man, is commonly called
the “knee” of the Horse, the joint between the metacarpal and the first
phalanx the “fetlock,” that between the first and second phalanges
the “pastern,” and that between the second and third phalanges the
“coffin-joint.”
In the hind limb the femur is marked, as in other Perissodactyles, by the
presence of a “third trochanter,” a flattened process, curving forwards,
arising from the outer side of the bone, about one-third of the distance
from the upper end. The fibula is reduced to a mere styliform rudiment
of the upper end; its lower part being absent or completely fused with
the tibia. The calcaneum has a long and compressed calcaneal process. The
astragalus has a large flat articular surface in front for the navicular,
and a very small one for the cuboid. The navicular and the external
cuneiform bones are very broad and flat. The cuboid is small, and the
internal and middle cuneiform bones are small and united together. The
metapodials and phalanges resemble very closely those of the fore limb,
but the principal metatarsal is more laterally compressed at its upper
end than is the corresponding metacarpal. The joint between the femur and
tibia, corresponding to the knee of man, is called the “stifle joint”;
while that between the tibia and tarsus, corresponding to the ankle of
man, is termed the “hock.” The bones and joints of the foot have the
same names as in the fore limb. The Horse is eminently “digitigrade,”
standing on the extremity of the single digit of each foot, which is kept
habitually in a position approaching to vertical.
The muscles[266] of the limbs are modified from those of the ordinary
mammalian type in accordance with the reduced condition of the bones
and the simple requirements of flexion and extension of the joints, no
such actions as pronation and supination, or opposition of digits, being
possible or needed. The muscles, therefore, which perform these functions
in other mammals are absent or rudimentary.
[Illustration: FIG. 164.—Section of foot of Horse. 1, Metacarpal bone;
2, first phalanx (_os suffraginis_); 3, second phalanx (_os coronæ_);
4, third or ungual phalanx (_os pedis_, or coffin-bone); 5, one of the
upper sesamoid bones; 6, lower sesamoid or “navicular” bone; 7, tendon
of anterior extensor of the phalanges; 8, tendon of superficial flexor
(_fl. perforatus_); 9, tendon of deep flexor (_fl. perforans_); 10,
suspensory ligament of fetlock; 11, inferior or short sesamoid ligament;
12, derma or skin of the foot, covered with hair, and continued into 13,
the coronary cushion, 14, the podophyllous or laminar membrane, and 15,
the keratogenous membrane of the sole; 16, plantar cushion; 17, hoof; 18,
fatty cushion of fetlock.]
Below the carpal and tarsal joints the fore and hind limbs correspond
almost exactly in structure as well as function. On the anterior or
extensor surface of the limb a powerful tendon (7 in Fig. 164), that of
the anterior extensor of the phalanges (corresponding to the _extensor
communis digitorum_ of the arm and _extensor longus digitorum_ of the
foot of man) passes down over the metacarpal bone and phalanges, to be
inserted mainly into the upper edge of the anterior surface of the last
phalanx or pedal bone. There is also a much smaller second extensor
on the outer side of this in each limb, the lateral extensor of the
phalanges. In the fore leg the tendon of this muscle (which corresponds
with the _extensor minimi digiti_ of man) receives a slip from that of
the principal extensor, and is inserted into the first phalanx. In the
hind leg (where it is the homologue apparently of the _peroneus brevis_
of man) the tendon becomes blended with that of the large extensor.
A very strong ligamentous band behind the metapodium, arising from near
the upper extremity of its posterior surface, divides into two at its
lower end, and each division, being first connected with one of the
paired upper sesamoid bones, passes by the side of the first phalanx to
join the extensor tendon of the phalanges. This is called in veterinary
anatomy the “suspensory ligament of the sesamoids,” or of the “fetlock”
(10 in Fig. 164); but its attachments and relations, as well as the
occasional presence of muscular fibres in its substance, show that it
is the homologue of the short flexor muscle of other mammals, curiously
modified both in structure and function to suit the requirements of the
Horse’s foot. Behind or superficial to this are placed the two strong
tendons of the long flexor muscles, the most superficial, or _flexor
perforatus_ (8), dividing to allow the other to pass through, and then
inserted into the middle phalanx. The _flexor perforans_ (9) is as
usual inserted into the terminal phalanx. In the fore leg these muscles
correspond with those similarly named in man. In the hind leg, the
perforated tendon is a continuation of that of the plantaris, passing
pulley-wise over the tuberosity of the calcaneum. The perforating tendon
is derived from the muscle corresponding with the long flexor of man, and
the smaller tendon of the oblique flexor (_tibialis posticus_ of man) is
united with it.
The hoof of the Horse corresponds to the nail or claw of other mammals,
but is so constructed as to form a complete and very solid case to the
expanded termination of the toe, giving a firm basis of support formed of
a nonsensitive substance, which is continually renewed by the addition of
material from within as its surface wears away by friction against the
ground. The terminal phalanx of the toe is greatly enlarged and modified
in form to support this hoof, and the size of the internal framework of
the foot is further increased by a pair of lateral fibro-cartilaginous
masses attached on each side to the hinder edges of the bone, and by a
fibro-cellular and adipose plantar cushion in the median part. These
structures are all enclosed in the keratogenous membrane or “subcorneous
integument,” a continuation of the ordinary derma of the limb, but
extremely vascular, and having its superficial extent greatly increased
by being developed into papillæ or laminæ. From this the horny material
which constitutes the hoof is exuded. A thickened ring encircling the
upper part, called coronary cushion (13), and the sole (15), are covered
with numerous thickly set papillæ or villi, and take the greatest share
in the formation of the hoof; the intermediate part constituting the
front and side of the foot (14), corresponding with the wall of the
hoof, is covered with parallel, fine longitudinal laminæ, fitting into
corresponding depressions in the inner side of the horny hoof.
The horny hoof is divided into a wall or crust consisting of the front
and sides, the flattened or concave sole, and the “frog,” a triangular
median prominence, notched posteriorly, with the apex turned forwards,
situated in the hinder part of the sole. It is formed of pavement
epithelial cells, mainly grouped in a concentric manner around the
vascular papillæ of the keratogenous membrane, so that a section near the
base of the hoof, cut transversely to the long axis of these papillæ,
shows a number of small circular or oval orifices, with cells arranged
concentrically round them. The nearer the surface of the hoof, or farther
removed from the seat of growth, the more indistinct the structure
becomes.
Small round or oval plates of horny epidermis called “chestnuts,” growing
like the hoof from enlarged papillæ of the skin, are found on the
inner face of the fore limb, above the carpal joint, in all species of
_Equidæ_, and in the Horse (_E. caballus_) alone similar formations occur
near the upper extremity of the inner face of the metatarsus. Their use
is unknown.
Behind the joint between the metapodium and the first phalanx is a
prominence formed by the fatty cushion of the fetlock (18 in Fig. 164).
On the middle of this is a small bare patch covered with thickened
epidermis, the _ergot_ or spur, generally concealed beneath the long hair
which grows around it. This is the functionless vestige of the large
callous pad found in this situation in the Tapir, and in fact in all
mammals in which this part reaches the ground in walking.
[Illustration: FIG. 165.—Longitudinal and transverse section of upper
incisor of Horse. _p_, Pulp cavity; _d_, dentine or ivory; _e_, enamel;
_c_, outer layer of cement; _c′_, inner layer of cement, lining _a_, the
pit or cavity of the crown of the tooth.]
_Dentition._—The dentition of the Horse, when all the teeth are in place,
is, as stated before, expressed by the formula _i_ ³⁄₃, _c_ ¹⁄₁, _p_
⁴⁄₃, _m_ ³⁄₃ = 42. The incisors of each jaw are placed in close contact,
forming a semicircle. The crowns are broad, somewhat awl-shaped, and of
nearly equal size. They have all the great peculiarity, not found in
the teeth of any other living mammal, of an involution of the external
surface of the tooth (see Fig. 165) forming a deep fossa or pit, the
bottom of which becomes partially filled up with cement. As the tooth
wears, the surface, besides the external enamel layer as in an ordinary
simple tooth, shows in addition a second inner ring of the same hard
substance surrounding the pit, thus of course adding greatly to the
efficiency of the tooth as an organ for biting tough, fibrous substances.
This pit, generally filled in the living animal with particles of food,
is conspicuous from its dark colour, and constitutes the “mark” by which
the age of the horse is judged, as in consequence of its extending only
to a certain depth, it becomes obliterated as the crown wears away,
when the tooth assumes the character of an ordinary incisor, consisting
only of a core of dentine surrounded by the external enamel layer. It
is not quite so deep in the lower as in the upper teeth. The canines
are either quite rudimentary or entirely absent in the female. In the
male they are compressed, pointed, and smaller than the incisors, from
which they are separated by a slight interval. The teeth of the cheek
series are all in contact with each other, but separated from the canines
by a considerable toothless space. The anterior premolars are quite
rudimentary, often, especially in the lower jaw, not developed at all,
and generally fall by the time the animal attains maturity, so that there
are but six functional grinding teeth—three that have predecessors in
the milk-dentition, and hence are considered as premolars, and three
true molars, but otherwise, except the first and last of the series,
not distinguishable in form or structure. These teeth in both upper and
lower jaws are extremely long-crowned or hypsodont (Fig. 158), successive
portions being pushed out as the surface wears away;—a process which
continues until the animal becomes advanced in age. The enamelled surface
is infolded in a complex manner (a modification of that found in other
Perissodactyles, see Figs. 155, 167), the folds extending quite to the
base of the crown, and the interstices being filled and the surface
covered with a considerable mass of cement, which binds together and
strengthens the whole tooth. As the teeth wear, the folded enamel,
being harder than the other constituents—the dentine and cement—forms
projecting ridges on the surface arranged in a definite pattern, which
give it great efficiency as a grinding instrument (see Fig. 157, _b_ and
_c_). The free surfaces of the upper teeth are quadrate, except the first
and last, which are nearly triangular. The lower teeth are much narrower
than the upper.
The milk dentition consists of _i_ ³⁄₃, _c_ ⁰⁄₀, _m_ ³⁄₃ = 24,—the
canines and first or rudimentary premolars having apparently no
predecessors. In form and structure they much resemble the permanent
teeth, having the same characteristic enamel-foldings. Their eruption
commences a few days after birth, and is complete before the end of the
first year, the upper teeth usually appearing somewhat earlier than those
of the lower jaw. The first teeth to appear are the first and second
milk-molars (about five days), then the central incisor (from seven to
ten days); this is followed by the second incisor (at one month), then
by the third molar, and finally by the third incisor. Of the permanent
teeth the first true molar appears a little after the end of the first
year, followed by the second molar before the end of the second year. At
about two and a half years the first premolar replaces its predecessor.
Between two and a half and three years the first incisor appears. At
three years the second and third premolars and the third true molar have
appeared; at from three and a half to four years the second incisor; at
four to four and a half years the canine; and, finally, at five years the
third incisor, completing the permanent dentition. Up to this period the
age of the horse is clearly shown by the state of the dentition, and for
some time longer indications can be obtained from the wear of the incisor
teeth, though this depends to a certain extent upon the hardness of the
food or other accidental circumstances. As a general rule, the depression
caused by the infolding of the surface of the incisor (the “mark”), is
obliterated in the first or central incisor at six years, in the second
at seven years, and in the third at eight years. In the upper teeth, as
the depressions are deeper, this obliteration does not take place until
about two years later. After this period no certain indications can be
obtained of the age of the horse from the teeth.
_Digestive Organs._—The lips are flexible and prehensile. The membrane
that lines them and the cheeks is quite smooth. The palate is long and
narrow; its mucous surface has seventeen pairs of not very sharply
defined oblique ridges, extending as far back as the last molar tooth,
beyond which the velum palati extends for about 3 inches, having a soft
corrugated surface, and ending posteriorly in an arched border without
uvula. This embraces the base of the epiglottis, and shuts off all
communication between the cavity of the mouth and the nasal passages,
respiration being, under ordinary circumstances, carried on exclusively
through the nostrils. Between the mucous membrane and the bone of the
hard palate is a dense vascular and nervous plexus. The membrane lining
the fauces is soft and corrugated. An elongated raised glandular mass,
3 inches long and 1 inch from above downwards, extending backwards from
the root of the tongue along the side of the fauces, with openings on the
surface leading into crypts with glandular walls, represents the tonsil.
The tongue, corresponding to the general form of the mouth, is long and
narrow. It consists of a compressed intermolar portion with a flat upper
surface, broad behind and becoming narrower in front; and of a depressed
anterior part rather shorter than the former, which is narrow behind but
widens towards the evenly rounded apex. The dorsal surface generally is
very soft and smooth. There are two large circumvallate papillæ near the
base, rather irregular in form, about a quarter of an inch in diameter
and half an inch apart. The conical papillæ are very small and close set,
though longer and more filamentous on the intermolar portion. There are
no fungiform papillæ on the dorsum, but a few not very conspicuous ones
scattered along the sides of the organ.
Of the salivary glands the parotid is by far the largest; elongated in
the vertical direction, and narrower in the middle than at either upper
or lower extremity. Its upper extremity embraces the lower surface of
the cartilaginous ear-conch; its lower end reaches the level of the
inferior margin of the mandible, along the posterior margin of which it
is placed. Its duct leaves the inferior anterior angle, at first descends
a little, and runs forward under cover of the rounded inferior border of
the mandibular ramus, then curves up along the anterior margin of the
masseter muscle, becoming superficial, pierces the buccinator, and enters
the mouth by a simple aperture opposite the middle of the crown of the
third premolar tooth. It is not quite so thick as a goose-quill when
distended, and nearly a foot in length.
The submaxillary gland is of very similar texture to the last, but much
smaller; it is placed deeper, and lies with its main axis horizontal.
It is elongated and slender, and flattened from within outwards. Its
posterior end rests against the anterior surface of the transverse
process of the atlas, from which it extends forwards and downwards,
slightly curved, to beneath the ramus of the jaw. The duct which runs
along its upper and internal border passes forwards in the usual course,
lying in the inner side of the sublingual gland, to open on the outer
surface of a distinct papilla, situated on the floor of the mouth, half
an inch from the middle line, and midway between the lower incisor teeth
and the attachment of the frænum linguæ. The sublingual is represented by
a mass of glands lying just beneath the mucous membrane of the floor of
the mouth on the side of the tongue, causing a distinct ridge, extending
from the frænum backwards, and the numerous ducts opening separately
along the summit of the ridge. The buccal glands are arranged in two rows
parallel with the molar teeth. The upper ones are the largest, and are
continuous anteriorly with the labial glands, the ducts of which open on
the mucous membrane of the upper lip.
The stomach of the Horse is simple in its external form, with a largely
developed right _cul de sac_, and is a good deal curved on itself,
so that the cardiac and pyloric orifices are brought near together.
The antrum pyloricum is small and not very distinctly marked off. The
interior is divided by the character of the lining membrane into two very
distinct portions, right and left. Over the latter the dense white smooth
epithelial lining of the œsophagus is continued, terminating abruptly
by a raised crenellated border. Over the right part (rather the larger
portion) the mucous membrane has a grayish-red colour and a velvety
appearance, and contains very numerous peptic glands, which are wanting
in the cardiac portion. The œsophageal orifice is very small, and is
guarded by a strong crescentic or rather horse-shoe-like band of muscular
fibres, which is supposed to be the cause of the difficulty of vomiting
in the Horse. The small intestine is of great length (80 to 90 feet),
its mucous membrane being covered with numerous fine villi. The cæcum is
of conical form, about 2 feet long and nearly a foot in diameter; its
walls are sacculated, especially near the base, having four longitudinal
fibrous bands; and its capacity is about twice that of the stomach. It
lies with its base near the lower part of the abdomen, and its apex
directed towards the thorax. The colon is about one-third the length
of the small intestine, and very capacious in the greater part of its
course. As usual, it may be divided into an ascending, transverse, and
descending portion; but the middle or transverse portion is folded into a
great loop, which descends as low as the pubis; so that the colon forms
altogether four folds, generally parallel to the long axis of the body.
The descending colon is much narrower than the rest, and not sacculated,
and being considerably longer than the distance it has to traverse, is
thrown into numerous folds.
The liver (Fig. 166) is tolerably symmetrical in its general arrangement,
being divided nearly equally into segments by a well-marked umbilical
fissure. Each segment is again divided by lateral fissures, which do
not extend quite to the posterior border of the organ; of the central
lobes thus cut off, the right is rather the larger, and has two fissures
in its free border subdividing it into lobules. The extent of these
varies, however, in different individuals, being not usually so marked
as in the figure, which is from a fœtal specimen. The two lateral lobes
are subtriangular in form. The Spigelian lobe is represented by a
flat surface between the portal fissure and the posterior border, not
distinctly marked off from the left lateral by a fissure of the ductus
venosus, as this vessel is buried deep in the hepatic substance, but
the caudate lobe is distinct and tongue-shaped, its free apex reaching
nearly to the border of the right lateral lobe. In most works on the
anatomy of the Horse this has been confounded with the Spigelian lobe of
man. There is no gall-bladder (as in all other Perissodactyles), and the
biliary duct enters the duodenum about 6 inches from the pylorus. The
pancreas has two lobes or branches—a long one passing to the left and
reaching the spleen, and a shorter right lobe. The principal duct enters
the duodenum with the bile-duct, and there is often a second small duct
which opens separately near to this.
[Illustration: FIG. 166.—Under surface of the liver of the Horse. _u_,
Umbilical fissure; _ll_, left lateral lobe; _lc_, left central lobe;
_rc_, right central lobe; _rl_, right lateral lobe; _s_, Spigelian lobe;
_c_, caudate lobe.]
_Circulatory and Respiratory Organs._—The heart has the form of a rather
elongated and pointed cone. There is one anterior vena cava, formed by
the union of the two jugular and two axillary veins. The aorta gives off
a large branch (the anterior aorta) very near its origin, from which
arise—first, the left axillary, and afterwards the right axillary and the
two carotid arteries.
Under ordinary circumstances the Horse breathes entirely by the nasal
passages, the communication between the larynx and the mouth being
closed by the velum palati. The nostrils are placed laterally, near
the termination of the muzzle, and are large and very dilatable, being
bordered by cartilages upon which several muscles act. Immediately within
the opening of the nostril, the respiratory canal sends off on its upper
and outer side a diverticulum or blind pouch (called “false nostril”) of
a conical form, and curved, 2 to 3 inches in depth, lying in the notch
formed between the nasal and premaxillary bones. It is lined by mucous
membrane continuous with that of the nasal passage, but its use is not
apparent. It is longer in the Ass than in the Horse. A similar structure
is found in the Rhinoceros, and in a much more developed condition in
the Tapir. Here may be mentioned the guttural pouches, large air sacs,
diverticula from the Eustachian tubes, and lying behind the upper part
of the pharynx. These are likewise found in other Perissodactyles,
but their use is also still not clearly understood. The larynx has the
lateral sacculi well developed, though entirely concealed within the alæ
of the thyroid cartilage. The trachea divides into two bronchi, one for
each lung.
_Nervous System._—The brain differs little, except in details of
arrangement of convolutions, from that of other Ungulates. The cerebral
hemispheres are rather elongated and subcylindrical, the olfactory
lobes are large and project freely in front of the hemispheres, and the
greater part of the cerebellum is uncovered. The eye is provided with a
nictitating membrane or third eyelid, at the base of which the ducts of
the Harderian gland open.
_Reproductive System._—The testes are situated in a distinct sessile or
slightly pedunculated scrotum, into which they descend from the sixth
to the tenth month after birth. The accessory generative glands are
the two vesiculæ seminales, with the median third vesicle, or _uterus
masculinus_, lying between them, the single bilobed prostate, and a pair
of globular Cowper’s glands. The penis is large, cylindrical, with a
truncated, expanded, flattened termination. When in a state of repose it
is retracted by a muscle arising from the sacrum, within the prepuce, a
cutaneous fold attached below the symphysis pubis.
The uterus is bicornuate. The vagina is often partially divided by a
membraneous septum or hymen. The mammæ (as in other members of the
suborder), are two, inguinally placed. The surface of the chorion is
covered evenly with minute villi, constituting a diffuse non-deciduate
placenta. The period of gestation is eleven months.
_Bibliography._—M. S. Arloing, “Organisation du pied chez le
cheval,” _Ann. Sci. Nat._ 1867, viii. pp. 55-81; H. Burmeister,
_Los caballos fosiles de la Pampa Argentina_, Buenos Ayres,
1875; Chanveau and Arloing, _Traité d’anatomie comparée des
animaux domestiques_, Paris, 1871, and English edition by G.
Fleming, 1873; E. Cuyer and E. Alix, _Le Cheval_, 1886; A.
Ecker, “Das Europäische Wildpferd und dessen Beziehungen zum
domesticirten Pferd,” _Globus_, Bd. xxxiv. Brunswick, 1878;
Forsyth-Major, “Beiträge zur Geschichte der fossilen Pferde
besonders Italiens,” _Abh. Schw. Pal. Ges._ iv. pp. 1-16, pt.
iv.; George, “Études zool. sur les Hémiones et quelques autres
espèces chevalines,” _Ann. Sci. Nat._ 1869, xii. p. 5; E. F.
Gurlt, _Anatomische Abbildungen der Haussäugethiere_, 1824,
and _Hand. der vergleich. Anat. der Haussäugethiere_, 2 vols.
1822; Huet, “Croisement des diverses espèces du genre cheval,”
_Nouv. Archives du Muséum_, 2d sér. tom. ii. p. 46, 1879;
Leisering, _Atlas der Anatomie des Pferdes_, Leipsic, 1861;
J. M’Fadyean, _The Anatomy of the Horse_, 1884; O. C. Marsh,
“Notice of New Equine Mammals from the Tertiary Formation,”
_Am. Journ. of Science and Arts_, vol. vii. March 1874; Id.
“Fossil Horses in America,” _Amer. Naturalist_, vol. viii.
May 1874; Id. “Polydactyle Horses,” _Am. Journ. of Science
and Arts_, vol. xvii. June 1879; Franz Müller, _Lehrbuch der
Anatomie des Pferdes_, Vienna, 1853; R. Owen, “Equine Remains
in Cavern of Bruniquel,” _Phil. Trans._ vol. clix. (1870), p.
535; W. Percivall, _The Anatomy of the Horse_, 1832; G. Stubbs,
_Anatomy of the Horse_, 1766. F. H. Huth’s _Bibliographical
Record of Hippology_ (1887) contains a list of nearly four
thousand works on Horses and Equitation, published in the
various languages of the civilised world.
_Family_ RHINOCEROTIDÆ.
Although the existing members of this family are readily distinguished
from the other living representatives of the suborder by the simple
crescentoid form assumed by the ridges of the lower cheek-teeth, yet
it is exceedingly difficult to give a definition by which they can be
distinguished from the _Lophiodontidæ_, from some members of which they
are, indeed, probably derived. The outer columns of the upper molars
(Fig. 167) are, however, so excessively flattened as to produce a
continuous thick and nearly straight outer wall, which is often produced
in advance of the anterior transverse ridge; both transverse ridges
being but little curved, and intimately connected with the outer wall.
The upper premolars are in most cases nearly or quite as complex as the
molars, and the ridges of the lower cheek-teeth are crescentoid. The
last lower molar has no third lobe. The height of the crowns of the
cheek-teeth is variable. The skull is large, with the orbit confluent
with the temporal fossa. There are either three or four digits in the
manus, and three in the pes. One or more dermal horns are attached to
the fronto-nasal region of the skull of existing forms, but these were
wanting in some of the fossil species.
[Illustration: FIG. 167.—A partially worn second right upper molar of
_Rhinoceros antiquitatis_. Letters as in Fig. 155 (p. 375), except _k_,
which indicates a prolongation of the median valley. (After Owen.)]
_Rhinoceros._[267]—Incisors variable, reduced in number, often quite
rudimentary, and early deciduous. Upper canines absent. Molar series,
consisting of the full number of four premolars and three molars above
and below, all in contact and closely resembling each other, except
the first, which is much smaller than the rest and often deciduous;
and the last, in which the hinder lobe is partly aborted, so that the
contour of the crown is triangular. Head large, skull elongated, elevated
posteriorly into a transverse occipital crest. No postorbital processes.
Nasal bones large and stout, co-ossified, and standing out freely above
the premaxillæ, from which they are separated by a deep and wide fissure;
the latter small, generally not meeting in the middle line in front,
often quite rudimentary. Tympanics small, not forming a bulla. Brain
cavity very small for the size of the skull. Vertebræ: C 7, D 19-20, L 3,
S 4, C about 22. Limbs stout, and of moderate length. Three completely
developed toes, with distinct broad rounded hoofs on each foot (Fig. 151,
p. 368), some fossil forms having a fourth in the manus. Eyes small. Ears
of moderate size, oval, erect, prominent, placed near the occiput. Skin
very thick, in many species thrown into massive folds. Hairy covering
scanty. When one horn is present it is situated over the conjoined nasal
bones; when two, the hinder one is over the frontals. These horns, which
are of a more or less conical form and usually recurved, often grow to
a great length (three or even four feet), and are composed of a solid
mass of hardened epidermic cells growing from a cluster of long dermal
papillæ. The cells formed on each papilla constitute a distinct horny
fibre, like a thick hair, and the whole are cemented together by an
intermediate mass of cells which grow up from the interspaces between
the papillæ. It results from this that the horn has the appearance of a
mass of agglutinated hairs, which, in the newly growing part at the base,
readily fray out on destruction of the softer intermediate substance; but
the fibres differ from true hairs in growing from a free papilla of the
derm, and not within a follicular involution of the same.
[Illustration: FIG. 168.—A partially worn second right upper molar of
(_A_) _Rhinoceros sondaicus_, and (_B_) _R. unicornis_. _k_, Fossette cut
off from median valley; _m_, crotchet; _n_, crista, or combining-plate;
_e_, anterior valley; _l_, anterior intermediate column. Other letters as
in Fig. 155, p. 375.]
The large lower cutting teeth of the typical Rhinoceroses have been
very generally regarded as incisors, but comparison with fossil allied
types, in which three lower incisors and canines are present, leaves
little doubt but that they are really canines. The upper molar teeth
present some amount of specific variation; thus while one type (Fig.
168, _A_) has only a simple “crotchet” projecting from the posterior
transverse ridge into the median valley, in others (Fig. 168, _B_)
this crotchet joins a “crista,” or “combing-plate,” projecting from
the outer wall to cut off a distinct fossette from the median valley.
Occasionally, however (as in Fig. 167), the crotchet and combing-plate
do not completely join, although the fossette is distinctly indicated.
The first upper premolar may occasionally be preceded by a milk-tooth.
The Rhinoceroses differ from the Horses and agree with the Tapirs in the
direction of the cæcum.
The living species of _Rhinoceros_ are all animals of large size, but
of little intelligence, generally timid indisposition, though ferocious
when attacked and brought to bay, using the nasal horns as weapons, by
which they strike and toss their assailant. Their sight is dull, but
their hearing and scent are remarkably acute. They feed on herbage,
shrubs, and leaves of trees, and, like so many other large animals which
inhabit hot countries, sleep the greater part of the day, being most
active in the cool of the evening or even during the night. They are fond
of bathing and wallowing in water or mud. None of the species have been
domesticated. Animals of the group have existed in both the Old and New
Worlds since the latter part of the Eocene period. In America they all
became extinct before the end of the Pliocene period. In the Old World
their distribution has become greatly restricted, and they are no longer
found in Europe and North Asia, but only in Africa and portions of the
Indian and Indo-Malayan region.
_Existing Species._—The existing (as well as many of the extinct) species
of Rhinoceroses naturally divide into three groups, which are regarded by
some zoologists as of generic value.
_Rhinocerotic, or Typical Group._—The adults with a single large
compressed incisor above on each side, and occasionally a small lateral
one; below, a very small incisor and a very large, procumbent, pointed
canine. Nasal bones pointed in front. A single nasal horn. Skin very
thick, and raised into strong, definitely arranged ridges or folds.
[Illustration: FIG. 169.—Indian Rhinoceros (_Rhinoceros unicornis_). This
figure, and also figures 170, 172, are reduced from drawings by J. Wolf,
from animals living in the London Zoological Society’s Gardens.]
There are two well-marked species of one-horned Rhinoceroses. (1) The
Indian Rhinoceros, _R. unicornis_ (Fig. 169) of Linnæus,[268] the
largest and best known, from being the most frequently exhibited alive
in England, is at present only met with in a wild state in the terai
region of Nipal and Bhutan, and in the upper valley of the Brahmaputra
or province of Assam, though it formerly had a wider range. The first
Rhinoceros seen alive in Europe since the time when these animals, in
common with nearly all the large remarkable beasts of both Africa
and Asia, were exhibited in the Roman shows, was of this species. It
was sent from India to Emmanuel, King of Portugal, in 1513; and from
a sketch of it, taken in Lisbon, Albert Dürer composed his celebrated
but rather fanciful engraving, which was reproduced in so many old
books on natural history. Both in this and the following species the
post-glenoid and post-tympanic processes of the squamosal bone of the
skull unite below so as to completely surround the external auditory
meatus. The molar teeth are hypsodont, and have a horizontal plane of
wear; those of the upper jaw (Fig. 168, _b_) being characterised by the
presence of a combing-plate joining the crotchet, and the absence of
a distinct buttress at the antero-external angle. The stomach departs
from the ordinary Perissodactyle type. The small intestine is beset
over most of its surface with long and fine villi; and the Spigelian
lobe of the liver is well developed. There is a gland behind the foot.
Teeth from the Pleistocene of the Narbada valley in India apparently
indicate the existence of the Indian Rhinoceros at that epoch. (2) The
Javan Rhinoceros (_R. sondaicus_, Fig. 170) is a smaller form, readily
distinguished by dental and internal characters, as well as by the
different arrangement of the plications of the skin (as seen in the
figures); the horn in the female appears to be very little developed, if
not altogether absent. This species has a more extensive geographical
range, being found in the Bengal Sunderbans near Calcutta, Burma, the
Malay Peninsula, Java, Sumatra, and probably Borneo. The molar teeth have
shorter crowns than in the preceding species, and wear into ridges;
those of the upper jaw (Fig. 168, _a_) having no combing-plate, and a
strongly marked buttress at the antero-external angle (not distinctly
shown in the figure). The visceral anatomy, according to Beddard,[269]
does not differ materially from that of the next species. In respect
to its dentition and anatomical characters this species is indeed more
nearly allied to the Sumatran than to the Indian Rhinoceros; and thereby
indicates that the division of the existing Rhinoceroses into separate
genera is not advisable.
[Illustration: FIG. 170.—Javan Rhinoceros (_Rhinoceros sondaicus_).]
_Ceratorhine Group._—The adults with a moderate-sized compressed incisor
above, and a laterally placed, pointed, procumbent canine below, which is
sometimes lost in old animals. Nasal bones narrow and pointed anteriorly.
A well-developed nasal, and a small frontal horn separated by an
interval. The skin thrown into folds, but these not so strongly marked
as in the former group. The smallest living member of the family, the
Sumatran Rhinoceros, _R. sumatrensis_, Cuvier, now represents this group.
Its geographical range is nearly the same as that of the Javan species,
though not extending into Bengal; but it has been found in Assam,
Chittagong, Burma, the Malay Peninsula, Sumatra, and Borneo. So far as
can be determined during the life of the type specimen, it appears that
the hairy form from Chittagong, described as _R. lasiotis_, is only a
variety of this species.[270] The molar teeth of the Sumatran Rhinoceros
are almost indistinguishable from those of the Javan species, and
reference has already been made to the resemblance between the visceral
anatomy of these species.[271] The form of the stomach is very similar to
that of the Horse. The liver (Fig. 171) has a comparatively large caudate
lobe, but is chiefly remarkable for the peculiar shape of the Spigelian
lobe, which mainly consists of a thin strip of tissue, 8 inches long, ¾
inch wide, and ¼ inch deep. The small intestine, in place of the villi of
_R. unicornis_, has throughout the greater part of its length a uniform
series of thin and nearly or quite continuous transverse foldings, like
the valvulæ conniventes of the human small intestine. There is no gland
behind the foot. The post-glenoid and post-tympanic processes of the
squamosal do not unite below the auditory meatus. The presence of a
lateral nasal diverticulum, like that of the Horses and Tapirs, has been
verified only in this species, although it doubtless occurs in the others.
[Illustration: FIG. 171.—Posterior aspect of the liver of _Rhinoceros
sumatrensis_. _rc_, Right central lobe; _rl_, right lateral lobe; _lc_,
left central lobe; _ll_, left lateral lobe; _c_, caudate lobe; _sp_,
Spigelian lobe. (From Garrod, _Proc. Zool. Soc._ 1873, p. 102.)]
_Atelodine Group._—In the adults the incisors and canines quite
rudimentary or entirely wanting. Nasal bones thick, rounded and truncated
in front. Well-developed anterior and posterior horns in close contact.
Skin without any definite permanent folds.
The two well-marked existing species are peculiar to the African
continent.
[Illustration: FIG. 172.—Common African Rhinoceros (_Rhinoceros
bicornis_).]
The common Two-horned Rhinoceros, _R. bicornis_, is the smaller of the
two, with a pointed prehensile upper lip, and a narrow compressed deep
symphysis of the lower jaw. It ranges through the wooded and watered
districts of Africa, from Abyssinia in the north to the Cape Colony, but
its numbers are yearly diminishing, owing to the inroads of European
civilisation, and especially of English sportsmen. It feeds exclusively
upon leaves and branches of bushes and small trees, and chiefly frequents
the sides of wood-clad rugged hills. Specimens in which the posterior
horn has attained a length as great as, or greater than, the anterior
have been separated under the name of _R. keitloa_, but the characters of
these appendages are too variable to found specific distinctions upon.
The Common African Rhinoceros is far more rarely seen in menageries in
Europe than either of the three Oriental species, but one has lived in
the gardens of the London Zoological Society since 1868. The molar teeth
of this species are of the general type of those of _R. sondaicus_,
having no combing-plate to join the crotchet in those of the upper jaw.
The conch of the ear is much rounded at its extremity, and edged by a
fringe of short hairs; while the nostrils are somewhat rounded. The eye
is placed immediately below the posterior horn.[272] Both in this and the
following species the post-glenoid and post-tympanic processes of the
squamosal do not unite below the auditory meatus. Nothing is known of the
anatomy of the soft parts of either of them.
Burchell’s or the Square-mouthed Rhinoceros (_R. simus_), sometimes
called the White Rhinoceros, though the colour (dark slate) is not
materially different from that of the last species, is the largest of the
whole group, and differs from all the others in having a square truncated
upper lip and a wide, shallow, spatulate symphysis to the lower jaw. In
conformity with the structure of the mouth, this species lives entirely
by browsing on grass, and is therefore more partial to open countries
or districts where there are broad grassy valleys between the tracts of
bush. It is only found in Africa south of the Zambesi, and of late years
has become extremely scarce, owing to the persecutions of sportsmen;
indeed, the time of its complete extinction cannot be far off. No
specimen of this species has ever been brought alive to Europe. Mr. F. C.
Selous[273] gives the following description of its habits from extensive
personal observation:—
“The square-mouthed rhinoceros is a huge ungainly-looking beast, with
a disproportionately large head, a large male standing 6 feet 6 inches
at the shoulder. Like elephants and buffaloes they lie asleep during
the heat of the day, and feed during the night and in the cool hours of
early morning and evening. Their sight is very bad; but they are quick of
hearing, and their scent is very keen; they are, too, often accompanied
by rhinoceros birds, which, by running about their heads, flapping their
wings, and screeching at the same time, frequently give them notice of
the approach of danger. When disturbed they go off at a swift trot, which
soon leaves all pursuit from a man on foot far behind; but if chased by
a horseman they break into a gallop, which they can keep up for some
distance. However, although they run very swiftly, when their size and
heavy build is considered, they are no match for an average good horse.
They are, as a rule, very easy to shoot on horseback, as, if one gallops
a little in front of and on one side of them, they will hold their
course, and come sailing past, offering a magnificent broadside shot,
while under similar circumstances a prehensile-lipped rhinoceros will
usually swerve away in such a manner as only to present his hind-quarters
for a shot. When either walking or running, the square-mouthed rhinoceros
holds its head very low, its nose nearly touching the ground. When a
small calf accompanies its mother it always runs in front, and she
appears to guide it by holding the point of her horn upon the little
animal’s rump; and it is perfectly wonderful to note how in all sudden
changes of pace, from a trot to a gallop or _vice versâ_, the same
position is always exactly maintained. During the autumn and winter
months (_i.e._ from March to August) the square-mouthed rhinoceros is
usually very fat; and its meat is then most excellent, being something
like beef, but yet having a peculiar flavour of its own. The part in
greatest favour among hunters is the hump, which, if cut off whole and
roasted just as it is in the skin, in a hole dug in the ground, would, I
think, be difficult to match either for juiciness or flavour.”
The molar dentition is of the type obtaining in _R. unicornis_, so that
in this respect _R. simus_ has the same relation to _R. bicornis_ as
is presented by _R. unicornis_ to _R. sondaicus_. The ear-conch of the
Square-mouthed Rhinoceros is very large, elongated, and pointed at its
extremity, which bears only a slight tuft of hair; it is much expanded
in the middle, and the lower portion has its edges united to form a
short tube. The nostrils have a long slit-like aperture; and the eye is
situated behind the posterior horn.
_Extinct Species._—Using the generic term _Rhinoceros_ in its widest
signification, a very large number of fossil forms may be referred to it,
the earliest of which date from the Upper Eocene (Oligocene) Phosphorites
of Central France. Only a few of the more important of these types can,
however, be even mentioned in this place.
In the Pliocene Siwaliks of India _R. sivalensis_ appears to have been
the direct ancestor of _R. sondaicus_; while _R. palæindicus_ was
probably nearly related to _R. unicornis_, although the upper molars had
not developed a combing-plate.
_R. schleirmacheri_, of the Lower Pliocene of Europe, falls into the
Ceratorhine group, although differing from _R. sumatrensis_ by the union
of the post-glenoid and post-tympanic processes of the squamosal beneath
the auditory meatus. The Middle Miocene _R. sansaniensis_ was a closely
allied if not identical form.
The Atelodine group was very widely spread in past epochs. Thus the
huge _R. platyrhinus_ of the Indian Pliocene, and the equally large _R.
antiquitatis_ of the Pleistocene of Europe, were specialised forms with
a dentition resembling that of _R. simus_, to which they were probably
allied. An upper molar of _R. antiquitatis_—the so-called Tichorine, or
Woolly Rhinoceros—is shown in the woodcut on p. 402. Of this species
nearly whole carcases, with the thick woolly external covering, have been
discovered associated with those of the Mammoth, preserved in the frozen
soil of the north of Siberia. In common with some other extinct species
it had a solid median wall of bone supporting the nasals, from which it
is inferred that the horns were of a size and weight surpassing that of
the modern species. In the Lower Pliocene of Attica _R. pachygnathus_
appears to have been closely allied to _R. bicornis_. Several species
such as _R. leptorhinus_ (Fig. 173), _R. megarhinus_, and _R. etruscus_,
occur in the European Pleistocene which do not present a marked
relationship to any of the living forms. This group is also represented
in the Pleistocene of Southern India by the small _R. deccanensis_ and
_R. karnuliensis_.
In the Upper Miocene, or Lower Pliocene, of North America numerous
Rhinoceroses with incisor teeth occur which have no nasal horn, although
in those forms of which the limbs are known the fore feet resembled
those of existing species in having only three digits. These species
have been generically separated as _Aphelops_, but so closely do they
resemble existing Rhinoceroses that at one time Professor Cope proposed
to refer the hornless female of _R. sondaicus_ (described by Lesson as
_R. inermis_) to the same genus. If these American types be included in
_Rhinoceros_ there seems no valid reason for separating the European
Lower Pliocene and Miocene forms described as _Aceratherium_, at least
some of which have four digits in the manus. This group is represented
in the Upper Eocene Phosphorites of France, and also by a very large
species in the Pliocene of India. Lastly, _R. minutus_, of the Lower
Miocene of France, and an allied North American species are distinguished
by carrying a pair of very small horns placed transversely across the
nasals, from which feature it has been proposed that they should be
separated genetically as _Diceratherium_.
[Illustration: FIG. 173.—Skull of _Rhinoceros leptorhinus_, from the
Pleistocene of Essex. About ⅛ natural size.]
_Extinct Generic Types._—The Tertiary deposits of different parts of the
world have yielded remains of many extinct forms more or less closely
related to the Rhinoceroses, and some of which should certainly be
included in the same family; although others perhaps form the types
of one or more distinct families. One of the most remarkable of these
extinct types is the huge _Elasmotherium_, from the Pleistocene of
Siberia, in which the dentition was reduced to two premolars and three
molars on either side of each jaw. The structure of the skeleton is
essentially rhinocerotic, the skull having an ossified nasal septum,
and a huge frontal prominence for the support of a very large horn. The
teeth are extremely hypsodont, with the enamel plicated to a remarkable
degree, and unlike those of _Rhinoceros_. The genus is evidently a very
specialised one.
The other genera we have to notice are more generalised types. Of these
the North American _Hyracodon_, with the full typical number of teeth,
and without nasal horn, appears to connect the Rhinoceroses with the
Lophiodont _Hyrachyus_. The genera _Amynodon_ and _Metamynodon_ (Fig.
174), from the American Tertiaries, are forms allied to the Rhinoceroses,
with the full number of incisors and canines, and the hinder lobe of the
last upper molar not aborted. The lower canines are either upright, or
less proclivous than in the Rhinoceroses; in _Metamynodon_ the premolars
are reduced to ³⁄₂. Molar teeth from the Phosphorites of Central France,
described under the name of _Cadurcotherium_, are constructed on the
general plan of those of the Rhinoceroses, although distinguished by
their extreme narrowness; this type of tooth being very similar to that
found in _Homalodontotherium_ from Tertiary deposits in Patagonia. The
latter has the full number of teeth, without any diastema in the series.
Until we have some knowledge of the skeleton of these remarkable forms
nothing definite can be said as to their serial position.
[Illustration: FIG. 174.—Right half of the palatal surface of the cranium
of _Metamynodon planifrons_, from the Upper Miocene of North America.
(After Scott and Osborn.)]
_Families_ LAMBDOTHERIIDÆ, CHALICOTHERIIDÆ, AND TITANOTHERIIDÆ.
These families contain a large number of more or less nearly related
extinct types from Tertiary beds of both the Old and New Worlds, some
of which present most remarkable deviations from the ordinary Ungulate
structure. All are characterised by their brachydont molars, which depart
widely from the normal lophodont type. The upper molars consist of four
columns, of which the two external ones are expanded to form an outer
wall; the posterior pair being connected in some cases by an oblique
transverse ridge, while there may be traces of an anterior ridge. The
premolars are simpler.
_Lambdotheriidæ._—This family is confined to the Upper Eocene and
Miocene of North America, where it is represented by _Lambdotherium_,
_Palæosyops_, and _Limnosyops_; it presents the normal type of foot
structure, and all the genera except the first have the full complement
of teeth. There were four digits in the manus. The last lower molar has
a third lobe. _Limnosyops_ differs from _Palæosyops_ in having two inner
columns to the last upper molar.
[Illustration: FIG. 175.—Anterior and distal aspects of a phalangeal bone
of _Chalicotherium sivalense_. (From the _Palæontologia Indica_.)]
_Chalicotheriidæ._—The genus _Chalicotherium_, which is found in the
Tertiaries of Europe, Asia, and North America, differs so remarkably
in the structure of the feet from all other Ungulates that it has
been proposed to regard it as the representative of a distinct order,
Ancylopoda. The molars are, however, almost indistinguishable from those
of the preceding and following families; while the cervical vertebræ
and portions of the limbs are of a Perissodactyle type. On the other
hand, the femur has lost its third trochanter; while the phalanges are
strangely modified, the terminal ones forming long curved claws, while
the others (Fig. 175) have strong ginglymoid distal articulations.
These phalanges were, indeed, long regarded as referable to Edentates,
being described in Europe as _Macrotherium_, and in the United States
as _Morotherium_ and _Moropus_. _Ancylotherium_, of the Grecian Pikermi
beds, is founded upon phalanges which indicate an allied genus. The
Indian species of _Chalicotherium_ is distinguished by the loss of the
incisors and the upper canine; while all the species want the first
premolar.
_Titanotheriidæ._—This exclusively North American family includes
gigantic forms closely allied to the _Lambdotheriidæ_, but with the last
upper premolar as complex as the molars, and frequently with large bony
protuberances in the nasal region. The best known genus, _Titanotherium_
(_Menodus_,[274] _Brontotherium_, _Symborodon_, _Allops_, etc.), may
either have the full complement of teeth, or the incisors may be reduced
to ²⁄₀. The canines and incisors are small, and there is no diastema
when the full dental series is developed. The skull is very like that of
the Rhinoceroses; but has a transverse pair of large bony prominences
on the nasal region, varying considerably in shape and size in the
different species, which in the living animal were probably covered with
horny sheaths. The third trochanter of the femur was aborted. These
huge animals—inferior in size only to the Elephant—appear to have been
abundant in the United States during the Miocene period.
_Family_ MACRAUCHENIIDÆ.
This extinct South American family is best known by the genus
_Macrauchenia_, as represented by _M. patachonica_ and _M. boliviensis_,
which are apparently from Pleistocene formations. They are very singular
and specialised forms, quite out of the line of descent of any of the
existing Perissodactyles, and the steps by which they are connected
with the rest of the group have not yet been discovered. Of the larger
species, _M. patachonica_, the skeleton is completely known. It had the
full number of forty-four teeth, forming an almost uninterrupted series.
The cervical vertebræ resemble those of the Camels in the position of
the vertebrarterial canal, but the ends of the centra are flat, and not
opisthocœlous as in the allied forms. In some of the limb characters it
resembles the _Equidæ_, but in the articulation of the fibula with the
calcaneum it agrees with the Artiodactyles. The structure of the feet is,
however, distinctly Perissodactylate, there being three toes on each. The
teeth approximate to a Rhinocerotine structure; and the incisors have an
infolding of the enamel of their crowns, as in those of the Horses. The
nares open on the top of the skull, and it is probable that the muzzle
was produced into a short proboscis. Several other South American forms
have been referred to this family, some of which have received distinct
generic names, but further evidence is required before many of them can
be accepted. Possibly _Homalodontotherium_ should be placed here.
_Family_ PROTEROTHERIIDÆ.
_Proterotherium._—Here may be noticed certain very remarkable
Perissodactyles from the South American Tertiaries, for which the name
_Proterotherium_ has been proposed. The cheek-teeth are so like those
of _Anchitherium_ that they have been described under that name. The
upper jaw has one pair of canine-like incisors and no canines, while the
lower jaw carries two pairs of incisors. In the skull the orbits were
completely closed, as in the Horses. The feet were tridactyle, like those
of _Hipparion_, but the tarsus was constructed on an Artiodactyle type.
SUBUNGULATA.
By far the greater number of the Subungulata are extinct, and of many of
those whose former existence has been revealed, chiefly by the labours
of the American palæontologists, our knowledge is at present necessarily
imperfect, though daily extending. It will only be possible here to give
details of some of the more interesting or best-known forms.
The characters by which the skeleton of the feet of the Subungulata
are distinguished from those of the Ungulata Vera have been already
mentioned on p. 275. In addition to these it may be observed that the
feet frequently have five functional digits, and may be plantigrade;
while the upper surface of the astragalus is generally flattened, instead
of presenting the strongly-marked pulley-like ridges and groove so
characteristic of the Ungulata Vera.
_Suborder_ HYRACOIDEA.
_Family_ HYRACIDÆ.
[Illustration: FIG. 176.—_Hyrax capensis._]
This division is constituted to receive a single family of mammals, the
affinities of which have long constituted a puzzle to zoologists. They
were first placed among the Rodents, to which animals their small size
and general appearance and habits give them much superficial resemblance.
Cuvier’s investigations into their anatomical structure, and especially
their dental characters, led him to place them among the Ungulates, near
the genus _Rhinoceros_, a position long accepted by many zoologists.
Further knowledge of their organisation and mode of development caused
Milne-Edwards, Huxley, and others to disassociate them from this
connection, and, failing to find any agreement with any other known
forms, to place them in an order entirely apart. Palæontology has thrown
no light upon the affinities of this anomalous and isolated group, as no
extinct animals possessing their distinctive characters have as yet been
discovered.
[Illustration: FIG. 177.—Skull and dentition of _Dendrohyrax dorsalis_. ×
⅔.]
The dentition, according to the usual interpretation, consists only of
incisors and molars, the formula in all known species being _i_ ¹⁄₂,
_c_ ⁰⁄₀, _p_ ⁴⁄₄, _m_ ³⁄₃. The upper incisors have persistent pulps,
and are curved longitudinally, forming a semicircle as in Rodents. They
are, however, not flattened from before backwards as in that order, but
prismatic, with an antero-external, an antero-internal, and a posterior
surface, the first two only being covered with enamel; their apices are
consequently not chisel-shaped, but sharp pointed. They are preceded by
functional, rooted milk-teeth. The outer lower incisors, which should
perhaps be regarded rather as canines, have long tapering roots, but not
of persistent growth. They are straight, procumbent, with awl-shaped,
trilobed crowns. Behind the incisors is a considerable diastema. The
molars and premolars are all contiguous, and formed almost exactly on
the pattern of some of the Perissodactyle Ungulates. The hyoid arch is
unlike that of any known mammal. The dorsal and lumbar vertebræ are very
numerous, 28 to 30, of which 21 or 22 bear ribs. The tail is extremely
short. There are no clavicles. In the fore foot the three middle toes are
subequally developed, the fifth is present, but smaller, and the hallux
is rudimentary, although, in one species at least, all its normal bones
are present. The ungual phalanges of the four outer digits are small,
somewhat conical, and flattened in form. The carpus has a distinct os
centrale. There is a slight ridge on the femur in the place of a third
trochanter. The fibula is complete, thickest at its upper end, where
it generally ankyloses with the tibia. The articulation between the
tibia and astragalus is more complex than in other mammals, the end
of the malleolus entering into it. The hind foot is very like that of
_Rhinoceros_, having three well-developed toes. There is no trace of a
hallux, and the fifth metatarsal is represented only by a small nodule.
The ungual phalanx of the inner (or second) digit is deeply cleft, and
has a peculiar long curved claw, the others have short broad nails. The
stomach is formed upon much the same principle as that of the Horse
or Rhinoceros, but is more elongated transversely and divided by a
constriction into two cavities—a large left _cul de sac_, lined by a very
dense white epithelium and a right pyloric cavity, with a very thick,
soft, vascular lining. The intestinal canal (Fig. 178) is long, and has
an arrangement perfectly unique among mammals, indeed among vertebrated
animals, for, in addition to the ordinary short, but capacious and
sacculated cæcum (_cm_) at the commencement of the colon, there is, lower
down, an additional pair of large, conical, pointed, supplemental cæca
(_c_). The liver is much subdivided, and there is no gall-bladder. The
brain resembles that of the typical Ungulates far more than the Rodents.
The testes are permanently abdominal. The ureters open into the fundus
of the bladder, as in some Rodents. The female has six teats, of which
four are inguinal and two axillary; and the placenta is zonary, as in the
Elephant and Carnivora.
[Illustration: FIG. 178.—Diagrammatic view of the alimentary canal
of _Hyrax capensis_, the intestines being somewhat abbreviated. _d_,
Duodenum; _i_, ileum; _cm_, cæcum; _c_, supplemental colic cæca; _r_,
rectum.]
There are two distinct forms of Hyrax, differing both in structure and
habits, which may be accorded generic rank.
_Hyrax._[275]—Molar teeth having the same pattern as those of
_Rhinoceros_. Interval between upper incisors less than the width of the
teeth. Lower incisors slightly notched at the cutting edge. Vertebræ:
C 7, D 22, L 8, S 6, C 6. Of this form the earliest known species, _H.
capensis_ (Fig. 176) is the type. There are several other species, as _H.
habessinicus_ and _syriacus_, from Eastern Africa and Syria. They inhabit
mountainous and rocky regions, and live on the ground.
_Dendrohyrax._[276]—Molar teeth having the same pattern as _Palæotherium_
(except that the third lower molar has but two lobes). Interval between
upper incisors exceeding the width of the teeth. Lower incisors with very
distinctly trilobed crowns. Vertebræ: C 7, D 21, L 7, S 5, C 10. The
members of this section frequent the trunks and large branches of trees,
sleeping in holes. There are several species, not distinctly defined,
from western and south Africa, as _D. arboreus_ and _D. dorsalis_. The
members of both groups appear to have a power like that possessed by the
Lizards called Geckos of clinging to vertical surfaces of rocks and trees
by the soles of their feet.
It should be added that some writers separate three of the African
species usually included in _Hyrax_ (viz. _H. bocagei_, _H. bakeri_, and
_H. blainvillei_) under the designation of _Heterohyrax_.[277]
_Suborder_ PROBOSCIDEA.
This name has been appropriated to a well-marked group of animals,
presenting some very anomalous characters, allied in many respects to
the typical Ungulata, but belonging neither to the Artiodactyle nor
Perissodactyle type of that order. It has been thought that they possess
some, though certainly not very close, affinities with the Rodentia, and
also with the Sirenia. It is certain, however, that the two species of
Elephant, which are the sole living representatives of the group, stand
quite alone among existing mammals, differing widely from all others
in many points of their structure. In some respects, as the skull,
proboscis, and dentition, they are highly specialised; but in others,
as in the presence of two anterior venæ cavæ and in the structure of
the limbs, they retain a low or generalised condition. A considerable
series of extinct forms, extending back through the Pliocene and Miocene
epochs, show the same type under different modifications, and in still
more generalised outlines; and certain forms from the Eocene of North
America, if their affinities are rightly interpreted, appear to link the
true Proboscidea to some unknown primitive type of Ungulata.
The following are the principal characters common to existing, and, by
inference, to the extinct, Proboscidea. The nose extended into a long,
muscular, very flexible and prehensile proboscis, at the end of which
the nostrils are situated, and from which the name given to the group
is derived. The teeth consisting of ever-growing incisors of very great
size, but never exceeding one pair in each jaw, and often present in
one jaw only; no canines; large and transversely ridged molars. No
clavicles. Limbs strong, the upper segment, especially in the hind limb,
the longer. Radius and ulna distinct, the latter articulating extensively
with the carpus. Fibula and tibia distinct. Astragalus very flat on both
surfaces. Manus and pes short, broad, and massive, each with five toes,
though the outer pair may be more or less rudimentary, all encased in
a common integument, though with distinct, broad, short hoofs. Third
digit the largest. Two anterior venæ cavæ entering the right auricle.
Stomach simple. A capacious cæcum. Testes permanently abdominal. Uterus
bicornuate. Placenta non-deciduate and zonary. Mammæ two, pectoral.
With regard to the teeth, the incisors,[278] which project largely out
of the mouth, and are commonly called “tusks,” are of an elongated
conical form, and generally curved. They are composed mainly of solid
dentine, the fine elastic quality and large mass of which renders it
invaluable as “ivory” for commerce and the arts. A peculiarity of the
dentine of most Proboscidea is that it shows, in transverse fractures or
sections, striæ proceeding in the arc of a circle from the centre to the
circumference in opposite directions, and forming by their decussations
curvilinear lozenges, as in the “engine-turning” of the case of a watch.
The enamel-covering in existing species is confined to the extreme
apex, and very soon wears off, but in some extinct species it forms
persistent longitudinal bands of limited breadth. The tusks have small
milk-predecessors, shed at an early age.
The molar teeth present a remarkable series of modifications, from the
comparatively simple form in _Dinotherium_, with two or three strongly
pronounced transverse ridges and a normal mode of succession, to the
extremely complex structure and anomalous mode of replacement found in
the true Elephants. The intermediate conditions occur in the various
species of _Mastodon_. In this genus the enamel-covered transverse
ridges of each tooth are generally more numerous than in _Dinotherium_,
and often complicated by notches dividing their edge or by accessory
columns attached to them, but in the unworn tooth they stand out freely
on the surface of the crown, with deep valleys between (Fig. 179, I).
In the Elephants the ridges are still further increased in number, and
consequently narrower from before backwards, and are greatly extended
in vertical height, so that, in order to give solidity to what would
otherwise be a laminated or pectinated tooth, it becomes necessary to
envelop and unite the whole in a large mass of cement, which completely
fills up the valleys, and gives a general smooth appearance to the organ
when unworn; but as the wear consequent upon the masticating process
proceeds, the alternate layers of tissue of different hardness—cement,
dentine, and enamel—which are disclosed upon the surface form a fine and
very efficient triturating instrument. The modification of the tooth of
a Mastodon into that of an Elephant is therefore precisely the same in
principle as that of the molar of a Palæotherium into that of a Horse,
or of the corresponding tooth of one of the primitive Artiodactyles into
that of an Ox. The intermediate stages, moreover, even in the present
state of our knowledge, are so numerous that it is not possible to draw a
definite line between the two types of tooth structure (see Fig. 179, I,
II, III, IV).
[Illustration: FIG. 179.—Longitudinal sections of the crown of a
molar tooth of various Proboscideans, showing stages in the gradual
modification from the simple to the complex form. I, _Mastodon
americanus_; II, _Elephas insignis_; III, _Elephas africanus_; IV,
_Elephas primigenius_. The dentine is indicated by transverse lines, the
cement by a dotted surface, and the enamel is black.]
As regards the mode of succession, that of modern Elephants is, as
before mentioned, very peculiar. During the complete lifetime of the
animal there are but six molar teeth on either side of each jaw, with
occasionally a rudimentary one in front, completing the typical number
of seven. The last three represent the true molars of ordinary mammals;
those in front appear to be milk-molars, which are never replaced by
permanent successors, but the whole series gradually moves forwards in
the jaw, and the teeth become worn away and their remnants cast out in
front, while development of others proceeds behind. The individual teeth
are so large, and the processes of growth and destruction by wear take
place so slowly, that not more than one, or portions of two, teeth are
ever in place and in use on either side of each jaw at one time, and the
whole series of changes coincides with the usual duration of the animal’s
life. On the other hand, the Dinotherium, the opposite extreme of the
Proboscidean series, has the whole of the molar teeth in place and use
at one time, and the milk-molars are vertically displaced by premolars
in the ordinary fashion. Among Mastodons transitional forms occur in
the mode of succession as well as in structure, many species showing a
vertical displacement of one or more of the milk-molars, and the same has
been observed in one extinct species of Elephant (_E. planifrons_) as
regards the posterior of these teeth.
All known Proboscideans are animals of comparatively large dimensions,
and some are the most colossal of land mammals. The head is of great
proportionate size; and, as the brain case increases but little in bulk
during growth, while the exterior wall of the skull is required to
be of great superficial extent to support the trunk and the huge and
ponderous tusks, and to afford space for the attachment of muscles of
sufficient size and strength to wield the skull thus heavily weighted,
an extraordinary development of air-cells takes place in the cancellous
tissue of nearly all the bones of the cranium (Fig. 180). These cells
are not only formed in the walls of the cranium proper, but are also
largely developed in the nasal bones and upper part of the premaxillæ and
maxillæ, the bones forming the palate and the basicranial axis, and even
extend into the interior of the ossified mesethmoid and vomer. Where two
originally distinct bones come into contact, the cells pass freely from
one to the other, and almost all the sutures become obliterated in old
animals. The intercellular lamellæ in the great mass which surrounds
the brain cavity superiorly and laterally mostly radiate from the inner
to the outer table, but in the other bones their direction is more
irregular. Like the similar but less developed air-cells in the skulls
of many other mammals, they all communicate with the nasal passages,
and they are entirely secondary to the original growth of the bones,
their development having scarcely commenced in the new-born animal, and
they gradually enlarge as the growth of the creature proceeds towards
maturity. The nasal bones are very short, and the anterior narial
aperture is situated high in the face. The zygomatic arch is slender and
straight, the jugal bone being small, and forming only the middle part of
the arch, the anterior part of which (unlike that of typical Ungulates)
is formed only by the maxilla. The maxillo-turbinals are but rudimentary,
the elongated proboscis supplying their place functionally in warming and
clearing from dust the inspired air.
[Illustration: FIG. 180.—A vertical section of the skull of the African
Elephant (_Elephas africanus_) taken to the left of the middle line, and
including the vomer (_Vo_) and the mesethmoid (_ME_). _an_, Anterior,
and _pn_, posterior narial aperture. ¹⁄₁₂ natural size. (From Flower’s
_Osteology of the Mammalia_.)]
The neck is very short. The limbs are long and stout, and remarkable for
the great length of the upper segment (especially the femur) as compared
with the distal segment, the manus, and pes. It is owing to this and
the vertical position of the femur that the knee-joint in the hind leg
is placed much lower, and is more conspicuous externally than in most
quadrupedal mammals; and this having been erroneously compared with the
hock-joint or ankle of typical Ungulates, the popular fallacy that the
joints of the Elephant’s leg bend in a contrary direction to that of
other mammals has arisen. There is no round ligament in the hip-joint,
or third trochanter to the femur. The radius and ulna are distinct,
though fixed in a crossed or prone position. The fibula also is quite
distinct from the tibia. The feet are short and broad, the carpal and
tarsal bones being very square, with flattened surfaces for articulation;
the astragalus especially differs from that of typical Ungulates in its
flatness, in the absence of a distinct pulley-like articular surface
at either extremity, and in having no articular facet for the cuboid.
The fibula articulates with the calcaneum, as in Artiodactyles. Of
the five toes present on each extremity (see Fig. 98), the middle one
is somewhat the largest, and the lateral ones smallest, and generally
wanting (especially in the hind foot) the complete number of phalanges.
The ungual phalanges are all small, irregular in form, and late in
ossification. The whole are encased in a common integument, with a flat,
subcircular, truncated sole, the only external indication of the toes
being the broad oval nails or hoofs arranged in a semicircle around the
front edge of the sole. The hind foot is smaller and narrower than the
front. The liver is small and simple, and there is no gall-bladder. In
form the brain resembles that of the Rodents and other lower orders
of mammals, the cerebellum being entirely behind and uncovered by the
cerebrum, but the hemispheres of the latter are richly convoluted.
The Proboscidea are exclusively vegetable feeders, living chiefly on
leaves and young branches of forest trees and various kinds of herbage,
which they gather and convey to their mouth by the very mobile proboscis,
an organ which combines in a marvellous manner strength with dexterity
of application, and is a necessary compensation for the shortness and
inflexibility of the neck, as by it many of the functions of the lips
of other animals are performed. By its means the Elephant is enabled
to drink without bending the head or limbs; the end of the trunk being
dipped into the stream or pool, a forcible inspiration fills the two
capacious air-passages in its interior with water, which, on the tip of
the trunk being turned upwards and inserted into the mouth, is ejected
by a blowing action, and swallowed; or if the animal wishes to refresh
and cool its skin, it can throw the water in a copious stream over any
part of its surface. Elephants can also throw dust and sand over their
bodies by the same means and for the same purpose, and wild animals
have been frequently observed fanning themselves with leafy boughs held
in the trunk. The species are at present limited in their geographical
distribution to the Ethiopian and Oriental regions, but they formerly had
a far more extensive range.
_Family_ ELEPHANTIDÆ.
Cheek-teeth succeeding one another in an arc of a circle, and portions of
only two, or at most three, of the hinder teeth in use at any one time.
Premolars frequently lost, and in any case of no functional importance.
_Elephas._[279]—Dentition: _i_ ¹⁄₀, _c_ ⁰⁄₀, _dm_ ³⁄₃, _m_ ³⁄₃ = 26. The
incisors variable, but usually of very large size, especially in the
male sex, directed somewhat outwards, and curved upwards, without enamel
except on the apex before it is worn. The molars composed of numerous
flattened enamel-covered plates or ridges of dentine, projecting from
a common many-rooted base, surrounded and united together by cement,
and extending straight across the crown, without (in most forms) any
median division into inner and outer columns. The number of plates
increases from the anterior to the posterior molar in regular succession,
varying in the different species, but the third and fourth (or the last
milk-molar and the first true molar), and these only, have the same
number of ridges, which always exceeds five. Premolars nearly always
wanting. Skull of adult very high and globular. Mandible ending in front
in a short, deflected, and spout-like symphysis. Vertebræ: C 7, D 19-21,
L 3-4, S 4, C 26-33.
The existing species of the genus differ so much that they have been
referred by some writers to distinct genera; fossil forms show, however,
such a transition from the one to the other that it is scarcely possible
to regard them even as the representatives of distinct groups.
[Illustration: FIG. 181.—Grinding surface of a half-worn lower molar of
the Indian Elephant (_Elephas indicus_). _d_, Dentine; _e_, enamel; _c_,
cement. (From Owen.)]
In the well-known Indian or Asiatic Elephant (_E. indicus_) the average
number of plates of the six successive molar teeth is expressed by the
“ridge-formula,” 4, 8, 12, 12, 16, 24. The plates are compressed from
before backwards, the anterior and posterior surfaces (as seen in the
worn grinding face of the tooth, Fig. 181) being nearly parallel. Ears of
moderate size. Upper margin of the end of the proboscis developed into
a distinct finger-like process, much longer than the lower margin. Five
nails on the fore feet, and four (occasionally five) on the hind feet.
[Illustration: FIG. 182.—Grinding surface of a partially worn right upper
molar of the African Elephant (_Elephas africanus_). Letters as in the
preceding figure. The left side of the figure is the front of the tooth,
and the lower side the outer border. (From Owen.)]
This species inhabits in a wild state the forest lands of India, Burma,
the Malay Peninsula, Cochin China, Ceylon, and Sumatra. The elephants
from the last-named islands, presenting some variations from those of
the mainland, have been separated under the name of _E. sumatranus_, but
the distinction has not been satisfactorily established. The appearance
of the Asiatic Elephant is familiar to all. Though rarely breeding in
captivity, it has been domesticated from the most remote antiquity, and
is still extensively used in the East as a beast of burden. In the wild
state it is gregarious, associating in herds of ten, twenty, or more
individuals, and though it may, under certain circumstances, become
dangerous, it is generally inoffensive and even timid, fond of shade and
solitude and the neighbourhood of water. The height of the male at the
shoulder when full grown is usually from 8 to 10 feet, but occasionally
as much as 11. The female is somewhat smaller.
[Illustration: FIG. 183.—African Elephant (_Elephas africanus_). From a
young specimen in the London Zoological Gardens.]
In the African Elephant (_E. africanus_) the molars (Fig. 182) are of
coarse construction, with fewer and larger plates and thicker enamel.
Ridge-formula: 3, 6, 7, 7, 8, 10. The plates not flattened, but thicker
in the middle than at the edges, so that their worn grinding surfaces
are lozenge-shaped. Ears very large. The upper and lower margins of the
end of the trunk forming two nearly equal prehensile lips. But three
hoofs on the hind foot. This species now inhabits the wooded districts
of the whole of Africa south of the Sahara, except where it has been
driven away by human settlements. Fossil remains of Pleistocene age,
undistinguishable specifically, have been found in Algeria, Spain, and
Sicily. It was trained for war and show by the ancient Carthaginians and
Romans, and recent experience of the species in captivity in England
shows that it is as intelligent as its Asiatic relative, if not more so,
while surpassing it in courage, activity, and obstinacy. Nevertheless,
in modern times, no people in Africa have been sufficiently civilised
or enterprising to care to train it for domestic purposes. It is hunted
chiefly for the sake of the ivory of its immense tusks, of which it
yields the principal source of supply to the European market, and the
desire to obtain which is rapidly leading to the extermination of the
species. In size the male African elephant often surpasses that of Asia,
but the female is usually smaller. The circumference of the fore foot is
half the height at the shoulder, a circumstance which enables the hunters
to judge from the footprints the exact size of the animals of which
they are in pursuit. The African Elephant also differs from its Indian
congener in having tusks in both sexes, whereas in the latter the male
only is so armed. Moreover, the eye is relatively larger, the forehead
more convex, and the colour somewhat darker. Whereas the Indian Elephant
frequents the depths of forests and seldom leaves their shade during the
daytime, the following account by Sir Samuel Baker indicates different
habits in the African species. This traveller observes: “In Africa, the
country being generally more open than in Ceylon, the Elephant remains
throughout the day either beneath a solitary tree or exposed to the sun
in the vast prairies, where the thick grass attains a height of from
nine to twelve feet. The general food of the African Elephant consists
of the foliage of trees, especially mimosas. Many of the mimosas are
flat-headed, about thirty feet high, and the richer portion of the
foliage confined to the crown. Thus the Elephant, not being able to
reach to so great a height, must overturn the tree to obtain the coveted
food. The destruction caused by a herd of Elephants in a mimosa forest
is extraordinary, and I have seen trees uprooted of so large a size that
I am convinced no single elephant could have overturned them. I have
measured trees four feet six inches in circumference and about thirty
feet high uprooted by elephants. The natives have assured me that the
elephants mutually assist each other, and that several engage together in
the work of overturning a large tree.”
[Illustration: FIG. 184.—Restored skeleton of the Mammoth (_Elephas
primigenius_). From Tilesius in _Mém. Acad. Imp. Sc. St. Pétersbourg_,
vol. v. (1815). _s_, Scapula; _h_, humerus; _r_, radius; _u_, ulna;
_c_, carpus; _rs_, ischium; _f_, femur; _t_, tibia; _fi_, fibula; _ta_,
tarsus.]
_Extinct Species of Elephant._—Abundant remains of Elephants are found
embedded in alluvial gravels, or secreted in the recesses of caves,
into which they have been washed by streams and floods, or dragged as
food by Hyænas and other carnivorous inhabitants of these subterranean
dens. Such remains belonging to the Pleistocene and Pliocene periods
have been found in many parts of Europe, including the British Isles,
in North Africa, throughout the North American continent from Alaska to
Mexico, and extensively distributed in Asia, where the deposits of the
sub-Himalayan Siwalik Hills, and equivalent deposits in the Punjab, Perim
Island,[280] and Burma, belonging to the earliest Pliocene, are rich
in the remains of Elephants of varied form. These species are chiefly
known and characterised at present by the skulls and teeth; some of the
latter resemble the existing Indian and some the African type, but the
majority are between the two, and make the distinction between the two
existing species as of generic importance quite impracticable. Others
again approach so closely in the breadth and coarseness of the ridges
and paucity of cement to _Mastodon_ as to have been placed by some
zoologists in that genus. These form the subgenus called _Stegodon_ by
Falconer, and may be regarded as a distinct group of the genus.
Among the best known extinct Elephants is _E. primigenius_, the
Mammoth,[281] very closely resembling the existing Indian species, and
one of the most recently extinct and extensively distributed of all the
fossil forms. Probably no animal which has not survived to the historic
period has left such abundant and well-preserved evidence of its former
existence. The discovery of immense numbers, not only, as in the case
of most extinct creatures, in the form of fragmentary bones and teeth,
but often as more or less nearly entire carcases, or “mummies,” as they
may be called, with the flesh, skin, and hair _in situ_, in the frozen
soil of the tundras of Northern Siberia, has for a long time given great
interest to the species, and been the cause of many legendary stories
among the natives of the lands in which they occur. Among these one of
the most prevailing is that the Mammoth was, or still is, an animal which
passes its life habitually in burrows below the surface of the ground,
and immediately dies if by any chance it comes into the upper air.
Of the whole group the Mammoth is in many respects, as in the size and
form of the tusks, and especially the characters of the molar teeth,
the farthest removed from the primitive Mastodon-like type, while its
nearest surviving relative, _E. indicus_, has retained the slightly more
generalised characters of the Mammoth’s contemporaries of more southern
climes, _E. columbi_ of America, and _E. armeniacus_ of the Old World,
if, indeed, it can be specifically distinguished from them.
The tusks or upper incisor teeth were doubtless present in both sexes,
but probably of smaller size in the female. In the adult males they
often attained the length of from 9 to 10 feet measured along the outer
curve. Upon leaving the head they were directed at first downwards and
outwards, then upwards and finally inwards at the tips, and generally
with a tendency to a spiral form not seen in other species of Elephant.
Different specimens, however, present great variations in curve, from
nearly straight to an almost complete circle.
It is chiefly by the characters of the molar teeth that the various
extinct modifications of the Elephant type are distinguished. Those of
the Mammoth (Fig. 185) differ from the corresponding organs of allied
species in the great breadth of the crown as compared with the length,
the narrowness and close approximation of the ridges, the thinness of the
enamel and its straightness, parallelism, and absence of “crimping,”
as seen on the worn surface, or in a horizontal section of the tooth.
Dr. Falconer gave the prevailing “ridge-formula” as 4, 8, 12, 12, 16,
24. Dr. Leith Adams, working from more abundant materials, has shown,
however, that the number of ridges of each tooth, especially those at
the posterior end of the series, is subject to very great individual
variation, ranging in each tooth of the series within the following
limits: 3 to 4, 6 to 9, 9 to 12, 9 to 15, 14 to 16, 18 to 27, excluding
the small plates called talons at each end of the tooth. Besides these
variations in the number of ridges or plates of which each tooth is
composed, the thickness of the enamel varies so much as to have given
rise to a distinction between a “thick-plated” and a “thin-plated”
variety—the latter being most prevalent among the specimens from the
Arctic regions, and most distinctively characteristic of the species.
From the specimens with thick enamel plates the transition to the other
species or varieties mentioned above, including _E. indicus_, is almost
imperceptible.
[Illustration: FIG. 185.—Grinding surface of upper molar of the Mammoth
(_Elephas primigenius_). _c_, Cement; _d_, dentine; _e_, enamel. (From
Owen.)]
The bones of the skeleton generally more resemble those of the Indian
Elephant than of any other known species, but the skull differs in the
narrower summit, narrower temporal fossæ, and more prolonged incisive
sheaths required to support the roots of the enormous tusks. Among the
external characters by which the Mammoth was distinguished from either
of the existing species of Elephant was the dense clothing, not only
of long coarse outer hair, but also of close woolly under hair, of a
reddish-brown colour, evidently in adaptation to the colder climate
which it inhabited. This character, for a knowledge of which we are
indebted to the well-preserved remains found in Northern Siberia, is also
represented in the rude but graphic drawings of prehistoric age found in
caverns in the south of France.[282] In size different individuals varied
considerably, but the average height does not appear to have exceeded
that of either of the existing species of Elephant.
The geographical range of the Mammoth was very extensive. There is
scarcely a county in England in which some of its remains have not been
found either in alluvial deposits of gravel or in caverns, and numbers
of its teeth are from time to time dredged up from the bottom of the sea
by the fishermen who ply their trade in the German Ocean, having been
washed out of the water-worn cliffs of the eastern counties of England.
In Scotland and Ireland its remains are less abundant, but they have been
found in vast numbers at various localities throughout the greater part
of Central Europe (as far south as Santander in Spain and Rome), Northern
Asia, and the northern part of the American continent, though the exact
distribution of the Mammoth in the New World is still a question of
debate. It has not hitherto been met with in any part of Scandinavia or
Finland.
In point of time, the Mammoth belongs exclusively to the Pleistocene
epoch, and it was undoubtedly contemporaneous with man in France, and
probably elsewhere. There is evidence to show that it existed in Britain
before, during, and after the glacial period.
As before indicated, it is in the northern part of Siberia that its
remains have been found in the greatest abundance, and in quite
exceptional conditions of preservation. For a very long period there
has been from that region a regular export of Mammoth ivory in a state
fit for commercial purposes, both eastward to China and westward to
Europe. In the middle of the tenth century an active trade was carried
on at Khiva in fossil ivory, which was fashioned into combs, vases, and
other objects, as related by Abu’l Kásim, an Arab writer of that period.
Middendorff reckoned that the number of tusks which have yearly come into
the market during the last two centuries has been at least a hundred
pairs, and Nordenskiöld, from personal observation, considers this
calculation as probably rather too low than too high. They are found at
all suitable places along the whole line of the shore between the mouth
of the Obi and Behring Straits, and the farther north the more numerous
do they become, the islands of New Siberia being now one of the most
favourite collecting localities. The soil of Bear Island and of Liachoff
Islands is said to consist only of sand and ice with such quantities of
Mammoth bones as almost to compose its chief substance. The remains are
not only found around the mouths of the great rivers, as would be the
case if the carcases had been washed down from more southern localities
in the interior of the continent, but are imbedded in the frozen soil in
such circumstances as to indicate that the animals had lived not far from
the localities in which they are now found, and they are exposed either
by the melting of the ice in unusually warm summers or by the washing
away of the sea cliffs or river banks by storms or floods. In this way
the bodies of more or less nearly perfect animals, often standing in the
erect position, with the soft parts and hairy covering entire, have been
brought to light.
References to the principal recorded discoveries of this kind, and to the
numerous speculations to which they have given rise, both among ignorant
peasants and learned academicians, will be found in Nordenskiöld’s
_Voyage of the Vega_ (English translation, vol. i. 1881, p. 398 _sq._)
and a series of papers in the _Geological Magazine_ for 1880 and 1881,
by H. H. Howorth, as well as in a separate work on the Mammoth by the
same writer. For the geographical distribution and anatomical characters,
see Falconer’s _Palæontological Memoirs_, vol. ii. 1868; Boyd Dawkins,
“_Elephas primigenius_, its Range in Space and Time,” _Quart. Journ.
Geol. Soc._ xxxv. p. 138 (1879); and Leith Adams, “Monograph of British
Fossil Elephants,” part ii., _Palæontographical Society_ (1879).
_E. antiquus_, of the European Pleistocene, has a lower ridge-formula
than in the Mammoth, the molars being narrower, and approximating to
those of the African Elephant in structure. Small allied forms occur in
the rock-fissures and caverns of Malta, and have been described as _E.
mnaidriensis_ and _E. melitensis_; some of the individuals of the latter
not exceeding 3 feet in height. The European _E. meridionalis_ is a
southern form of somewhat earlier age, very common in the Upper Pliocene
of Italy and France, and also in the so-called Forest-bed of the Norfolk
coast. It attained very large dimensions, its height being estimated at
upwards of 15 feet. The ridge-formula is lower than in _E. antiquus_, the
molars are broad, with the worn enamel-discs generally expanded in the
middle, and the enamel itself is crenulated.
Elephant remains are very abundant in the Pleistocene and Pliocene
deposits of India, those from the latter beds being the oldest
representatives of the genus. Of these the Pleistocene _E. namadicus_
appears closely allied to _E. antiquus_, from which it is distinguished
by a bold ridge across the forehead. Among the Pliocene forms _E.
hysudricus_ may be an ancestral type allied to the Indian Elephant;
while _E. planifrons_ is closely related to _E. meridionalis_, although
retaining the ancestral feature of developing premolars.
The Stegodont group is peculiar to the eastern parts of the Old World,
and, as already observed, connects the true Elephants intimately with
the Mastodons. The molars (Fig. 179, II) are characterised by the
lowness of the ridges, while the intervening valleys may have but little
cement, and there may be a more or less distinct longitudinal groove
in the crown dividing each ridge into an inner and an outer moiety. In
species like _E. insignis_ the ridge-formula is nearly the same as in
_E. meridionalis_, but in _E. clifti_ some of the molars carry only six
ridges, and premolars were present, so that we thus have such a complete
transition to the next genus that it is very difficult to know where to
draw the line between the two.
_Mastodon._[283]—Dentition: _i_ ¹⁄₁₋₀, _c_ ⁰⁄₀, _dm_ ³⁄₃, _m_ ³⁄₃. Upper
incisors large, as in _Elephas_, sometimes with longitudinal bands of
enamel, more or less spirally disposed. Lower incisors variable; when
present comparatively small and straight, sometimes persistent, sometimes
early deciduous, and in some species never present. Grinding surface of
molars with transverse ridges, the summits of which are divided more
or less into conical or mammillary cusps, and often with secondary or
additional cusps between and clustering against the principal ridges;
enamel thick; cement very scanty, never filling up the interspaces
between the ridges. The third, fourth, and fifth cheek-teeth (_i.e._ the
last milk-molar, and the first and second molars) having the same number
of ridges,[284] which never exceeds five.
In the upper jaw the incisors, though of large size, were apparently
never so much curved as in some species of Elephant, and they often
have longitudinal bands of enamel, more or less spirally disposed upon
their surface, which are not met with in Elephants. Lower incisors were
present throughout life in some species, which have the symphysis of the
lower jaw greatly elongated to support them (as in _M. angustidens_, _M.
pentilici_, and _M. longirostris_). In the common North American species
(_M. americanus_) the mandibular symphysis is short, but it may have a
small incisor on one side. In other species no inferior tusks have been
found, at all events in adult life (see figure of _M. arvernensis_).
The molar teeth increase in size from before backwards, but as many as
three of these teeth may be in place in each jaw at one time. There is
in many species a true vertical succession, affecting either the third,
or the third and second, or (in _M. productus_) the first, second, and
third of the six molariform teeth. These three are therefore reckoned as
milk-molars, and their successors as premolars, while the last three,
which are never changed, correspond to the true molars of those animals
in which the typical dentition is fully developed. The study of the mode
of succession of the teeth in the different species of Mastodons is
particularly interesting, as it exhibits so many stages of the process
by which the very anomalous dentition of the modern Elephants may have
been derived by gradual modification from the typical heterodont and
diphyodont dentition of the ordinary mammal. It also shows that the
anterior molars of Elephants do not correspond to the premolars of other
Ungulates, but to the milk-molars, the early loss of which in consequence
of the peculiar process of horizontal forward-moving succession does
not require, or allow time for, their replacement by premolars. It must
be noted, however, that, in the Mastodon in some respects the least
specialised in tooth-structure, the _M. americanus_ of North America, no
vertical succession of the molars has yet been observed, although vast
numbers of specimens have been examined.
[Illustration: FIG. 186.—Restoration of the skeleton of _Mastodon
arvernensis_, from the Pliocene of Europe, (After Sismonda.)]
The Mastodons have fewer ridges on their molar teeth than the
Elephants; the ridges are also less elevated, wider apart, have a
thicker enamel-covering, and scarcely any cement filling up the space
between them. Sometimes (as in _M. americanus_) the ridges are simple
transverse wedge-shaped elevations, with straight or concave edges. In
other species the summits of the ridges are more or less subdivided into
conical cusps, and may have accessory cusps clustering around them (as
in _M. americanus_, see Fig. 187). When the apices of these are worn by
mastication, their surfaces present circles of dentine, surrounded by a
border of enamel, and as the attrition proceeds different patterns are
produced by the union of the bases of the cusps, a trilobed or trefoil
form being characteristic of some species (Fig. 188).
[Illustration: FIG. 187.—Oblique side and crown view of the last upper
molar of _Mastodon arvernensis_. (From Owens.)]
As already mentioned, certain of the molariform teeth of the middle of
the series in Mastodons have the same number of principal ridges, those
in front of them having fewer and those behind a greater number. These
teeth were distinguished as “intermediate” molars by Dr. Falconer, and
are three in number, namely the last milk-molar and the first and second
true molars (or the third, fourth, and fifth of the whole series). The
number of ridges on these intermediate molars is nearly always three or
four, and the tooth in front has usually one fewer and that behind one
more, so that the ridge-formula of most Mastodons can be reduced either
to 1, 2, 3, 3, 3, 4, or 2, 3, 4, 4, 4, 5. The former characterises the
section called _Trilophodon_ (of which an intermediate molar is shown in
Fig. 188), and the latter that called _Tetralophodon_ by Dr. Falconer.
These divisions are very useful, as under one or the other all the
present known species of Mastodon can be ranged, but observations upon a
larger number of individuals have shown that the number of ridges upon
the teeth is not quite so constant as implied by the formulæ given above.
Their exact enumeration is even difficult in many cases, as “talons” or
small accessory ridges at the hinder end of the teeth occur in various
stages of development, until they take on the character of true ridges.
Transitional conditions have also been shown, at least in some of the
teeth, between the trilophodont and the tetralophodont forms, and again
between the latter and what has been called a “pentalophodont” type,
which leads on towards the condition of dental structure characteristic
of the true Elephants.
[Illustration: FIG. 188.—Grinding surface of the partially worn last left
lower milk-molar of _Mastodon angustidens_, from the Upper Miocene of
India. The lower side of the figure is the outer border of the tooth.]