GENERAL ACCOUNT OF THE SKELETON IN MAMMALIA.
THE EXOSKELETON AND VERTEBRAL COLUMN.
EPIDERMAL EXOSKELETON.
=Hair=, which forms the characteristic Mammalian exoskeleton, varies
much in different animals, and in different parts of the same animal.
A large proportion of mammals have the surface fairly uniformly
covered with hair of one kind only. In some forms however there are
two kinds of hair, a longer and stiffer kind alone appearing on the
surface, and a shorter and softer kind forming the under fur. In most
mammals hairs of a special character occur in certain regions, such as
above the eyes, on the margins of the eyelids, and on the lips and
cheeks, here forming the vibrissae or whiskers.
Sometimes as in _Hippopotamus_, _Orycteropus_ and the Sirenia, the
hair, though scattered over the whole surface, is extremely scanty,
while in the Cetacea it is limited to a few bristles in the
neighbourhood of the mouth, or may even be absent altogether in the
adult. In most mammals the hairs are shed and renewed at intervals,
sometimes twice a year, before and after the winter. The vibrissae or
large hairs which occur in many animals upon the upper lip, and the
mane and tail of Equidae are probably persistent.
In the hedgehogs, porcupines and _Echidna_ certain of the hairs are
modified and greatly enlarged, forming stiff spines. Similar spines
occur in the young of _Centetes_, and in _Acanthomys_ among the
Muridae.
Several other forms of epidermal exoskeleton are met with in mammals,
including:--
(_a_) =Scales=. These overlie the bony scutes of armadillos and occur
covering the tail in several groups of mammals, such as beavers and
rats. In the Manidae the body is covered by flat scales which overlap.
(_b_) The =horns= of Bovine Ruminants. These, which must on no account
be confused with antlers, are hollow cases of hardened epidermis
fitting on to bony outgrowths of the frontals. In almost every case
they are unbranched structures growing continuously throughout life,
and are very rarely shed entire. In the Prongbuck _Antilocapra_
however they are bifurcated and are periodically shed. Horns are
nearly always limited to a single pair, but the four-horned antelope
_Tetraceros_ has two pairs, the anterior pair being the smaller.
(_c_) The =horns of Rhinoceroses=. These are conical structures
composed of a solid mass of hardened epidermal cells growing from a
cluster of long dermal papillae. From each papilla there grows a fibre
which resembles a thick hair, and cementing the whole together are
cells which grow from the interspaces between the papillae. These
fibres differ from true hairs in not being developed in pits in the
dermis. Rhinoceros horns may be either one or two in number, and are
borne on the fronto-nasal region of the skull. They vary much in
length, the longest recorded having the enormous length of fifty-seven
inches.
(_d_) _Nails_, _hoofs_ and =claws=. In almost all mammals except the
Cetacea, these are found terminating the digits of both limbs. =Nails=
are more or less flattened structures, =claws= are pointed and
somewhat curved. In most mammals the nails tend to surround the ends
of the digits much more than they do in man. Sometimes the nail of one
digit differs from that of all the others; thus the second digit of
the pes in the Hyracoidea and Lemuroidea is terminated by a long claw,
the other digits having flat nails. In the Felidae the claws are
retractile, the ungual phalanx with claw attached folding back when
the animal is at rest into a sheath, above, or by the side of the
middle phalanx. In the Sloths and Bats enormously developed claws
occur, forming hooks by which the animals suspend themselves. In
_Notoryctes_ the third and fourth digits of the manus bear claws of
great size; similar claws occur in _Chrysochloris_, being correlated
in each case with fossorial habits. The nail at its maximum
development entirely surrounds the terminal phalanx of the digit to
which it is attached, and is then called a =hoof=. Hoofs are specially
characteristic of the Ungulata.
(_e_) =Spurs= and =beaks= are structures which are hardly represented
among mammals, while so characteristic of birds. They are however both
found in the Monotremata. In both _Echidna_ and _Ornithorhynchus_ the
male has a peculiar hollow horny spur borne on a sesamoid bone
articulated to the tibia. The jaws in _Ornithorhynchus_ are cased in
horny beaks similar to those of birds, and are provided with horny
pads which act as teeth.
(_f_) =Horny plates= of a ridged or roughened character occur upon the
anterior portion of the palate, and of the mandibular symphysis in all
three genera of recent Sirenia; also upon the toothless anterior
portion of the palate in Ruminants.
(_g_) The =baleen of whales= also belongs to the epidermal
exoskeleton. It consists of a number of flattened horny plates
arranged in a double series along the palate. The plates are somewhat
triangular in form and have their bases attached to the palate at
right angles to its long axis, while their apices hang downwards into
the mouth cavity. The outer edge of each plate is hard and smooth,
while the inner edge and apex fray out into long fibres which look
like hair. At the inner edge of each principal plate are subsidiary
smaller plates. The plates are formed of a number of fibres each
developed round a dermal papilla in the same way as are the fibres
forming the horns of _Rhinoceros_. Baleen and Rhinoceros horn likewise
agree in that the fibres are bound together by less hardened
epithelial cells, which readily wear away and allow the harder fibres
to fray out. The greatest development of baleen occurs in the Northern
Right whale, _Balaena mysticetus_, in which the plates number three
hundred and eighty or more on each side, and reach a length of ten or
twelve feet near the middle of the series.
DERMAL EXOSKELETON.
Mammals show two principal kinds of exoskeletal structures which are
entirely or partially dermal in origin, viz. the bony scutes of
armadillos, and teeth.
The =bony scutes of armadillos= are quadrate or polygonal in shape and
are in general aggregated together, forming several shields protecting
various regions of the body. The head is generally protected by a
_cephalic_ shield, the anterior part of the body by a _scapular_, and
the posterior by a _pelvic_ shield. The tail is also generally encased
in bony rings, and scutes are irregularly scattered over the surface
of the limbs. The mid-body region is protected by a varying number of
bands of scutes united by soft skin, so as to allow of movement.
Corresponding to each dermal scute is an epidermal plate. In
_Chlamydophorus_ the scutes are mainly confined to the posterior
region where they form a strong vertically-placed shield which
coalesces with the pelvis. The anterior part of the body is mainly
covered by horny epidermal plates with very little ossification
beneath. In the gigantic extinct Glyptodonts the body is covered with
a solid carapace formed by the union of an immense number of plates,
and there are no movable rings. The top of the head is defended by a
similar plate, the tail is generally encased in an unjointed bony
tube, and there is commonly a ventral plastron.
In _Phocaena phocaenoides_ the occurrence of vestigial dermal ossicles
has been described, and in _Zeuglodon_ the back was probably protected
by dermal plates.
TEETH[145].
Teeth are well developed in the vast majority of mammalia, and are of
the greatest morphological and systematic importance, many extinct
forms being known only by their teeth. Mammalian teeth differ from
those of lower animals in various well-marked respects. (1) They are
attached only to the maxillae, premaxillae and mandible, never to the
palatines, pterygoids or other bones. (2) They frequently have more
than one root. (3) They are always, except in some Odontoceti, placed
in distinct sockets. (4) They are hardly ever ankylosed to the bone.
(5) They are in most cases markedly heterodont. (6) They are commonly
developed in two sets, the milk dentition and permanent dentition.
It sometimes happens that teeth after being formed are reabsorbed
without ever cutting the gum. This is the case, for instance, with the
upper incisors of Ruminants.
The form of mammalian teeth varies much, some are simple conical
structures comparable to those of most reptiles, and these may either
have persistent pulps, as in the case of the upper canines of the
Walrus and the tusks of Elephants, or may be rooted as in most canine
teeth. Some teeth have chisel-shaped edges, and this may be their
original form, as in the human incisors, or may, as in those of
Rodents, be brought about by the more rapid wearing away of the
posterior edge, the anterior edge being hardened by a layer of enamel.
Then, again, the crown may, as in the majority of grinding teeth, be
more or less flattened. The various terms used in describing some of
the forms of the surface of grinding teeth are defined on page 345.
[Illustration FIG. 81. SKULL OF A YOUNG INDIAN RHINOCEROS (_R.
unicornis_), SHOWING THE CHANGE OF THE DENTITION × 1/7. (Brit. Mus.)
1. nasal.
2. frontal.
3. parietal.
4. zygomatic process of squamosal.
5. jugal.
_mI{1}._ milk incisor.
_mc._ milk canine.
_mpm{1}._ milk premolar.
_I{1}._ first incisor.
_c._ canine.
_pm{2}_, _pm{3}_, _pm{4}_. 2nd, 3rd and 4th premolars.
_m{1}_, _m{2}_. first and second molars.]
The teeth of the Aard Varks are compound, and differ completely from
those of all other mammals (see p. 425).
As a rule, the higher the general organisation of an animal the
better are its milk teeth developed, and the more do they form a
reproduction on a small scale of the permanent set. This fact is well
seen in the Primates, Carnivora and Ungulata. The method of notation
by which the dentition of any mammal can be briefly expressed as a
formula has been already described. The regular mammalian arrangement
of teeth for each side is expressed by the formula
_i_ 3/3 _c_ 1/1 _pm_ 4/4 _m_ 3/3 × 2; total, 44.
MONOTREMATA. In _Echidna_ teeth are quite absent. In the young
_Ornithorhynchus_[146] functional molar teeth of a multi-tubercular
type resembling those of some Mesozoic mammalia are present, but in
the adult they disappear, their office being discharged by horny
plates.
MARSUPIALIA[147] have a heterodont dentition, which has generally been
regarded as almost monophyodont, the only tooth which has an obvious
deciduous predecessor being the last premolar. The researches of
Röse[148] and Kükenthal[149] tend to show that the teeth of Marsupials
are developed in the same way as in other mammals, and are diphyodont.
In the case of the premolars, teeth which are homologous with the
permanent teeth of other mammals begin to develop as lateral
outgrowths from the milk teeth, but afterwards become absorbed, so
that the teeth which actually persist belong to the milk series. The
last premolar, however, does as a rule develop and replace its milk
predecessor; sometimes, however, as in _Didelphys_, it takes its place
among the milk molars without replacing one of them.
The types of dentition characteristic of the different groups of
placental mammals may mostly be paralleled among the Marsupials. Thus
among the polyprotodont forms the Didelphyidae or opossums, and some
of the Dasyuridae, such as _Sarcophilus_ and _Thylacinus_, have a
typical carnivorous dentition with small incisors, large canines, and
molars with pointed compressed crowns. The dental formula of
_Thylacinus_, is _i_ 4/3 _c_ 1/1 _pm_ 3/3 _m_ 4/4, total 46.
In _Myrmecobius_ five or six molar teeth occur on each side, and the
total number of teeth reaches fifty-two or fifty-six. The teeth bear
rows of tubercles, and resemble those of the Multituberculate mesozoic
Mammals[150], more than do those of any other living form. Calcified
teeth have recently been described[151] in _Myrmecobius_ earlier than
the functional or milk set. This would relegate the milk teeth of
mammals in general to a second series, and the permanent teeth to a
third. In _Notoryctes_ the dental formula[152] is given as _i_ 3/2 _c_
1/1 _pm_ 2/3 _m_ 4/4, total 40. The canines are small, and the
anterior molars have strongly developed cusps, and much resemble those
of _Chrysochloris_ (Insectivora).
Among the diprotodont types the Phascolomyidae, or Wombats, have a
dentition recalling that of the Rodents. All the teeth grow from
persistent pulps, and the incisors have enamel only on the anterior
surface as in Rodents. The dental formula is _i_ 1/1 _c_ 0/0 _pm_ 1/1
_m_ 4/4, total 24. There are indications of a vestigial second pair of
incisors.
The Macropodidae, or Kangaroos, have a herbivorous dentition with the
formula _i_ 3/1 _c_ (0--1)/0 _pm_ 2/2 _m_ 4/4. The incisors are sharp
and cutting, and are separated by a long diastema or gap from the
molars, which have their crowns marked by ridges or cusps. There are
indications of several vestigial incisors.
_Coenolestes_, a remarkable form recently described from America,
belongs to the diprotodont section, and is the only living member of
the section known outside the Australian region[153]. An exceptional
dentition is seen in the case of the extinct _Thylacoleo_, in which
the functional teeth are reduced to two pairs; one pair of large
cutting incisors and one of compressed sharp-edged premolars.
EDENTATA. Some Edentata, viz. the ant-eaters (Myrmecophagidae) are, as
far as is known, absolutely toothless at all stages of their
existence; being the only mammals except _Echidna_ in which no tooth
germs have been discovered; others, viz. the Manidae, though showing
foetal tooth germs, are quite toothless in post-foetal life; others,
viz. some of the armadillos, have the largest number of teeth met with
in land mammals. The teeth are homodont except in the Aard Varks, and
grow from persistent pulps. In the sloths (Bradypodidae) and the
Megatheriidae, there are five pairs of teeth in the upper and four in
the lower jaw. The teeth of sloths consist of a central axis of
vasodentine, surrounded firstly by a thin coating of hard dentine, and
secondly by a thick coating of cement.
In no living Edentate have the teeth any enamel; it has, however, been
described as occurring in certain early Megatheroid forms from S.
America[154], and an enamel organ has also been discovered in an
embryo _Dasypus_[155]. In the Armadillos (Dasypodidae) the number of
teeth varies from 8/8 or 7/7 in _Tatusia_, to upwards of 25/25 in
_Priodon_, which therefore may have upwards of a hundred teeth, the
largest number met with in any land mammal. In _Tatusia_ all the teeth
except the last are preceded by two-rooted milk teeth. The Aard Varks
are diphyodont, and milk teeth are also known in a species of
_Dasypus_, but with these exceptions Edentates are, as far as is
known, monophyodont. In _Glyptodon_ the teeth are almost divided into
three lobes by two deep grooves on each side.
The Aard Varks (Orycteropodidae) are quite exceptional as regards
their teeth, which are cylindrical in shape, and are made up of a
number of elongated denticles fused together. Each denticle contains a
pulp cavity from which a number of minute tubes radiate outwards.
These teeth are diphyodont and somewhat heterodont, eight to ten pairs
occur in the upper jaw and eight in the lower, but they are not all in
place at one time. The last three teeth in each jaw are not preceded
by milk teeth[156].
SIRENIA. The teeth of Sirenia show several very distinct types, the
least modified being that of the extinct Halitheriidae, which have
large incisors in the upper jaw, and five or six pairs of tuberculated
grinding teeth in each jaw, the anterior ones being preceded by milk
teeth.
In both the living genera the dentition is monophyodont. In _Manatus_
the dentition is _i_ 2/2 _pm_ and _m_ 11/11. The incisors are
vestigial, and disappear before maturity. The grinding teeth have
square enamelled crowns marked by transverse tuberculated ridges. They
are not all present in the jaw at the same time. In _Halicore_ the
upper jaw bears a pair of straight tusklike incisors; in the male
these have persistent pulps and project out of the mouth; in the
female they soon cease to grow and are never cut. They are separated
by a long diastema from the grinding teeth which have tuberculated
crowns and are 5/5 or 6/6 in number, but are not all in place at once.
Several other pairs of slender teeth occur in the young animal, but
are absorbed or fall out before maturity. In _Rhytina_ teeth are
altogether absent.
CETACEA.
_ARCHAEOCETI._ _Zeuglodon_ has the following dentition, _i_ 3/3 _c_
1/1 _pm_ and _m_ 5/5, total 36. The incisors and canines are simple
and conical; the cheek teeth are compressed and have serrated cutting
edges like those in some seals.
In the _MYSTACOCETI_, or whalebone whales, calcified tooth germs
probably belonging to the milk dentition are present in the embryo,
but they are never functional, and are altogether absent in the adult.
The anterior of these germs are simple, the posterior ones are
originally complex, but subsequently split up into simple teeth like
those of the anterior part of the jaw. Hence according to Kükenthal,
who described these structures, the Cetacean dentition was originally
heterodont.
In the living _ODONTOCETI_ the dentition is homodont and monophyodont.
In some cases traces occur of a replacing dentition which never comes
to maturity, and renders it probable that the functional teeth of the
Odontoceti are really homologous with the milk teeth of other mammals.
Some of the dolphins afford the apparently simplest type of mammalian
dentition known. The teeth are all simple, conical, slightly recurved
structures, with simple tapering roots and without enamel. The
dentition is typically _piscivorous_, being adapted for seizing active
slippery animals such as fish. The prey is then swallowed entire
without mastication. Sometimes the teeth are excessively numerous,
reaching two hundred or more (fifty to sixty on each side of each jaw)
in _Pontoporia_. This multiplication of teeth is regarded by Kükenthal
as due to the division into three parts of numbers of trilobed teeth
similar to those of some seals.
In the Sperm whale, _Physeter_, the lower jaw bears a series of twenty
to twenty-five stout conical recurved teeth, while in the upper jaw
the teeth are vestigial and remain imbedded in the gum. An extinct
form, _Physodon_, from the Pliocene of Europe and Patagonia is allied
to the Sperm whale, but has teeth in both jaws. In the Grampus _Orca_,
the teeth number about 12/12, and are very large and strong. In some
forms the teeth are very much reduced in number; thus in _Mesoplodon_
the dentition consists simply of a pair of conical teeth borne in the
mandible. In the Narwhal _Monodon_ the dentition is practically
reduced to a single pair of teeth, which lie horizontally in the
maxillae, and in the female normally remain permanently in the alveoli.
In the male the right tooth remains rudimentary, while the left is
developed into an enormous cylindrical tusk marked by a spiral groove.
Occasionally both teeth develop into tusks, and there is reason for
thinking that two-tusked individuals are generally or always female.
In the extinct _Squalodon_ the dentition is decidedly heterodont, and
the molars have two roots. The dental formula is
_i_ 3/3 _c_ 1/1 _pm_ 4/4 _m_ 7/7, total 60.
It is probable that the homodont condition of modern Odontoceti is not
primitive, but due to retrogressive evolution.
UNGULATA.
Just as in the Cetacea a piscivorous dentition is most typically
developed, so the Ungulata are, as a group, the most characteristic
representatives of a _herbivorous_ dentition in its various forms.
UNGULATA VERA.
_ARTIODACTYLA_. As regards the living forms, the Artiodactyla can be
readily divided into two groups, namely those with bunodont and those
with selenodont teeth. It has, however, been shown that selenodont
teeth always pass through an embryonic bunodont stage[157]. The
bunodont type is best seen in Pigs and Hippopotami and such extinct
forms as _Hyotherium_. In _Hippopotamus_ the dental formula is _i_
(2-3)/(1-3) _c_ 1/1 _pm_ 4/4 _m_ 3/3.
The incisors and canines of _Hippopotamus_ are very large and grow
continuously. The genus _Sus_, which affords a good instance of an
_omnivorous_ type of dentition, has the regular unmodified Mammalian
dental formula _i_ 3/3 _c_ 1/1 _pm_ 4/4 _m_ 3/3, total 44. The
canines, specially in the male, are large and have persistent pulps,
and the upper canines do not have the usual downward direction but
pass outwards and upwards. In the Wart Hog, _Phacochaerus_, they are
enormously large, but a still more extraordinary development of teeth
is found in _Babirussa_. In the male _Babirussa_ the canines, which
are without enamel, are long, curved and grow continuously. Those of
the upper jaw never enter the mouth, but pierce the skin of the face
and curve backwards over the forehead. The dental formula of
_Babirussa_ is _i_ 2/3 _c_ 1/1 _pm_ 2/2 _m_ 3/3, total 34.
The Wart Hog has a very anomalous dentition, for as age advances all
the teeth except the canines and last molars show signs of
disappearing; both pairs of persisting teeth are however very large.
Various extinct Ungulata such as _Anoplotherium_ have teeth which are
intermediate in character between the bunodont and selenodont types.
_Anoplotherium_ has the regular mammalian series of forty-four teeth.
The crowns of all the teeth are equal in height, and there is no
diastema--an arrangement found in no living mammal but man.
We come now to the selenodont Artiodactyla.
The Tylopoda--camels (Camelidae) and Llamas (Aucheniidae) when young
have the full number of incisors, but in the adult the two upper
middle ones are lost. The molars are typically selenodont and
hypsodont[158]. In the Camel the dental formula is _i_ 1/3 _c_ 1/1
_pm_ 3/2 _m_ 3/3, total 34. The upper incisors, canines and first
premolars of the Camel are very small teeth, and the first premolar
is separated by a long diastema from the others.
The Tragulina or Chevrotains have no upper incisors, while the canines
are largely developed, especially in the male.
The Ruminantia or Pecora are very uniform as regards their dentition.
The upper incisors are always absent, for though their germs are
developed they are reabsorbed without ever becoming visible, and as a
rule the upper canines are absent too, while the lower canines are
incisiform. The grinding teeth are typically selenodont, and in the
lower jaw form a continuous series separated by a wide diastema from
the canines. The dental formula is usually
_i_ 0/3 _c_ 0-1/1 _pm_ 3/3 _m_ 3/3.
The canines are largely developed in the male Muskdeer (_Moschus_) and
in _Hydropotes_.
_PERISSODACTYLA_. The premolars and molars have a very similar
structure and form a continuous series of large square teeth with
complex crowns. The crowns are always constructed on some modification
of the bilophodont[159] plan, as is easily seen in the case of the
forms with brachydont teeth, but in animals like the Horse, in which
the teeth are very hypsodont, this arrangement is hard to trace. All
four premolars in the upper jaw are preceded by milk teeth, while in
Artiodactyla the first has no milk predecessors.
In the Tapiridae the grinding teeth are brachydont and the lower ones
are typically bilophodont. The last two upper molars have the
transverse ridges united by an outer longitudinal ridge. The dentition
is _i_ 3/3 _c_ 1/1 _pm_ 4/3 _m_ 3/3, total 42.
In some of the extinct Perissodactyles such as _Lophiodon_[160], the
dentition is brachydont and bilophodont, the grinding teeth in general
resembling the posterior upper molars of the Tapir. The same type of
brachydont tooth is seen in _Palaeotherium_ but the transverse ridges
are crescentic instead of straight, and are separated from one another
by shallow valleys without cement. Some of the Palaeotheridae have the
regular series of forty-four teeth.
A complete series of forms is known showing how from the simple
brachydont teeth of the Palaeotheridae, were derived the complicated
hypsodont teeth of the Equidae. The increase in depth of the tooth was
accompanied by increase in the depth and complexity of the enamel
infoldings, and of the cement filling them.
Both upper and lower grinding teeth of the Equidae are much
complicated by enamel infoldings, but their derivation from the
bilophodont type can still be recognised. The diastema in front of the
premolars is longer in the living Equidae than in their extinct
allies. In the adult horse the dental formula is _i_ 3/3 _c_ 1/1 _pm_
3/3 _m_ 3/3, total 40, with often a vestigial first upper premolar
(fig. 82, _pm_ 1). The last molar is not more complex than the others,
and in the female the canine is quite vestigial. The incisors are
large and adapted for cutting and have the enamel curiously folded in
forming a deep pit. The milk dentition is _di_ 3/3 _dc_ 0/0 _dpm_ 3/3,
total 24. The last milk premolar is not more complex than the premolar
that succeeds it. The horse affords an excellent instance of a
typically _herbivorous_ type of dentition, the cutting incisors,
reduced canines and series of large square flat-crowned grinding teeth
being most characteristic.
In _Rhinoceros_ the grinding teeth are much like those of _Lophiodon_,
having an outer longitudinal ridge from which two crescentic
transverse ridges diverge. The upper premolars are as complex as the
molars, and there are no canines; in some species incisors also are
absent. The dental formula is
_i_ (0--2)/(0--1) _c_ 0/(0--1) _pm_ 4/4 _m_ 3/3.
[Illustration FIG. 82. PALATAL ASPECT OF THE CRANIUM AND MANDIBLE OF A
DONKEY (_Equus asinus_) × 1/5. (Camb. Mus.)
1. supra-occipital.
2. occipital condyle.
3. basi-occipital.
4. vacuity representing the confluent foramen lacerum posterius
and foramen lacerum medium.
5. auditory bulla.
6. glenoid surface.
7. vomer.
_i_ 1, _i_ 3. first and third incisors.
_c._ canine.
_pm_ 1, _pm_ 2. first and second premolars.
_m_ 1. first molar.]
Among the Titanotheriidae _Palaeosyops_[161] has very brachydont teeth
whose crowns have been described as _buno-selenodont_, the inner pair
of columns being bunodont, the outer, selenodont. Similar grinding
teeth occur in _Chalicotherium_. Some of the Titanotheriidae have the
regular mammalian series of forty-four teeth.
SUBUNGULATA.
_TOXODONTIA._ _Nesodon_ has the regular dental formula; its grinding
teeth are rooted and the upper ones resemble those of Rhinoceros. The
second upper and third lower incisors form ever-growing tusks. There
is a marked difference between the deciduous and permanent dentition.
_Astrapotherium_ likewise has large rooted cheek teeth of a
rhinocerotic type, and each jaw bears a pair of permanently growing
tusks, those of the lower jaw being the canines. The dental formula is
_i_ 1/3 _c_ 0/1 _pm_ 2/1 _m_ 3/3, total 28.
In _Toxodon_ the upper incisors and molars are large and curved and
all the teeth have persistent pulps. In _Typotherium_ there are no
tusks, but the upper incisors are chisel-like, recalling those of
Rodents.
The _CONDYLARTHRA_ have brachydont, generally bunodont teeth, with the
premolars simpler than the molars. They generally have the regular
dental formula.
_HYRACOIDEA._ The dental formula of _Procavia_ is usually given as _i_
1/2 _c_ 0/0 _pm_ 4/4 _m_ 3/3, total 34; in young individuals however
there occur a second pair of upper incisors which early fall out. The
upper incisors resemble those of Rodents in being long and curved and
growing from persistent pulps. They are however triangular in
transverse section, not rectangular, having two antero-lateral faces
covered with enamel and a posterior face without enamel. Their
terminations are pointed, not chisel-shaped as in Rodents. The lower
incisors (fig. 83, _i_ 1) are pectinate or partially divided by
vertical fissures, and the grinding teeth are of the rhinocerotic
type.
[Illustration FIG. 83. SKULL OF _Procavia (Dendrohyrax) dorsalis_ ×
2/3. (Camb. Mus.)
1. nasal.
2. parietal.
3. external auditory meatus.
4. paroccipital process of the exoccipital.
5. jugal.
6. lachrymal foramen.
_i_ 1. first incisor.]
_AMBLYPODA._ Two of the best known forms belonging to this extinct
group differ much as regards dentition. For while _Coryphodon_ has the
regular dental formula, and the canines of both jaws of moderate size,
in _Uintatherium_ the dentition is very specialised, there are no
upper incisors, and the upper canines form a pair of enormous tusks.
The grinding teeth form a continuous series marked by =V=-shaped ridges
and the dental formula is _i_ 0/3 _c_ 1/1 _pm_ 3/3 _m_ 3/3 total 34.
_PROBOSCIDEA._ The incisors are composed entirely of dentine and have
the form of conical tusks projecting greatly from the mouth. In
living forms they are confined to the upper jaw, in some species of
the extinct _Mastodon_ however they occur in the lower jaw also. In
_Dinotherium_ they are probably absent from the upper jaw, but form a
pair of downwardly and backwardly-directed tusks growing from the
elongated symphysis of the mandible.
The grinding teeth in the various Proboscidea show a very remarkable
series of modifications. In _Dinotherium_ they are bilophodont or else
are marked by three straight transverse ridges. The dental formula is
_i_ 0?/1 _c_ 0/0 _pm_ 2/2 _m_ 3/3, and the teeth have the normal
method of succession. In _Mastodon_ as in _Dinotherium_ the grinding
teeth are marked by transverse ridges, but the ridges are subdivided
into conical or mammillary cusps, and similar cusps often occur
between the ridges. These cusps are covered with very thick enamel and
the spaces between them are not filled up with cement. There are six
of these grinding teeth for each side of each jaw but only three are
in place at once. The first three are milk teeth as they may be
succeeded vertically by others.
In the true Elephants the number and depth of the enamel folds is much
increased, and the spaces between the folds are filled up with cement.
A very complete series of extinct forms is known with teeth
intermediate in character between those of _Mastodon_ and those of the
Mammoth and living elephants. The dental formula of _Elephas_ is
_di_ 1/0 _i_ 1/0 _c_ 0/0 _dm_ 3--4/3--4 _m_ 3/3.
Sir W.H. Flower describes[162] the mode of succession of teeth in
Elephants as follows: "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 each
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 each
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."
In the TILLODONTIA the grinding teeth are of Ungulate type, while the
second incisors are large and grow from persistent pulps, so as to
resemble those of Rodents.
RODENTIA have a most characteristic and very constant dentition, the
common dental formula being
_i_ 1/1 _c_ 0/0 _pm_ (0--1)/(0--1) _m_ 3/3, total 18 or 20.
The incisors always have chisel-like edges and persistent pulps, and
are separated by a wide diastema from the premolars. Canines are
always absent, and there are generally three grinding teeth not
preceded by milk teeth; their surface may be grooved, or may be
bunodont. Teeth are most numerous in the Duplicidentata (Hares and
Rabbits), in which the formula is _i_ 2/1 _c_ 0/0 _pm_ 3/2 _m_ 3/3,
total 28, and fewest in Hydromys and certain other forms, in which
the formula is _i_ 1/1 _c_ 0/0 _pm_ 0/0 _m_ 2/2, total 12. The hares
and rabbits are the only rodents which have well developed deciduous
incisors, though a vestigial milk incisor has been described in the
Mouse (_Mus musculus_). The last upper molar of _Hydrochaerus_ is very
complicated, its structure approaching that of the teeth of Elephants.
[Illustration FIG. 84. CARNASSIAL OR SECTORIAL TEETH OF CARNIVORA
(from FLOWER).
_Upper sectorial teeth_ of I. _Felis_, II. _Canis_, III. _Ursus_. 1.
anterior, 2. middle, 3. posterior cusp of blade, 4. inner lobe
supported on distinct root, 5. inner lobe posterior in position and
without distinct root, characteristic of the Ursidae.
_Lower sectorial teeth._ 1. _Felis_, 2. _Canis_, 3. _Herpestes_. 1.
anterior, 2. posterior lobe of blade, 3. inner tubercle, 4. heel.]
CARNIVORA have the teeth rooted and markedly diphyodont and
heterodont. The canines are greatly developed, and the incisors are
small.
In _CARNIVORA VERA_ the incisors are almost always 3/3. The fourth
upper premolar and first lower molar are differentiated as carnassial
teeth (see p. 436), and retain fundamentally the same characters
throughout the suborder. The upper carnassial (fig. 84, I. II. III.)
consists of a more or less compressed, commonly trilobed blade borne
on two roots, with an inner tubercle borne on a third root. The lower
carnassial has only two roots; its crown consists of a bilobed blade
with generally an inner cusp, and a heel or talon (fig. 84, 4) behind
the blade.
The most thoroughly carnivorous type of dentition is seen in the
Æluroidea, and especially in the cat tribe (Felidae). In the genus
_Felis_ the dental formula is _i_ 3/3 _c_ 1/1 _pm_ 3/2 _m_ 1/1, total
30. The incisors are very small, so as not to interfere with the
action of the large canines, the lower carnassial is reduced to simply
the bilobed blade (fig. 84, IV), and the cheek teeth are greatly
subordinated to the carnassial. The extinct _Machaerodus_ has the
upper canines comparable in size to those of the Walrus.
The Civets and Hyaenas have a dentition allying them closely to the
cats. The hyaena-like _Proteles_ has, however, the grinding teeth
greatly reduced.
In the Cynoidea[163] the general dentition is _i_ 3/3 _c_ 1/1 _pm_ 4/4
_m_ 2/3, total 42. This differs from the regular mammalian dentition
only in the absence of the last upper molar. The upper carnassial
tooth (fig. 84, II.) consists of a larger middle and smaller posterior
lobe with hardly any trace of an anterior lobe. The lower carnassial
(fig. 84, V.) is typical, consisting of a bilobed blade with inner
cusp and posterior talon.
The dentition of the Cynoidea is most closely linked with that of the
Arctoidea by means of fossil forms.
[Illustration FIG. 85. MANDIBLE OF ISABELLINE BEAR (_Ursus
isabellinus_) × 1/2. (Camb. Mus.)
1. condyle.
2. coronoid process.
_i_ 1. first incisor.
_c._ canine.
_pm_ 1, _pm_ 2. first and second premolars.
_m_ 1. first molar. The dotted
line is pointing to the posterior
half of the tooth.
This specimen has only
three premolars, there
should be four.]
In the Arctoidea the dentition is not so typically carnivorous as in
the Æluroidea and Cynoidea. In the bears, Ursidae, the molars have
broad flat tuberculated crowns (fig. 85). The dental formula in
_Ursus_ is _i_ 3/3 _c_ 1/1 _pm_ 4/4 _m_ 2/3, total 42. The upper
carnassial (fig. 84, III.) differs from that of the Æluroidea and
Cynoidea in having no inner lobe supported on a third root. In the
large group of Mustelidae there are generally two molars in the lower
and one in the upper jaw. The grinding teeth commonly have large,
flattened, more or less tuberculated crowns, and the upper molar may
be as large or much smaller than the carnassial.
In the _CREODONTA_ there are no specially differentiated carnassial
teeth.
[Illustration FIG. 86. LEFT MANDIBULAR RAMUS OF THE SEA LEOPARD
(_Ogmorhinus leptonyx_) WITH THE ROOTS OF THE TEETH EXPOSED × 1/3.
(Camb. Mus.)
1. condyle.
2. coronoid process.
_i_ 3. third incisor.
_c._ canine.
_pm_ 1, _pm_ 4. first and fourth premolars.
_m._ molar.]
In the _PINNIPEDIA_ the dentition differs considerably from that of
the Carnivora vera. The milk dentition is always vestigial, and the
teeth are frequently absorbed before birth. There are four premolars
and one molar, forming an uniform series of cheek teeth, all of which
except in the Walrus have compressed and pointed, never flattened,
crowns. There is no special carnassial tooth, and the incisors are
always fewer than 3/3. In _Otaria_ the dentition is
_i_ 3/2 _c_ 1/1 _pm_ 4/4 _m_ 1 or 2/1, total 34 or 36.
In the Walrus the upper canines form immense tusks. The other teeth
are all small and one-rooted, and the molars have flat crowns. In the
true seals the dentition is strikingly piscivorous, the cheek teeth
often having accessory cusps (fig. 86).
The INSECTIVORA are diphyodont and heterodont, having well-developed
rooted teeth. The canines are usually weak, the incisors pointed, and
those of the two jaws often meet like a pair of forceps. The crowns of
the molars are characteristically studded with short cusps. Some
genera, such as _Gymnura_ and the mole, _Talpa_, have the regular
mammalian dentition. In the hedgehog, _Erinaceus_, the dentition is
_i_ 3/2 _c_ 1/1 _pm_ 3/2 _m_ 3/3, total 36.
In the genus _Sorex_ (Shrews) the teeth differ in the following two
marked respects from those of most other Monodelphia, (1) they are
monophyodont, (2) the lower incisors sometimes become fused to the
jaws. Most Insectivora have square molar teeth, but in _Potamogale_,
_Chrysochloris_, _Solenodon_ and the Centetidae the molar teeth are
triangular in section. Four molars occur in _Centetes_.
In the aberrant genus _Galeopithecus_ the dentition is _i_ 2/3 _c_ 1/1
_pm_ 2/2 _m_ 3/3, total 34. The upper incisors are placed at some
distance from the anterior end of the jaw, and the outer upper
incisors and canines of both jaws have two roots,--a very unusual
character. The lower incisors are deeply grooved or pectinated in the
same way as are the lower incisors of _Procavia_. The upper incisors
and canines of both jaws bear many cusps, and are very similar in
appearance to the cheek teeth of some Seals.
The dentition of the CHIROPTERA is diphyodont and heterodont, and the
dental formula never exceeds
_i_ 2/3 _c_ 1/1 pm 3/3 _m_ 3/3, total 38.
The milk teeth are very slender and have sharp recurved cusps; they
are quite unlike the permanent teeth. The permanent teeth are of two
types. In the Insectivorous forms the molar teeth are cusped, and
resemble those of Insectivora. In the blood-sucking Vampire bat
_Desmodus_, the teeth are peculiarly modified; the canines and the
single pair of upper incisors are much enlarged and exceedingly sharp,
while all the other teeth are much reduced in size.
In the Frugivorous bats the molar teeth have nearly always smooth
crowns. The dental formula in the chief genus _Pteropus_ is _i_ 2/2
_c_ 1/1 _pm_ 3/3 _m_ 2/3, total 34.
The PRIMATES have a diphyodont and heterodont dentition, generally of
an omnivorous type, with cheek teeth adapted for grinding. The
incisors are generally 2/2, and the molars, except in the Hapalidae,
are 3/3. In the Lemurs the upper canines are large, and the lower
incisors slender and directed almost horizontally forwards. The Aye
Aye, _Chiromys_, has the following singular dentition: _i_ 1/1 _c_ 0/0
_pm_ 1/0 _m_ 3/3, total 18. The incisors much resemble those of
rodents having persistent pulps, and enamel only on the anterior face.
In Man and in the Anthropoid and Old World Apes the dental formula is
always _i_ 2/2 _c_ 1/1 _pm_ 2/2 _m_ 3/3, total 32.
In the Cebidae there is an extra premolar in each jaw bringing the
number up to 36. In the Hapalidae, as in the Cebidae, there is a third
premolar, but the molars are reduced to 2/2. Man is the only Primate
that has the teeth arranged in a continuous series. In all the others
there is a gap or diastema of larger or smaller size between the
incisors and canines. In all except man also the canines are enlarged,
especially in the males.
The Exoskeletal structures of mammals may be summarised in the
following table:
I. _Epidermal exoskeletal structures._
1. Hairs (_a_) ordinary hair,
(_b_) vibrissae and bristles,
(_c_) spines of hedgehog, porcupine, _Echidna_,
_Centetes_, _Acanthomys_.
2. Scales { of Manidae,
{ on tails of rats, beavers, &c.
3. Horns of Rhinoceros.
4. Horns of Bovine Ruminants.
5. Nails, claws, hoofs.
6. Spurs of male _Ornithorhynchus_ and _Echidna_.
7. Horny beak and teeth of _Ornithorhynchus_.
8. Horny pads on jaws of Sirenians and Ruminants.
9. Baleen of whales.
10. Enamel of teeth.
II. _Dermal exoskeletal structures._
1. Dentine and cement of teeth.
2. Bony scutes of Armadillos.
ENDOSKELETON.
VERTEBRAL COLUMN.
CERVICAL VERTEBRAE.
The cervical vertebrae of all mammals have certain characters in
common. However long the neck may be, the number of cervical
vertebrae, with very few exceptions, is seven. Movable ribs are
generally absent, and if present are small and do not reach the
sternum. The transverse processes are generally wide but not long, and
are perforated near the base by the vertebrarterial canals, through
which the vertebral arteries pass; they generally bear downwardly
directed inferior lamellae which are sometimes as in the seventh human
cervical seen to ossify from centres distinct from those forming the
rest of the transverse process, and are really of the nature of ribs.
The atlas and axis always differ much from the other vertebrae.
We may pass now to the special characters of the cervical vertebrae in
the different groups. In MONOTREMES and MARSUPIALS the number of
cervical vertebrae is always seven. With the exception of the atlas of
_Echidna_ the cervical vertebrae of Monotremes are without
zygapophyses. In Monotremes the transverse processes ossify from
centres distinct from that forming the body, and remain suturally
connected with the rest of the vertebra until the adult condition is
reached. The method of the ossification of the atlas in Marsupials
varies considerably, thus in some forms such as the Wombats
(_Phascolomys_) there is an unossified gap in the middle of the
inferior arch of the atlas, which may remain permanently open; in
_Thylacinus_ this gap is filled up by a distinct heart-shaped piece of
bone, while in _Didelphys_ and _Perameles_ the atlas is ossified below
in the same way as in other mammals. In _Notoryctes_ the second to
sixth cervical vertebrae are ankylosed together.
The cervical vertebrae of the EDENTATA have some remarkable
peculiarities. In the three-fingered Sloth, _Bradypus_, there are nine
cervical vertebrae, all except the last of which have their transverse
processes perforated by the vertebrarterial canals. In a two-fingered
sloth, _Choloepus hoffmanni_, there are only six cervical vertebrae.
In the Megatheriidae, Anteaters (Myrmecophagidae), Pangolins
(Manidae), and Aard Varks (Orycteropodidae), the cervical vertebrae
are normal, but in the Armadillos (Dasypodidae), and still more in the
Glyptodonts, several of them are commonly fused together. The fusion
affects not only the centra, but also the neural arches, so that the
neural canals form a continuous tube.
In the Glyptodonts there is a complex joint at the base of the neck to
allow the partial retraction of the head within the carapace. This
arrangement recalls that in Tortoises.
As a rule the SIRENIA possess seven short cervical vertebrae, not
fused together and not presenting any marked peculiarities. In
_Manatus_ however there are only six cervical vertebrae and they are
very variable.
[Illustration FIG. 87. CERVICAL VERTEBRAE OF A YOUNG FIN WHALE
(_Balaenoptera musculus_) × 1/10. (Camb. Mus.)
1. surface on the atlas for articulation
with the occipital condyle of the skull.
2. foramen for exit of the first spinal nerve.
3. upper transverse process.
4. lower transverse process.
In the fresh specimen these two transverse processes are united by
cartilage, in adult individuals the whole transverse process is
ossified.
5. epiphyses of centrum.
6. neural spine.]
In the CETACEA there are invariably seven cervical vertebrae, but they
are always very short and are frequently even before birth fused
together by their centra into one continuous mass (see fig. 67).
Sometimes the last one or two are free. In the Rorquals
(_Balaenoptera_) however, the cervical vertebrae are quite separate
and distinct (fig. 87), and in the fluviatile Odontoceti,
_Platanista_, _Inia_, and _Pontoporia_, and also in _Beluga_ and
_Monodon_, though very short they are free. In _Physeter_ the first
vertebra is free while the others are fused. An odontoid process is
not commonly present even in Cetaceans with free cervical vertebrae,
but a very short one occurs in the Rorquals. The cervical vertebrae of
Rorquals give off on each side two transverse processes (fig. 87, 3
and 4) which enclose between them a wide space. These processes are
not completely ossified till the animal is adult.
[Illustration FIG. 88. ATLAS (B) AND AXIS (A) VERTEBRAE OF AN OX (_Bos
taurus_) × 1/4. (Camb. Mus.)
1. neural canal.
2. transverse process.
3. surfaces for articulation with the occipital condyles of the skull.
4. spout-like odontoid process.
5. hypapophysis.
6. anterior opening of the vertebrarterial canal.
7. foramen for the exit of the second spinal nerve.
8. neural spine.
9. postzygapophysis.]
In all UNGULATA the number of cervical vertebrae is seven. Among the
Artiodactyla two forms of the odontoid process of the axis occurs; in
the Suina and Tragulina it is conical, in the Ruminantia and Tylopoda
it is spout-like (fig. 88, 4). The atlas in the Suina and to a less
extent in the Ruminantia has long flattened transverse processes, and
the remaining cervical vertebrae are opisthocoelous. Those of the
Giraffe and Llama (fig. 103) are noticeable for their great length. In
the Tylopoda the posterior half of the vertebrarterial canal is
confluent with the neural canal.
The Perissodactyla have remarkably opisthocoelous cervical vertebrae.
Those of _Macrauchenia_ have the posterior half of the vertebrarterial
canal confluent with the neural canal as in Tylopoda. In the
Proboscidea they are short flattened discs slightly opisthocoelous;
the axis and seventh vertebra and to a less extent the sixth have high
neural spines.
In the RODENTIA the atlas generally has broad wing-like transverse
processes, and the axis a large and long neural spine, while the
odontoid process is much developed. In the Jerboas (_Dipus_) all the
cervical vertebrae except the atlas are fused together, a condition
recalling that in armadillos.
In the CARNIVORA the wings of the atlas are well developed (fig. 69,
A, 1), and it is deeply cupped for articulation with the condyles of
the skull. The axis has a long odontoid process and a high compressed
neural spine (fig. 69, B, 4). The third to sixth cervical vertebrae
have large transverse processes with prominent perforated inferior
lamellae, whose ventral margins in the third and fourth vertebrae
diverge as they pass backwards, while in the fifth they are parallel
and in the sixth convergent. The transverse processes of the seventh
vertebra have no inferior lamellae and are not perforated.
Metapophyses are often developed.
In the INSECTIVORA the cervical vertebrae vary considerably. The
neural spines except in the case of the axis are generally very small
and in the Shrews and Moles the neural arches are exceedingly slender.
In the CHIROPTERA all the cervical vertebrae are broad and short with
slender neural arches.
PRIMATES. In Man the cervical vertebrae have short blunt transverse
processes and small often bifid neural spines. The neural and
vertebrarterial canals are large. The atlas forms a ring surrounding a
large cavity, and has a very slender inferior arch and small
transverse processes. Traces of a pro-atlas have been described in
_Macacus_ and _Cynocephalus_. The axis has a prominent spine and
odontoid process and short transverse processes. In most Primates the
cervical vertebrae are very similar to those of man, but the inferior
lamellae of the transverse processes are better developed. In the
Anthropoid Apes the neural spines are as a rule much elongated.
THORACO-LUMBAR, OR TRUNK VERTEBRAE.
In the MONOTREMATA there are nineteen thoraco-lumbar vertebrae,
sixteen (_Echidna_) or seventeen (_Ornithorhynchus_) of which bear
ribs. The transverse processes are very short and do not articulate
with the ribs, which are united to the centra only.
In the MARSUPIALIA there are always nineteen thoraco-lumbar vertebrae,
thirteen of which generally bear ribs. The lumbar vertebrae frequently
have large metapophyses and anapophyses, these being specially well
seen in the Kangaroos and Koala (_Phascolarctus_).
The EDENTATA are very variable as regards their trunk vertebrae. The
two genera of Sloths differ much as regards the number, for while
_Bradypus_ has only nineteen, fifteen or sixteen of which bear ribs,
_Choloepus_ has twenty-seven, twenty-four of which are thoracic, and
bear ribs. In _Bradypus_ a small outgrowth from the transverse process
articulates with the neural arch of the succeeding vertebra. In both
genera the neural spines are all directed backwards.
In the Megatheriidae as in the sloths the neural spines are all
directed backwards, and in the lumbar region additional articulating
surfaces occur, better developed than are those in _Bradypus_.
In the ant-eaters (Myrmecophagidae) there are seventeen or eighteen
thoraco-lumbar vertebrae, all of which except two or three bear ribs.
The posterior thoracic and anterior lumbar vertebrae articulate in a
very complex fashion, second, third, and fourth pairs of zygapophyses
being progressively developed in addition to the ordinary ones, as the
vertebrae are followed back.
In the Armadillos the lumbar vertebrae have long metapophyses which
project upwards and forwards and help to support the carapace. In
_Glyptodon_ almost all the thoraco-lumbar vertebrae are completely
ankylosed together.
In the Manidae there are no additional zygapophyses but the normal
ones of the lumbar and posterior thoracic regions are very much
developed, the postzygapophyses being semi-cylindrical and fitting
into the deep prezygapophyses of the succeeding vertebra.
In the SIRENIA the number of lumbar vertebrae is very small; in the
dugong there are nineteen thoracic and four lumbar, and in the manatee
seventeen thoracic and two lumbar.
In the CETACEA the number of thoracic vertebrae varies from nine in
_Hyperoödon_ to fifteen or sixteen in _Balaenoptera_, and the number
of lumbar vertebrae from three in _Inia_ to twenty-four or more in
_Delphinus_. The lumbar vertebrae are often very loosely articulated
together and the zygapophyses sometimes as in the Dolphins are placed
high up on the neural spines. The centra are large, short in the
anterior region but becoming longer behind. The epiphyses are
prominent, and so are the neural spines and to a less extent the
metapophyses. The transverse processes are well developed, anteriorly
they arise high up on the neural arch, but when the vertebral column
is followed back they come gradually to be placed lower down, till in
the lumbar region they project from the middle of the centra. This can
be well traced in the Porpoise (_Phocaena_). In the Physeteridae the
transverse processes of the anterior thoracic vertebrae are similar to
those of most Cetacea, but when followed back, instead of shifting
their position on the vertebrae, they gradually disappear, and other
processes gradually arise from the point where the capitulum of the
rib articulates.
UNGULATA. In the Ungulata vera the thoraco-lumbar vertebrae are
slightly opisthocoelous. The anterior thoracic vertebrae commonly have
exceedingly high backwardly-projecting neural spines (fig. 89, 1); but
those of the lumbar and posterior thoracic vertebrae often point
somewhat forwards so that the spines all converge somewhat to a point
called the _centre of motion_ (cp. fig. 101). In the Artiodactyla
there are always nineteen thoraco-lumbar vertebrae, and in the
Perissodactyla twenty-three.
_Procavia_ sometimes has thirty thoraco-lumbar vertebrae, a greater
number than occurs in any other terrestrial mammal; twenty-two of
these are thoracic and eight lumbar. In _Phenacodus_ the convergence
of the neural spines to a centre of motion is well seen.
[Illustration FIG. 89. FIRST AND SECOND THORACIC VERTEBRAE OF AN OX
(_Bos taurus_) × 1/3. (Camb. Mus.)
1. neural spine.
2. neural canal.
3. prezygapophysis.
4. facet for articulation with the tuberculum of the rib.
5. facet for articulation with the capitulum of the rib.
6. postzygapophysis.
7. foramen for exit of spinal nerve.]
In the Proboscidea there are twenty-three thoraco-lumbar vertebrae, of
which nineteen or twenty bear ribs.
In the RODENTIA there are generally nineteen thoraco-lumbar vertebrae
but occasionally the number rises as high as twenty-five. In the Hares
(Leporidae) the number is nineteen, twelve or thirteen of which are
thoracic. The anterior thoracic vertebrae have short centra and high
backwardly-directed neural spines, the lumbar vertebrae have large
forwardly-and downwardly-directed transverse processes with expanded
ends. Metapophyses, anapophyses and hypapophyses are all present. In
the Agouti (_Dasyprocta_) the convergence of the neural spines to a
centre of motion is very strongly marked.
In the CARNIVORA the trunk vertebrae are nearly always twenty or
twenty-one in number; in the genera _Felis_ and _Canis_ thirteen of
these are thoracic and seven lumbar. The anterior thoracic vertebrae
have long backwardly-projecting neural spines, while the posterior
thoracic and lumbar vertebrae have shorter and thicker neural spines
which project slightly forwards. In the Pinnipedia there is no change
in the direction of the neural spines, and anapophyses are but little
developed.
In the INSECTIVORA the number of trunk vertebrae varies much from
nineteen--thirteen thoracic and six lumbar--in _Tupaia_, to
twenty-four--nineteen thoracic and five lumbar--in _Centetes_. The
development of the various processes varies in accordance with the
habits of the animals, being great in the active forms, slight in the
slowly moving or burrowing forms. In _Talpa_ and _Galeopithecus_ the
intervertebral discs of the thoraco-lumbar region instead of being
cartilaginous have ossified forming inter centra, a condition met with
in very few mammals.
In the CHIROPTERA there are seventeen or eighteen thoraco-lumbar
vertebrae, eleven to fourteen of which may bear ribs. The development
of processes is slight.
Among PRIMATES the number of trunk vertebrae is generally nineteen, of
which twelve to fourteen bear ribs; in man and the Gorilla and
Chimpanzee the number is, however, seventeen, and in the Orang
(_Simia_) sixteen. In some of the Lemuroidea there are as many as
twenty-three or twenty-four. In most cases the neural spines converge
more or less to a centre of motion, and this is especially marked in
some of the Lemurs; it does not occur in man and the anthropoid apes.
SACRAL AND CAUDAL VERTEBRAE.
At the posterior end of the trunk in all mammals a certain number of
vertebrae are found fused together forming the sacrum. But of these
only two or three answer to the definition of true sacral vertebrae in
being united to the ilia by small ribs. The others which belong to the
caudal series may be called pseudosacral vertebrae. In different
individuals of the same species it sometimes happens that different
vertebrae are attached to the pelvis and form the sacrum. Sometimes
even different vertebrae are attached to the pelvis at successive
periods in the life history of the individual. This is owing to a
shifting of the pelvis and has been especially well seen in man. In
young human embryos the pelvis is at a certain stage attached to
vertebra 30, but as development goes on it becomes progressively
attached to the twenty-ninth, twenty-eighth, twenty-seventh,
twenty-sixth and twenty-fifth vertebrae. As the attachment to these
anterior vertebrae is gained, the attachment to the posterior ones
becomes lost, so that in the adult the pelvis is generally attached to
vertebrae 25 and 26. But there are no absolutely pre-determined sacral
vertebrae, as sometimes the pelvis does not reach vertebra 25,
remaining attached to vertebrae 26 and 27; sometimes it becomes
attached even to vertebra 24. This shifting of the pelvis is seen in
_Choloepus_ in a more marked degree even than in man.
Of the MONOTREMATA, _Ornithorhynchus_ has two sacral vertebrae
ankylosed together, while _Echidna_ has three or four[164].
In MARSUPIALIA as a rule only one vertebra is directly united to the
ilia, but one or two more are commonly fused to the first. In the
Wombats there may be as many as four or five vertebrae fused together
in the sacral region. In _Notoryctes_ there is extensive fusion in the
sacral region, six vertebrae, owing mainly to the great development of
their metapophyses, being united with one another, and with the ilia,
and the greater part of the ischia.
In most EDENTATA there is an extensive fusion of vertebrae in the
sacral region. This is especially marked in the Armadillos and
Megatheriidae, and to a less extent in the Sloths and Aard Varks.
In the SIRENIA the vestigial pelvis is attached by ligament to the
transverse processes of a single vertebra, which hence may be regarded
as sacral.
In CETACEA there is no sacrum, the vestigial pelvis not being
connected with the vertebral column.
In most UNGULATA the sacrum consists of one large vertebra united to
the ilia, and having a varying number of smaller vertebrae fused with
it behind.
The same arrangement obtains in most RODENTIA, but in the Beavers
(Castoridae) all the fused vertebrae are of much the same size, the
posterior ones having long transverse processes which nearly meet the
ilia.
In CARNIVORA there may be two sacral vertebrae as in the Hyaena, three
as in the Dog, four or five as in Bears and Seals.
In INSECTIVORA from three to five are united, while in many CHIROPTERA
all the sacral and caudal vertebrae have coalesced. Among PRIMATES, in
Man and Anthropoid Apes there are usually five fused vertebrae forming
the sacrum, but of these only two or three are connected to the ilia
by ribs. In most of the other Anthropoidea there are two or three
fused vertebrae, and in the Lemuroidea two to five.
FREE CAUDAL VERTEBRAE. The free caudal vertebrae vary greatly in
number and character. When the tail is well developed, the anterior
vertebrae are comparatively short and broad, with well-developed
neural arches and zygapophyses; but as the tail is followed back, the
centra gradually lengthen and become cylindrical, and at the same time
the neural arches and all the processes gradually become reduced and
disappear, so that the last few vertebrae consist of simple rod-like
centra. Chevron bones are frequently well-developed.
Of the MONOTREMES _Echidna_ has twelve caudal vertebrae, two of which
bear irregular chevron bones. In _Ornithorhynchus_ there are twenty or
twenty-one caudal vertebrae with well-developed hypapophyses, but no
chevron bones.
In MARSUPIALS there is great diversity as regards the tail. In the
Wombat and Koala the tail is small and without chevron bones. In most
other Marsupials it is very long, having sometimes as many as
thirty-five vertebrae in the prehensile-tailed opossums. In the
Kangaroos the tail is very large and stout. Chevron bones are almost
always present, and in _Notoryctes_ are large and expanded.
Most EDENTATES have large tails with well-developed chevron bones. The
length of the tail varies greatly from the rudimentary condition in
Sloths to that in the Pangolins, one of which has forty-six to
forty-nine caudal vertebrae--the largest number in any known mammal.
Chevron bones are much developed, sometimes they are Y-shaped,
sometimes as in _Priodon_, they have strong diverging processes. The
caudal vertebrae of Glyptodonts, though enclosed in a continuous bony
sheath, have not become ankylosed together.
The SIRENIA have numerous caudal vertebrae with wide transverse
processes. In the CETACEA also the tail is much developed, and the
anterior vertebrae have large chevron bones and prominent straight
transverse processes; the posterior caudal vertebrae, which in life
are enclosed in the horizontally expanded tail fin, are without
transverse processes.
In UNGULATA the tail is simple, formed of short cylindrical vertebrae,
which in living forms are never provided with chevron bones. The
number of caudal vertebrae varies from four, sometimes met with in
_Procavia_, to thirty-one in the Elephant. The tail is exceedingly
long in _Anoplotherium_ and in _Phenacodus_, in which there are thirty
caudal vertebrae.
In RODENTIA the tail is variable. In the Hares, Guinea pig (_Cavia_)
and _Capybara_ it is very small, in _Pedetes_ and the Beaver it is
very long and has well-developed chevron bones.
Most of the CARNIVORA except the Bears and Seals have very long tails,
the greatest number of vertebrae, thirty-six, being met with in
_Paradoxurus_. Bears have only eight to ten caudal vertebrae. Chevron
bones are not often much developed.
In INSECTIVORA the tail is very variable as regards length, the number
of vertebrae varying from eight in _Centetes_ to forty-three in
_Microgale_.
In CHIROPTERA the tail is sometimes quite rudimentary, and as in
_Pteropus_, composed of a few coalesced vertebrae, sometimes it is
formed of a large number of slender vertebrae.
In PRIMATES also the tail is very variable. In Man all the four caudal
vertebrae are rudimentary and are fused together, forming the
_coccyx_. In the Anthropoid apes, too, there are only four or five
caudal vertebrae. In many monkeys of both the eastern and western
hemispheres the tail is very long, having thirty-three vertebrae in
_Ateles_, in which genus it is also prehensile. Chevron bones are
present in all Primates with well-developed tails. In the Lemuroidea
the number of caudal vertebrae varies from seven to twenty-nine.
FOOTNOTES:
[145] See W.H. Flower, "Remarks on the homologies and notation of the
teeth in Mammalia," _J. Anat. and Physiol. norm. path._, Vol. III., p.
262; R. Owen, _Odontography_, London, 1840--45; C.S. Tomes, _Manual of
Dental Anatomy_, London, 1876. See also H.F. Osborn, "Recent
researches on succession of teeth in Mammals," _Amer. Natural._,
XXVII., p. 493, and "Rise of Mammalia in N. America," _Stud. Biol.
Lab. Columb. Coll._, Zool. I., no. 2.
[146] See E.B. Poulton, _P.R.S._, Feb. 1888, and _Quart. J. Micr.
Sci._, Vol. XXIX. 1889; also Oldfield Thomas, _P.R.S._, XLVI. (1889).
[147] W.H. Flower, _Phil. Trans._, vol. 156, pp. 631--641, 1867; also
Oldfield Thomas, _Phil. Trans._, pp. 443--462, 1887.
[148] C. Röse, _Anat. Anz._ VII., p. 639.
[149] W. Kükenthal, _Anat. Anz._ VI., p. 364.
[150] See p. 348.
[151] W. Leche, _Morph. Jahrb._ XX., pp. 113--142 (1893).
[152] E.C. Stirling, _P.Z.S._ 1891, p. 327.
[153] O. Thomas, _P.Z.S._, 1895, p. 870.
[154] F. Ameghino, _Bull. Ac. Argen._ XII. p. 437. According to H.
Burmeister, _Annal. Mus. Buenos Aires_, III. 401 (1891), enamel does
not occur, osteodentine having been mistaken for it.
[155] E. Ballowitz, _Arch. Mikr. Anat._ XL. p. 133.
[156] See Oldfield Thomas, _P.R.S._, vol. XLVII., p. 246 (1890).
[157] J. Taeker, "_Fur Kenntniss der Odontogenese bei Ungulaten_."
Dorpat, 1892.
[158] See p. 345.
[159] See p. 345.
[160] According to H.F. Osborn, _Amer. Natural._, XXVI. p. 763, a
number of not very closely allied forms have been included under
_Lophiodon_.
[161] C. Earle, _J. Ac. Philad._, vol. IX., 1892, p. 267.
[162] _Encyclopaedia Britannica_, article _Mammalia_, p. 424.
[163] See T.H. Huxley, "The dental and cranial characters of the
Canidae," _P.Z.S._, 1880, p. 238.
[164] G.B. Howes, _Journ. of Anat. and Phys._ XXVII., p. 544.