THE ORDER PRIMATES 680
_Suborder_ LEMUROIDEA 682
_Family_ LEMURIDÆ, 683; _Indris_, 684; _Propithecus_,
684; _Avahis_, 686; _Lemur_, 687; _Hapalemur_, 689;
_Lepidolemur_, 689; _Chirogaleus_, 689; _Galago_, 690;
_Nycticebus_, 691; _Loris_, 692; _Perodicticus_, 693.
_Family_ TARSIIDÆ, 694; _Tarsius_, 694. _Family_ CHIROMYIDÆ,
694; _Chiromys_, 695. EXTINCT LEMUROIDS, 696.
_Suborder_ ANTHROPOIDEA 699
_Family_ HAPALIDÆ, 709; _Hapale_, 710; _Midas_, 710.
_Family_ CEBIDÆ, 711; _Mycetes_, 711; _Pithecia_, 712;
_Uacaria_, 712; _Callithrix_, 713; _Chrysothrix_, 714;
_Nyctipithecus_, 714; _Ateles_, 715; _Eriodes_, 715;
_Lagothrix_, 716; _Cebus_, 717. _Family_ CERCOPITHECIDÆ,
718; _Cynocephalus_, 719; _Theropithecus_, 722;
_Cynopithecus_, 722; _Macacus_, 722; _Cercocebus_, 723;
_Cercopithecus_, 724; _Nasalis_, 725; _Semnopithecus_, 726;
_Colobus_, 727; Extinct genera, 727. _Family_ SIMIIDÆ,
728; _Hylobates_, 728; _Simia_, 731; _Gorilla_, 734;
_Anthropopithecus_, 736. _Family_ HOMINIDÆ, 739; _Homo_,
740. Classification of the varieties of Man, 743.
AN INTRODUCTION TO THE STUDY OF MAMMALS LIVING AND EXTINCT
CHAPTER I
_INTRODUCTORY REMARKS_
Mammalia (French, _Mammifères_; German, _Säugethiere_) is the name
invented by Linnæus (from the Latin _mamma_), and now commonly used by
zoologists, for one of the five great classes of vertebrated animals,
which, though the best known and undoubtedly the most important group
of the animal kingdom, has never received any generally accepted
vernacular designation in our language. The unity of structure of the
animals composing this class, and their definite demarcation from other
vertebrates, were not recognised until comparatively modern times,
and hence no word was thought of to designate what zoologists now
term a mammal. The nearest equivalents in common use are “beast” and
“quadruped,” both of which, however, cover a different ground, since they
are often used to include the larger four-footed reptiles, and to exclude
certain undoubted mammals, as Man, Bats, and Whales.
The limits of the class as now understood by zoologists are perfectly
well defined, and, although certain forms still existing on the earth
(but not those mentioned above as excluded by the popular idea) are
of exceedingly aberrant structure, and exhibit several well-marked
characters connecting them with the lower vertebrated groups, common
consent retains them in the class with which the great proportion of
their characters ally them, and hitherto no traces of any species showing
still more divergent or transitional characters have been discovered.
There is thus an interval, not bridged over by any known forms, between
mammals and other vertebrates; although recent discoveries have shown
evidence of a more or less marked affinity between the most generalised
mammals and a peculiar group of extinct reptiles known as the Anomodontia
(or Theromora), which are themselves nearly related to the equally
extinct Labyrinthodont amphibians of the Palæozoic and Mesozoic epochs.
In the gradual order of evolution of living beings, mammals, taken
altogether, are certainly the highest in organisation, as, with the
possible exception of birds, they were the last to appear on the
earth’s surface. But, as in speaking of all other large and greatly
differentiated groups, this expression must not be understood in too
limited a sense. The tendency to gradual perfection for their particular
station in life, which all groups manifest, leads to various lines of
specialisation, or divergence from the common or general type, which
may or may not take the direction of elevation. A too complex and
sensitive condition of organisation may in some circumstances of life be
disadvantageous, and modification may then take place in a retrograde
direction. Thus in mammals, as in other classes, there are low as well as
high forms, but by any tests that can be applied—especially those based
on the state of development of the central nervous system—it will be seen
that the average exceeds that of any other class; that the class contains
many species far excelling those of any other in perfection of structure,
and especially one form which is unquestionably the culminating point yet
arrived at amongst organised beings.
With regard to the time of the first appearance of mammals upon
the earth, the geological record is provokingly imperfect. At the
commencement of the Tertiary period they were abundant, and already
modified into most of the leading types at present existing. It was at
one time thought that they first came into being at this date, but the
discovery of more or less fragmentary remains of numerous and generally
small species has revealed the existence of some forms of the class at
various periods throughout almost the whole of the age of the deposition
of the Secondary or Mesozoic rocks. This subject will be reverted to
later on.
It hardly need be said that mammals are vertebrated animals, and possess
all the characteristics common to the members of that division of the
animal kingdom. They are separated from the _Ichthyopsida_ (fishes and
amphibians), and agree with the _Sauropsida_ (reptiles and birds), in
the possession during their development of an amnion and allantois, and
in never having external branchiæ or gills. They differ from reptiles
and resemble birds in being warm-blooded, and having a heart with four
cavities and a complete double circulation. They differ from both birds
and reptiles in the red corpuscles of the blood being non-nucleated
and, with very few exceptions, circular in outline; in the lungs being
freely suspended in a thoracic cavity, separated from the abdomen by a
complete muscular partition—the diaphragm—which is the principal agent
in inflating the lungs in respiration; in having but one aortic arch,
which curves over the left bronchus; in the skin being more or less
clothed with hair; in the greater perfection of the commissural system of
the cerebral hemispheres, which has either a complete corpus callosum,
or an incomplete one associated with a very large anterior commissure;
in having no syrinx or inferior vocal organ, but a complete larynx at
the upper end of the trachea; in having a mandible of which each ramus
(except in very early developmental conditions) consists of a single bone
on each side, articulating to the squamosal without the intervention of
a quadrate bone; in having a pair of laterally placed occipital condyles
instead of one median one; and in the very obvious character of the
female being provided with mammary glands, by the secretion of which the
young (usually produced alive, although in the lowest forms by means of
externally hatched eggs) are nourished for some time after birth.
In common with all vertebrated animals, mammals never have more than
two pairs of limbs; as the larger number live ordinarily on the surface
of the earth, in the great majority of the class both pairs are
well-developed and functional, and adapted for terrestrial progression.
Mammals are, however, by no means limited to this situation. Thus some
species spend the greater part of their lives beneath the surface, their
fore limbs being specially modified for burrowing; others, again, are
habitually arboreal, their limbs being fitted for climbing or hanging
to boughs of trees; some are as aerial as birds, the fore limbs being
developed into wings of a special character; while in others which are as
aquatic as fishes, the limbs assume the form of fins or paddles. In many
of the latter the hinder extremities are either completely suppressed,
or present only in a rudimentary state. In no known mammal are the fore
limbs absent.
The hinder extremity of the axis of the body is usually prolonged into
a tail, which may be a mere pendent appendage, or may be modified to
perform various functions, as grasping boughs in climbing, or even
gathering food, in the case of the prehensile-tailed Monkeys and
Opossums, swimming in the Cetacea, and acting as a flap to drive away
troublesome insects from the skin in the Ungulata.
The state of development of the young at the time of birth varies greatly
in the different groups. Thus among the Marsupials where there is no
connection during intra-uterine life between the circulatory systems
of the parent and the fœtus, the young are born in an exceedingly
imperfectly developed condition. For their protection the mother, in
a large number of cases, has a special pouch enclosing the mammæ, into
which the young are transferred at birth, and in which they remain till
they are well developed. Among the higher, or Placental types, however,
where a connection exists between the maternal and fœtal circulations
previous to birth, the young are always born in a much more highly
developed state than among the Marsupials, although we meet with great
variations in this respect. In those forms which habitually live in
holes, like many Rodents, the young are always very helpless at birth;
and the same is also true of many of the Carnivora, which are well able
to defend their young from attack. In the great order of Ungulate,
or Hoofed Mammals, where in the majority of cases defence from foes
depends upon fleetness of foot, or upon huge corporeal bulk, the young
are born in a very highly developed condition, and are able almost at
once to run by the side of the parent. This state of relative maturity
at birth reaches its highest development in the Cetacea, where it is
evidently associated with the peculiar conditions under which these
animals pass their existence. In the Primates, however, we again find
the young produced in a more or less helpless condition, and requiring
a long period before they attain their full development, this being
more especially the case with those higher forms which approximate in
structure to man.
In point of size mammals vary to a greater extent than the existing
members of any one class of animals, and include the largest living
inhabitants of the earth. The extremes of size are marked on the one
hand by the whale known as Sibbald’s Rorqual, which attains a length of
eighty feet and a weight of nearly as many tons, and on the other by the
Pigmy-Shrew and the minute Harvest-mouse, which can climb a stem of wheat.
Of all the living creatures inhabiting our globe, mammals are by far
the most important in their economic uses, since, in addition to being
the only animals capable of labour for human benefit, they furnish the
greater portion of the animal food of many races of man, and likewise a
large amount of their clothing. In these respects the Ungulates hold the
first place.
As regards employment for labour, with the exception of the Dogs used
for sleighing by the Esquimaux, and those which among some European
nations draw light carts, all the mammals in general use are Ungulates.
Of the first importance are the Horses and Asses, which are employed
as beasts of draught or burden over nearly the whole globe. Among many
nations, however, cattle, as represented by the true Oxen, the Buffalos,
and the Yaks of Tibet, occupy a still more important position, while in
the highlands of Tibet, Sheep are largely used for carrying burdens. In
other regions, again, the place of the Horse and the Ass is taken by the
Camels, which are peculiarly fitted for traversing parched and arid
deserts, while in the Andes we find the Llamas serving the same office.
In Lapland and other parts of the northern regions the Reindeer is the
main agent employed in draught. Lastly, we must not omit to mention the
Indian Elephant, which, from its vast strength, is so useful in transport
through the wilder parts of its native country.
As regards food, we again find the Ungulates, and more especially the
Artiodactyle division, taking the foremost place; and in this connection
we have only to mention, among animals kept in a domestic condition,
Swine, Cattle, Sheep, and Goats—the three latter affording not only their
flesh, but also milk and its resulting cheese and butter. To many races,
however, Mares and Camels are the chief milk producers, while the Laps
make use of the milk of the Reindeer. The Rodents, as represented by
Hares and Rabbits, occupy a minor position as furnishers of food.
In relation to clothing, the Ungulates are likewise of paramount
importance, as exemplified by the wool of the Sheep, which is so valuable
on account of its peculiar property of felting. Furs, however, are
mostly yielded by mammals of other orders, among which the Fur-seals are
perhaps the most important at the present day. Many other Carnivores
yield valuable furs, among which may be mentioned Bears, Foxes, Raccoons,
Skunks, Minks, Otters, and Ermines. Of less importance are certain
Rodents, such as the Squirrels, Rabbits, Hares, etc., while the hair of
the Beaver was formerly much sought after for the manufacture of hats.
Returning to the Ungulates, we may notice the importance of horse-hair,
the employment of camel’s hair for brushes, and the many uses of the
bristles of the pig. Some of the Monkeys yield fur which has been
extensively used. Leather, again, is almost exclusively supplied by
mammals, and mainly by the Ungulates.
Three other important products, namely horn, buck’s-horn, and ivory, are
likewise obtained solely from the same great order. Horn, as we shall
notice in the sequel, is the sheath covering the bony horn-cores of the
Oxen, while buck’s-horn is the commercial term applied to the antlers of
the Deer, which are largely used for knife-handles and other purposes.
True ivory is the product of the two species of Elephant; but other kinds
of ivory are obtained from the teeth of the Sperm Whale and the tusks
of the Walrus and Hippopotamus, the latter kind having been extensively
employed some years ago for artificial teeth. For many purposes the place
of ivory is taken by bone, this being mostly obtained from Ungulates.
The bones of Camels are of an especially firm texture and good colour,
and are largely employed in India for inlaying. Other important uses of
bones are in the form of bone-dust as manure, and also as a source of
phosphoric acid. The horns of the African Rhinoceros and the hide of the
Hippopotamus are occasionally manufactured into small canes or whips.
Horns and hoofs are also largely employed in the manufacture of glue.
Formerly the so-called whalebone, or more properly baleen, was much
used, especially to form the ribs of umbrellas and in stiffening ladies’
apparel, but the gradual destruction of the Right Whales, its only source
of supply, has largely restricted its use of late years.
The Cetacea are also of great economical importance from the abundance
of oil yielded by the thick layer of blubber underlying the skin.
Large quantities of valuable oil are also furnished by the Walrus and
the Seals. Spermaceti, which was at one time extensively used in the
manufacture of candles, is obtained from a large cavity in the head
of the Sperm Whale or Cachalot, and also from the _Hyperoödon_ or
Bottle-nosed Whale.
The nature of ambergris, a peculiar substance found floating on the
surface of the sea and employed in perfumery, was long a matter of
controversy; but it appears to be an intestinal concretion of the Sperm
Whale. Other substances of more importance to the perfumer are musk, the
product of the Musk-Deer of the Himalaya, and civet, which is obtained
from the so-called Civet Cat and other allied Carnivores. A secretion of
the Beaver has also been used in perfumery and in medicine.
CHAPTER II
GENERAL ANATOMICAL CHARACTERS
I. TEGUMENTARY STRUCTURES
_Hair._—The external surface of the greater number of members of the
class is thickly clothed with a peculiarly modified form of epidermis,
commonly called hair. This consists of hard, elongated, slender,
cylindrical or tapering, filiform, unbranched masses of epidermic
material, growing from a short papilla sunk at the bottom of a follicle
in the derm or true skin. Such hairs upon different parts of the same
animal, or upon different animals, assume various forms, and are of
various sizes and degrees of rigidity,—as seen in the delicate soft
velvety fur of the Mole, the stiff bristles of the Pig, and the spines
of the Hedgehog and Porcupine, all modifications of the same structures.
Each hair is composed usually of a cellular pithy internal portion,
containing much air, and a denser or more horny cortical part. In some
animals, as Deer, the substance of the hair is almost entirely composed
of the medullary or cellular substance, and it is consequently very
easily broken; in others the horny part prevails almost exclusively, as
in the bristles of the Wild Boar. In the Three-toed Sloth (_Bradypus_)
the hairs have a central horny axis and a pithy exterior. Though
generally nearly smooth, or but slightly scaly, the surface of some hairs
is strongly imbricated, notably so in some Bats; while in the Two-toed
Sloth (_Cholœpus_) the hairs are longitudinally grooved or fluted.
Though usually more or less cylindrical or circular in section, hairs
are often elliptical or flattened, as in the curly-haired races of men,
the terminal portion of the hair of Moles and Shrews, and conspicuously
in the spines of the Rodents _Xerus_ and _Platacanthomys_. Hair having
a property of mutual cohesion or “felting,” which depends upon a
roughened scaly surface and a tendency to curl, as in domestic Sheep (in
which animal this property has been especially cultivated by selective
breeding), is called “wool.”
In a large number of mammals hairs of one kind only are scattered pretty
evenly over the surface; but in many there are two kinds, one longer,
stiffer, and alone appearing on the surface, and the other shorter,
finer, and softer, constituting the under fur, analogous to the down
of birds. This under fur, or _pashm_ as it is called by the natives
of Kashmir, is especially abundant in the mammals inhabiting the cold
plateau of Tibet and the adjacent regions. In many cases hairs of a
different character from those of the general surface grow in special
regions, forming ridges or tufts on the median dorsal or ventral surface
or elsewhere. The tail is very often completed in this way by variously
disposed elongated hairs. The margins of the eyelids are almost always
furnished with a special row of stiffish hairs, called _cilia_ or
eyelashes; and in most mammals specially modified hairs, constituting
the _vibrissæ_ or whiskers, and endowed, through the abundant nerve
supply of their basal papillæ, with special tactile powers, grow from
the lips and cheeks. In some mammals the hairy covering is partial and
limited to particular regions; in others, as the Hippopotamus and the
Sirenia, though scattered over the whole surface, it is extremely short
and scanty; but in none is it reduced to so great an extent as in the
Cetacea, in which it is limited to a few small bristles confined to the
neighbourhood of the lips and nostrils, and often only present in the
young or even fœtal condition.
Some kinds of hairs, as those of the mane and tail of the Horse, appear
to persist throughout the lifetime of the animal; but more generally,
as in the case of the body hair of the same animal, they are shed and
renewed periodically, generally annually. Many mammals have a longer
hairy coat in winter, which is shed as summer comes on; and some few,
which inhabit countries covered in winter with snow, as the Arctic Fox,
Variable Hare, and Ermine, undergo a complete change of colour in the
two seasons, being white in winter, and gray or brown in summer. The
several species of Cape Mole (_Chrysochloris_), the Desmans or Water
Moles (_Myogale_), and _Potamogale velox_, are remarkable as being the
only mammals whose hair reflects those iridescent tints so common in the
feathers of tropical birds.
The principal and most obvious purpose of the hairy covering is to
protect the skin against external influences, especially cold and damp.
Its function in the hairless Cetacea is supplied by the specially
modified and thickened layer of adipose tissue beneath the skin, called
“blubber.”
_Colour._—From the consideration of hair we are easily led to that of
colour. As a general rule, bright and primary colours are absent in
the class; but among the Baboons we find brilliant patches of scarlet
or blue on some of the bare portions of the body, and one of the South
American Monkeys (_Brachyurus_) has its whole face of a bright crimson.
The most general colours are various shades of gray, brown, and tawny,
with a frequent tendency to whiteness of the ventral surface of the
body; but among the Squirrels, and more especially those provided with
a parachute for flying, we find brilliant russets, passing into orange
and red. Dark brown or black is also not very uncommon, as in the Bears
and the Sable Antelope of South Africa. Entirely white mammals are rare,
and mostly characteristic of the polar regions, or of countries having
a long and snowy winter. An entirely white Bat (_Diclidurus albus_)
occurs, however, in South America. In the large majority of mammals that
exhibit a varied coloration, the upper and most exposed parts of the
surface present the richest and darkest colours, the under parts being
pale or often quite white. The Ratels, Gluttons, _Ælurus_, Hamsters,
and some others are exceptions to this rule. A large number of mammals
having a ground colour of gray, tawny, or dun are marked by stripes or
spots, which are generally of a darker hue than the ground colour, as in
many Carnivora, but more rarely are lighter, as in the Fallow and Axis
Deer and several species of Antelope. These stripes very generally run
transversely to the axis of the body, as in the Tasmanian Thylacine, the
Tiger, and the Zebra; but they may be longitudinal, as in several of
the Civet family. There has been considerable discussion as to whether
the striped or the spotted is the more primitive type of coloration;
but no very conclusive arguments have been brought forward in favour of
either view. It is, however, manifest that in several groups of mammals
there is a tendency to lose the spots, and more rarely the stripes, and
to assume a uniform colour. Thus the young of nearly all the species of
Deer are spotted, whereas the adults of only the Fallow and Axis Deer
are so marked. The same is true of most of the Pigs; and the young of
the Malayan and American Tapirs are marked by light-coloured stripes and
spots on a dark ground. In like manner the young of the Lion and the
Puma exhibit distinct spots which disappear with advancing age. In most
of our domestic horses of various shades of bay and brown we may detect
“dappling” on the under hair when the outer coat has been removed, which
is not apparent on the surface of the latter. Many varieties of the Ass
and the Horse also exhibit a tendency to the presence of stripes on the
legs, which would seem to indicate a descent from a striped Zebra-like
type.
A peculiar feature, which is, however, common to many other groups of
animals, is the tendency to what is known as melanism, or the production
of black or dark individuals or races of particular species, due to an
excess of pigment in the skin and hair. Thus we may have black Leopards
and Jaguars, black Wolves, and black Rabbits.
The opposite to melanism, and of more frequent occurrence, is albinism—a
condition in which the pigment or colouring matter usually present in
the tissues constituting the external coverings of the body, and which
gives them their characteristic hue, is absent. When it occurs the hair
is of an opaque white, the claws, hoofs, etc., of a pale horn-colour,
and the skin and eyes pink, in consequence of the colour of the blood
which circulates through them being no longer concealed by the stronger
hues of the pigments. An animal in this condition is called an _albino_.
In complete albinism there is a total absence of pigment throughout the
system. This condition occurs occasionally as an individual peculiarity
among wild animals of many kinds; but it has never been perpetuated among
them in distinct races or species. The disadvantage of absence of pigment
in the eye, causing a certain amount of intolerance of light, is probably
sufficient to account for this. Several races of true albinos, as White
Ferrets, Rabbits, Rats, and Mice, have, however, been established under
the protection of man, and in them this abnormal condition is propagated
from generation to generation.
Partial albinism—a condition in which the absence of pigment is limited
to portions of the surface, or, at all events, does not extend to the
eyes—is much more common as an individual variation both in domestic and
in wild animals. It is possible that the artificial conditions incident
to domestication increase the tendency to its occurrence; but, whether
this be so or not, it certainly becomes perpetuated more frequently among
domesticated than among wild animals. This may be accounted for partly
by its proving of no disadvantage to them, and partly by the frequent
selection by man of animals of such colour in preference to others. The
result is that there is no completely domestic animal of which white
races do not exist. On the other hand, to most wild animals even partial
albinism seems to be a disadvantage in the struggle for existence, since,
except in the case of species inhabiting lands continually covered with
snow, it renders them more conspicuous objects both to their enemies and
their prey, and hence it is rarely perpetuated. In northern regions,
however, a large proportion of species are regularly and normally of a
white colour, either, as the Polar Bear, all the year through, or, as the
Ermine or Stoat, Arctic Fox, and Alpine Hare, during the winter season.
The coloration in these cases is obviously protective, as it is also to a
great extent in many other instances throughout the class.
Among conspicuously coloured mammals, it has been observed that the
vertical black and tawny stripes of the Tiger harmonise so well with the
brown and green grasses of its native jungle as to render the animal
almost invisible when lying among them; while the dappled hide of the
Giraffe is said to agree equally well with the chequered splashes of
light and shade in the clumps of tall mimosas among which it feeds. The
uniformly tawny hue of the Lion accords well with the prevailing tint
of its native desert; and any one who has seen an Elephant or Buffalo in
the deep shades of an Indian forest will realise how perfectly adapted is
their dull, slaty colour to concealment in such a spot. The dun colour
of the Wild Ass of India is equally well suited to the sandy deserts
of Kutch; it is also stated that the brilliant stripes of the Zebras
of Africa are arranged in such proportion as exactly to match the pale
tint which arid ground possesses when seen by moonlight.[1] The most
remarkable instance of protective coloration is, however, to be found in
the Sloths of South America, in which the coarse gray hairs so closely
resemble a mass of lichenous growth that it is almost impossible to
distinguish these animals when at rest from the gnarled and lichen-clad
boughs from which they suspend themselves. This resemblance is increased
by the fact that the hairs actually develop a growth of lichens upon
themselves. That the sombre coloration of these animals has been produced
to harmonise with their present surroundings seems to be evident by the
circumstance that when the long hair is plucked off the under fur is seen
to present a bold alternation of black and yellow stripes, which may
probably be regarded as the original primitive coloration of this group.
_Scales, etc._—True scales, or flat imbricated plates of horny material,
covering the greater part of the body, so frequently occurring in
reptiles, are found only in one family of mammals, the _Manidæ_ or
Pangolins; but these are also associated with hairs growing from the
intervals between the scales, or on the parts of the skin not covered
by them. Similarly, imbricated epidermic productions form the covering
of the under surface of the tail of the flying Rodents of the genus
_Anomalurus_; and flat scutes, with the edges in apposition, and not
overlaid, clothe both surfaces of the tail of the Beaver, Rats, and
others of the same order, and also of some Insectivores and Marsupials.
The Armadillos alone have an ossified exoskeleton, composed of plates
of true bony tissue, developed in the derm or corium, and covered with
scutes of horny epidermis. Other epidermic appendages are the horns of
Ruminants and Rhinoceroses,—the former being elongated, tapering, hollow
caps of hardened epidermis of fibrillated structure, fitting on and
growing from conical projections of the frontal bone, and always arranged
in pairs, while the latter are of similar structure, but solid and
without any internal bony support, and (in all existing species) situated
in the median line. Callosities, or bare patches covered with hardened
and thickened epidermis, are found covering the pads under the soles of
the feet and undersurfaces of the toes of nearly all mammals, upon the
ischial tuberosities of many Apes, the sternum of Camels, on the inner
side of the limbs of the _Equidæ_, the grasping under surface of the
tail of the prehensile-tailed Monkeys, etc. The greater part of the skin
of both species of one-horned Asiatic Rhinoceros is immensely thickened
and stiffened by increase of the tissue both of the derm and epiderm,
constituting the well-known jointed “armour-plated” hide of those animals.
_Nails, Claws, and Hoofs._—With very few exceptions, the terminal
extremities of the digits of both limbs are more or less protected or
armed by epidermic plates or sheaths, constituting the various forms
of nails, claws, or hoofs. These are wanting in the Cetacea alone. A
perforated spur, with a special secreting gland in connection with it,
is found attached to the hind leg of the males of the three genera of
Monotremata, _Ornithorhynchus_, _Proechidna_, and _Echidna_.
_Odour-secreting Glands._—Besides the universally distributed sebaceous
glands connected with the pilose system, most mammals have special glands
situated in modified portions of the integument, often involuted to form
a shallow recess or a deep sac with a narrow opening, situated in various
parts of the surface of the body, and secreting odorous substances,
by the aid of which individuals appear to recognise one another, and
probably affording the principal means by which wild animals are able
to become aware of the presence of other members of the species, even
at great distances. Although the commencement of the modifications of
portions of the external covering for the formation of special secretions
may be at present difficult to understand, the principle of natural
selection will readily explain how such organs become fixed and gradually
increase in development in any species, especially as there would
probably be a corresponding modification and increased sensibility of the
olfactory organs. Such individuals as by the intensity and peculiarity
of their scent had greater power of attracting the opposite sex would
certainly be those most likely to leave descendants to inherit and in
their turn propagate the modification.
To this group of structures belong the suborbital gland or “crumen” of
Antelopes and Deer, the frontal gland of the Muntjac and of Bats of
the genus _Hipposiderus_, the submental gland of the Chevrotains and
of _Taphozous_ and some other Bats, the post-auditory follicle of the
Chamois, the temporal gland of the Elephant, the lateral glands of the
Musk-Shrew, the dorsal gland of the Peccary, the inguinal glands of
Antelopes, the preputial glands of the Musk-Deer and Beaver (already
alluded to in connection with the use made of their powerfully odorous
secretion in medicine and perfumery) and also of the Swine and Hare,
the anal glands of Carnivora, the perineal gland of the Civet (also of
commercial value), the caudal glands of the Fox and Goat, the gland on
the humeral membrane of Bats of the genus _Saccopteryx_, the post-digital
gland of the Rhinoceros, the interdigital glands of the Sheep and many
Ruminants, and numerous others. In some of these cases the glands are
peculiar to, or more largely developed in, the male; in others they are
found equally developed in both sexes.
II. DENTAL SYSTEM
The dental system of mammals may be considered rather more in detail
than space permits for some other portions of their structure, not only
on account of the important part it plays in the economy of the animals
of this class, but also for its interest to zoologists as an aid in the
classification and identification of species. Owing to the imperishable
nature of their tissues, teeth are preserved for an indefinite time, and
in the case of extinct species frequently offer the only indications
available from which to derive an idea of the characters, affinities,
and habits of the animals to which they once belonged. Hence even
their smallest modifications have received great attention from
comparative anatomists, and they have formed the subject of many special
monographs.[2]
Teeth are present in nearly all mammals, and are applied to various
purposes. They are, however, mainly subservient to the function of
alimentation, being used either in procuring food, by seizing and
killing living prey or gathering and biting off portions of vegetable
material, and more indirectly in tearing or cutting through the hard
protective coverings of food substances, as the husks and shells of nuts,
or in pounding, crushing, or otherwise mechanically dividing the solid
materials before swallowing, so as to prepare them for digestion in the
stomach. Certain teeth are also in many animals most efficient weapons of
offence and defence, and for this purpose alone, quite irrespective of
subserviency to the digestive process, are they developed in the male sex
of many herbivorous animals, in the females of which they are absent or
rudimentary.
Teeth belong essentially to the tegumentary or dermal system of organs,
and, as is well seen in the lower vertebrates, pass by almost insensible
gradations into the hardened spines and scutes formed upon the integument
covering the outer surface of the body; but in mammals they are more
specialised in structure and limited in locality. In this class they
are developed only in the gums or fibro-mucous membrane covering the
alveolar borders of the upper and lower jaws, or, in other words, the
premaxillary and maxillary bones and the mandible. In the process of
development, for the purpose of giving them that support which is needful
for the performance of their functions, they almost always become
implanted in the bone,—the osseous tissue growing up and moulding itself
around the lengthening root of the tooth, so that ultimately they become
apparently parts of the skeleton. In no mammal, however, does ankylosis
or bony union between the tooth and jaw normally take place, as in
many fishes and reptiles,—a vascular layer of connective tissue, the
alveolo-dental membrane, always intervening.[3] The presence of two or
more roots, frequently met with in the cheek-teeth of mammals, implanted
in corresponding distinct sockets of the jaw, is now peculiar to animals
of this class.[4]
_Structure._—The greater number of mammalian teeth when fully formed are
not simple and homogeneous in structure, but are composed of several
distinct tissues, which are enumerated below.
The _pulp_, a soft substance, consisting of a very delicate gelatinous
connective tissue, in which numerous cells are imbedded, and abundantly
supplied with blood-vessels and nerves, constitutes the central axis
of all the basal part of the tooth, and affords the means by which the
vitality of the whole is preserved. The nerves which pass into the pulp
and endow the tooth with sensibility are branches of the fifth pair of
cranial nerves. The pulp occupies a larger relative space, and performs
a more important purpose, in the young growing tooth than afterwards, as
by the calcification and conversion of its outer layers the principal
hard constituent of the tooth, the dentine, is formed. In teeth which
have ceased to grow the pulp occupies a comparatively small space, which
in the dried tooth is called the pulp-cavity. This communicates with the
external surface of the tooth by a small aperture at the apex of the
root, through which the branches of the blood-vessels and nerves, by
which the tooth receives its nutrition and sensitiveness, pass in to be
distributed in the pulp. In growing teeth the pulp-cavity is widely open,
while in advanced age it often becomes obliterated, and the pulp itself
entirely converted into bone-like material.
The _dentine_ or _ivory_ forms the principal constituent of the greater
number of teeth. When developed in its most characteristic form, it is
a very hard but elastic substance, white, with a yellowish tinge, and
slightly translucent. It consists of an organic matrix, something like,
but not identical with, that of bone, richly impregnated with calcareous
salts (chiefly calcium phosphate), these constituting in a fresh human
tooth 72 per cent of its weight. When subjected to microscopical
examination it is seen to be everywhere permeated by nearly parallel
branching tubes which run, in a slightly curving or wavy manner, in a
general direction from the centre towards the free surface of the tooth.
These tubes communicate by open mouths with the pulp-cavity, and usually
terminate near the periphery of the dentine by closed ends or loops,
though in Marsupials and certain other mammals they penetrate into the
enamel. They are occupied in the living tooth by soft gelatinous fibrils
connected with the cells of the pulp. A variety of dentine, permeated
by canals containing blood-vessels, met with commonly in fishes and
in some few mammals, as the _Megatherium_, is called vaso-dentine.
Other modifications of this tissue occasionally met with are called
osteodentine and secondary dentine,—the latter being a dentine of
irregular structure which often fills up the pulp-cavity of old animals.
The _enamel_ constitutes a thin investing layer, complete or partial, of
the outer or exposed and working surface of the dentine of the crown of
the teeth of most mammals. This is the hardest tissue met with in the
animal body, containing from 95 to 97 per cent of mineral substances
(chiefly calcium phosphate and some carbonate, with traces of fluoride).
Its ultimate structure consists of prismatic fibres, placed generally
with their long axes at right angles to the free surface of the tooth.
Enamel is easily distinguished from dentine with the naked eye by its
clear, bluish-white, translucent appearance.
The _cement_ or _crusta petrosa_ is always the most externally placed
of the hard tissues of which teeth are composed, as will be understood
when the mode of development of these organs is considered. It is often
only found as a thin layer upon the surface of the root; but sometimes,
as in the complex-crowned molar teeth of the Horse and Elephant, it is a
structure which plays a very important part, covering and filling in the
interstices between the folds of the enamel. In appearance, histological
structure, and chemical composition it is closely allied to osseous
tissue, containing lacunæ and canaliculi, though only when it is of
considerable thickness are Haversian canals present in it.
_Development._—The two principal constituents of the teeth, the dentine
and the enamel, are developed from the two layers of the mucous membrane
of the jaw—the dentine from the deeper or vascular, the enamel from the
superficial or epithelial layer. The latter dips down into the substance
of the gum, and forms the enamel-organ or germ, the first rudiment of the
future tooth, which is constantly present even in those animals in which
the enamel is not found as a constituent of the perfectly-formed tooth.
Below the mass of epithelial cells thus embedded in the substance of the
gum, and remaining connected by a narrow neck of similar structure with
the epithelium of the surface, a portion of the vascular areolar tissue
becomes gradually separated and defined from that which surrounds it,
and assumes a distinct form, which is that of the crown of the future
tooth,—a single cone in the case of simple teeth, or with two or more
eminences in the complex forms. This is called the dental papilla or
dentine germ, and by the gradual conversion of its tissue into dentine
the bulk of the future tooth is formed, the uncalcified central portion
remaining as the pulp. The conversion of the papilla into hard tissue
commences at the outer surface of the apex, and gradually proceeds
downwards and inwards, so that the form of the papilla exactly determines
the form of the future dentine, and no alteration either in shape or
size of this portion of the tooth, when once calcified, can take place
by addition to its outer surface. In the meanwhile, calcification of a
portion of the cells of the enamel-organ, which adapts itself like a cap
round the top of the dentinal papilla, and has assumed a somewhat complex
structure, results in the formation of the enamel-coating of the crown of
the tooth. While these changes are taking place the tissues immediately
surrounding the tooth-germ become condensed and differentiated into a
capsule, which appears to grow up from the base of the dental papilla,
and encloses both this and the enamel-germ, constituting the follicle
or tooth-sac. By the ossification of the inner layer of this follicle
the cement is formed. This substance, therefore, unlike the dentine,
increases from within outwards, and its growth may accordingly be the
cause of considerable modification of form and enlargement, especially
of the roots, of certain teeth, as those of Seals and some Cetacea. The
delicate homogeneous layer coating the enamel surface of newly-formed
teeth, in which cement is not found in the adult state, and known as
Nasmyth’s membrane, is considered by Tomes as probably a film of this
substance, too thin to exhibit its characteristic structure, though
by others it is believed to be derived from the external layer of the
enamel-organ. The homology of the teeth with the dermal appendages,
hairs, scales, and claws, has already been alluded to, and it will now
be seen that in both cases two of the primary embryonic layers are
concerned in their development—the mesoblast and epiblast—although in
very different proportions respectively. Thus in the hair or nail the
part derived from the epiblast forms the principal bulk of the organ, the
mesoblast only constituting the papilla or matrix. But in the tooth the
epiblastic portion is limited to the enamel, and is always of relatively
small bulk and often absent, while the dentine (the principal constituent
of the tooth) and the cement are formed from the mesoblast.
When more than one set of teeth occur in mammals, those of the second
set are developed in a precisely similar manner to the first, but the
enamel-germ, instead of being derived directly from an independent
part of the oral epithelium, is formed from a budding out of the neck
of the germ of the tooth succeeded. In the case of the true molars,
which have no predecessors, the germ of the first has an independent
origin, but that of the others is derived from the neck of the germ of
the tooth preceding it in the series. The foundations of the permanent
teeth are thus laid as it were almost simultaneously with those of their
predecessors, although they remain in many cases for years before they
are developed into functional activity.
Although the commencement of their formation takes place at an early
period of embryonic life, teeth are in nearly all mammals still concealed
beneath the gum at the time of birth. The period of eruption, or
“cutting” of the teeth as it is called, that is, their piercing through
and rising above the surface of the mucous membrane, varies much in
different species. In some, as Seals, the whole series of teeth appears
almost simultaneously; but more often there are considerable intervals
between the appearance of the individual teeth, the front ones usually
coming into place first, and those at the back of the mouth at a later
period.
_Forms of Teeth._—The simplest form of tooth may be exemplified on a
large scale by the tusk of the Elephant (Fig. 1, I.) It is a hard mass
almost entirely composed of dentine, of a conical shape at first, but
during growth becoming more and more cylindrical or uniform in width.
The enamel-covering, present on the apex in its earliest condition, soon
disappears, but a thin layer of cement covers the circumference of the
tooth throughout life. In section it will be seen that the basal portion
is hollow, and contains a large conical pulp, as broad at the base as the
tooth itself, and deeply imbedded in the bottom of a recess, or socket,
in the maxillary bone. This pulp continues to grow during the lifetime
of the animal, and at the same time is converted at its surface into
dentine. The tooth therefore continually elongates, but the use to which
the animal subjects it in its natural state causes the apex to wear away,
at a rate generally proportionate to the growth at the base, otherwise it
would become of inconvenient length and weight. Such teeth of indefinite
growth are said to be “rootless,” or to have “persistent pulps.”
[Illustration: FIG. 1.—Diagrammatic Sections of various forms of Teeth.
I. Incisor or tusk of Elephant, with pulp-cavity persistently open
at base. II. Human incisor during development, with root imperfectly
formed, and pulp-cavity widely open at base. III. Completely formed human
incisor, with pulp-cavity contracted to a small aperture at the end of
the root. IV. Human molar, with broad crown and two roots. V. Molar
of the Ox, with the enamel covering the crown deeply folded, and the
depressions filled up with cement. The surface is worn by use; otherwise
the enamel coating would be continuous at the top of the ridges. In all
the figures the enamel is black, the pulp white, the dentine represented
by horizontal lines, and the cement by dots.]
One of the corresponding front teeth of man (Fig. 2, II. and III.) may
be taken as an example of a very different condition. After its crown is
fully formed by calcification of the germ, the pulp, though continuing to
elongate, begins to contract in diameter; a neck or slight constriction
is formed; and the remainder of the pulp is converted into the root
(often, but incorrectly, called “fang”), a tapering conical process
imbedded in the alveolar cavity of the bone, and having at its extremity
a minute perforation, through which the vessels and nerves required to
maintain the vitality of the tooth enter the pulp-cavity, which is very
different from the widely open cavity at the base of the growing tooth.
When the crown of the tooth is broad and complex in character, instead of
having a single root, it may be supported by two or more roots, each of
which is implanted in a distinct alveolar recess or socket, and to the
apex of which a branch of the common pulp-cavity is continued (Fig. 1,
IV.) Such teeth are called “rooted teeth.” When they have once attained
their position in the jaw, with the neck a little way above the level
of the free margin of the alveolus, and embraced by the gum or tough
fibrovascular membrane covering the alveolar border, and having the root
fully formed, they can never increase in length or alter their position;
if they appear to do so in old age, it being only in consequence of
absorption and retrocession of the surrounding alveolar margins. If,
as often happens, their surface wears away in mastication, it is never
renewed. The open cavity at the base of the imperfectly developed tooth
(Fig. 1, II.) causes it to resemble the persistent condition of the
rootless tooth. The latter is therefore a more primitive condition,
the formation of the root being a completion of the process of tooth
development. Functionally it is, however, difficult to say that the one
is a higher form than the other, since they both serve important and
different purposes in the animal economy.
As is almost always the case in nature, intermediate conditions between
these two forms of teeth are met with. Thus some teeth, as the molars of
the Horse, and of many Rodents, are for a time rootless, and have growing
pulps producing very long crowns with parallel sides, the summits of
which may be in use and beginning to wear away while the bases are still
growing; but ultimately the pulp contracts, forms a neck and distinct
roots, and ceases to grow. The canine tusks of the Musk Deer and of the
Walrus have persistent pulps, and are open at their base until the animal
is of advanced age, when they close, and the pulp ceases to be renewed.
The same sometimes happens in the tusks of very old Boars.
The simplest form of the crown of a tooth is that of a cone; but this may
be variously modified. Thus it may be flattened, with its edges sharp
and cutting, and pointed at the apex, as in the laterally compressed
premolars of most Carnivora; or it may be chisel- or awl-shaped, with a
straight truncated edge, as in the human incisors; or it may be broad,
with a flat or rounded upper surface. Very often there is a more or less
prominent ridge encircling the whole or part of the base of the crown
just above the neck, called the cingulum, which serves as a protection to
the edge of the gum in masticating, and is most developed in flesh-eating
and insectivorous animals, in which the gums are liable to be injured by
splinters of bone or other hard fragments of their food. The form of the
crown is frequently rendered complex by the development upon its surface
of elevations or tubercules called cusps or cones, or by ridges usually
transverse, but sometimes variously curved or folded. When the crown
is broad and the ridges are greatly developed, as in the molars of the
Elephant, Horse, and Ox (Fig. 1, V.), the interspaces between them are
filled with cement, which supports them and makes a solid compact mass of
the whole tooth. When such a tooth wears away at the surface by friction
against the opposed tooth of the other jaw, the different density of
the layers of the substances of which it is composed—enamel, dentine,
and cement—arranged in characteristic patterns, causes them to wear
unequally, the hard enamel ridges projecting beyond the others, and thus
giving rise to a grinding surface of great mechanical advantage.
_Succession._—The dentition of all mammals consists of a definite set
of teeth, almost always of constant and determinate number, form,
and situation, and, with few exceptions, persisting in a functional
condition throughout the natural term of the animal’s life. In many
species these are the only teeth which the animal ever possesses,—the
set which is first formed being permanent, or, if accidentally lost,
or decaying in extreme old age, not being replaced by others. These
animals are called Monophyodont. But in the larger number of mammals,
certain of the teeth are preceded by others, which may be only of a
very transient, rudimentary, and functionless character (being in the
Seals, for example, shed either before or within a few days after
birth), or may be considerably developed, and functionally occupy the
place of the permanent teeth for a somewhat lengthened period, during
the growth and development of the latter and of the jaws. In all cases
these teeth disappear (by the absorption of their roots and shedding
of the crowns) before the frame of the animal has acquired complete
maturity, as evidenced by the coalescence of the epiphyses of the
osseous system. As these teeth are, as a general rule, present during
the period in which the animal is nourished by the milk of the mother,
the name of “milk-teeth” (French _dents de lait_, German _milchzähne_)
has been commonly accorded to them, although it must be understood that
the epoch of their presence is by no means necessarily synchronous with
that of lactation. Animals possessing such teeth are called Diphyodont.
No mammal is known to have more than two sets of teeth; and the definite
and orderly replacement of certain members of the series is a process
of quite a different nature from the indefinite succession which takes
place in all the teeth continuously throughout the lifetime of the lower
vertebrates.
When the milk-teeth are well developed, and continue in place during the
greater part of the animal’s growth, as is especially the case with the
Ungulata, and, though to a less degree, with the Primates and Carnivora,
their use is obvious, since taken all together they form structurally
a complete epitome on a small scale of the more numerous and larger
permanent set (see Fig. 3), and, consequently, are able to perform the
same functions, while time is allowed for the gradual maturation of
the latter, and especially while the jaws of the growing animal are
acquiring the size and strength sufficient to support the permanent
teeth. Those animals, therefore, that have a well-developed and tolerably
persistent set of milk-teeth may be considered to be in a higher state
of development, as regards their dentition, than those that have the
milk-teeth absent or rudimentary.
It is a very general rule that individual teeth of the milk and permanent
set have a close relationship to one another, being originally formed, as
mentioned above, in exceedingly near proximity, and with, at all events
so far as the enamel-germ is concerned, a direct connection. Moreover,
since the latter ultimately come to occupy the position in the alveolar
border temporarily held by the former, they are spoken of respectively as
the predecessors or successors of each other. But it must be understood
that milk-teeth may be present which have no successors in the permanent
series, and, what is far more general, permanent teeth may have no
predecessors in the milk series.
The complete series of permanent teeth of most mammals forms a complex
machine, with its several parts adapted for different functions,—the
most obvious structural modification for this purpose being an increased
complexity of the individual components of the series from the anterior
towards the posterior extremity of such series. Since, as has just been
said, the complete series of the milk teeth often presents structurally
and functionally a similar machine, but composed of fewer individual
members, and the anterior of which are as simple, and the posterior as
complex as those occupying corresponding positions in the permanent
series,—and since the milk-teeth are only developed in relation to
the anterior or lateral, never to the most posterior of the permanent
series,—it follows that the hinder milk-teeth are usually more complex
than the teeth of which they are the predecessors in the permanent
series, and represent functionally, not their immediate successors, but
those more posterior permanent teeth which have no direct predecessors.
This character is clearly seen in those animals in which the various
members of the molar series are well differentiated from each other in
form, as the Carnivora, and also in Man.
In animals which have two sets of teeth the number of those of the
permanent series which are preceded by milk-teeth varies greatly, being
sometimes, as in Marsupials and some Rodents, as few as one on each side
of each jaw, and sometimes including the larger portion of the series.
Although there are difficulties in some cases in arriving at a
satisfactory solution of the question, it is, on the whole, safest to
assume that when only one set of teeth is present, this corresponds to
the permanent teeth of the Diphyodonts. When this one set is completely
developed, and remains in use throughout the animal’s life, there can
be no question on this subject. When, on the other hand, the teeth are
rudimentary and transient, as in the Whalebone Whales, it is possible
to consider them as representing the milk series; but there are weighty
reasons in favour of the opposite conclusion.[5]
_Arrangement, Homologies, and Notation of Teeth._—The teeth of the two
sides of the jaws are always alike in number and character, except in
cases of accidental or abnormal variation, and in the one remarkable
instance of constant deviation from bilateral symmetry among mammals,
the tusks of the Narwhal (_Monodon_), in which the left is of immense
size, and the right rudimentary. In certain mammals, such as the Dolphins
and some Armadillos, which have a very large series of similar teeth,
not always constant in number in different individuals, there may be
differences in the two sides; but, apart from these, in describing the
dentition of any mammal, it is quite sufficient to give the number and
characters of the teeth of one side only. Since the teeth of the upper
and the lower jaws work against each other in masticating, there is a
general correspondence or harmony between them, the projections of one
series, when the mouth is closed, fitting into corresponding depressions
of the other. There is also a general resemblance in the number,
characters, and mode of succession of both series, so that, although
individual teeth of the upper and lower jaws may not be in any strict
sense of the term homologous parts, there is a great convenience in
applying the same descriptive terms to the one as are used for the other.
[Illustration: FIG. 2.—Upper and Lower Teeth of one side of the Mouth
of a Dolphin (_Lagenorhynchus_) as an example of the homodont type of
dentition. The bone covering the outer side of the roots of the teeth has
been removed to show their simple character.]
The simplest dentition as a whole is that of many species of Dolphin
(Fig. 2), in which the crowns are single-pointed, slightly curved cones,
and the roots also single and tapering, and all alike in form from the
anterior to the posterior end of the series, though it may be with
some slight difference in size, those at the two extremities of the
series being rather smaller than the others. Such a dentition is called
Homodont, and in the case cited, as the teeth are never changed, it is
also Monophyodont. Such teeth are adapted only for catching slippery
living prey, as fish.
In a very large number of mammals the teeth of different parts of the
series are more or less differentiated in character, and have different
functions to perform. The front teeth are simple and one-rooted, and
are adapted for cutting and seizing. They are called “incisors.” The
back- or cheek-teeth have broader and more complex crowns, tuberculated
or ridged, and are supported on two or more roots. They crush or grind
the food, and are hence called “molars.” Many animals have, between
these two sets, a tooth at each corner of the mouth, longer and more
pointed than the others, adapted for tearing or stabbing, or for fixing
struggling prey. From the conspicuous development of such teeth in the
Carnivora, especially the Dogs, they have received the name of “canines.”
A dentition with its component parts so differently formed that these
distinctive terms are applicable to them is called Heterodont. In most
cases, though by no means invariably, animals with Heterodont dentition
are also Diphyodont.
This general arrangement is extremely obvious in a considerable number of
mammals; and closer examination shows that, under very great modification
in detail, there is a remarkable uniformity of essential characters in
the dentition of a large number of members of the class belonging to
different orders and not otherwise closely allied; so much so indeed that
it has been possible (chiefly through the researches of Sir Richard Owen)
to formulate a common plan of dentition from which the others have been
derived by the alteration of some and suppression of other members of the
series, and occasionally, but very rarely, by addition. The records of
palæontology fully confirm this view, as by tracing back many groups now
widely separated in dental characters we find a gradual approximation to
a common type. In this generalised form of mammalian dentition (which is
best exemplified in the genera _Anoplotherium_ and _Homalodontotherium_)
the entire number of teeth present is 44, or 11 above and 11 below on
each side. Those of each jaw are placed in continuous series without
intervals between them; and, although the anterior teeth are simple and
single-rooted, and the posterior teeth complex and with several roots,
the transition between the two kinds is gradual.
In dividing and grouping such teeth for the purpose of description and
comparison, more definite characters are required than those derived
merely from form or function. The first step towards a classification
has been made by the observation that the upper jaw is composed of two
bones, the premaxilla and the maxilla, and that the suture between these
bones separates the three anterior teeth from the others. These three
teeth, then, which are implanted by their roots in the premaxilla,
form a distinct group, to which the name of “incisor” is applied. This
distinction is, however, not so important as it appears at first sight,
for, as mentioned when speaking of the development of the teeth, their
connection with the bone is only of a secondary nature, and, although it
happens conveniently for our purpose that in the great majority of cases
the segmentation of the bone coincides with the interspace between the
third and fourth tooth of the series, still, when it does not happen to
do so, as in the case of the Mole, we must not give too much weight to
this fact, if it contravenes other reasons for determining the homologies
of the teeth. The eight remaining teeth of the upper jaw offer a natural
division, inasmuch as the posterior three never have milk-predecessors;
and, although some of the anterior teeth may be in the same case, the
particular one preceding these three always has such a predecessor. These
three then are grouped apart as the “molars,” or, since some of the teeth
in front of them often have a molariform character, “true molars.” Of the
five teeth between the incisors and molars the most anterior, or that
which is usually situated close behind the premaxillary suture, almost
always, as soon as any departure takes place from the simplest and most
homogeneous type, assumes a lengthened and pointed form, and is the tooth
so developed as to constitute the “canine” or “laniary” tooth of the
Carnivora, the tusk of the Boar, etc. It is customary therefore to call
this tooth, whatever its size or form, the “canine.” The remaining four
are the “premolars” or “false molars.” This system of nomenclature has
been objected to as being artificial, and in many cases not descriptive,
the distinction between premolars and canine especially being sometimes
not obvious; but the terms are now in such general use, and are so
practically convenient—especially if, as it is best to do in all such
cases, we forget their original signification and treat them as arbitrary
signs—that it is not likely they will be superseded by any that have been
proposed as substitutes for them.
With regard to the lower teeth the difficulties are greater, owing to the
absence of any suture corresponding to that which defines the incisors
above; but since the number of the teeth is the same, the corresponding
teeth are preceded by milk-teeth, and in the large majority of cases it
is the fourth tooth of the series which is modified in the same way as
the canine (or fourth tooth) of the upper jaw, it is quite reasonable
to adopt the same divisions as with the upper series, and to call the
first three, which are implanted in the part of the mandible opposite
to the premaxilla, the incisors, the next the canine, the next four the
premolars, and the last three the molars. It may be observed that when
the mouth is closed, especially when the opposed surfaces of the teeth
present an irregular outline, the corresponding upper and lower teeth
are not exactly opposite, otherwise the two series could not fit into
one another; but as a rule the points of the lower teeth shut into the
interspaces in front of the corresponding teeth of the upper jaw. This
is seen very distinctly in the canine teeth of the Carnivora, and is a
useful guide in determining the homologies of the teeth of the two jaws.
Objections have certainly been made to this view, because, in certain
rare cases, the tooth which, according to it, would be called the lower
canine has the form and function of an incisor (as in Ruminants and
Lemurs), and on the other hand (as in _Cotylops_, an extinct Ungulate
from North America) the tooth that would thus be determined as the
first premolar has the form of a canine; but it should not be forgotten
that, as in all such cases, definitions derived from form and function
alone are quite as open to objection as those derived from position and
relation to surrounding parts, or still more so.
_Dental formulæ._—For the sake of brevity the complete dentition,
arranged according to these principles, is often described by the
following formula, the numbers above the line representing the teeth of
the upper, those below the line those of the lower jaw:—incisors ³⁻³⁄₃₋₃,
canines ¹⁻¹⁄₁₋₁, premolars ⁴⁻⁴⁄₄₋₄, molars ³⁻³⁄₃₋₃ = ¹¹⁻¹¹⁄₁₁₋₁₁; total
44. Since, however, initial letters may be substituted for the names of
each group, and it is quite unnecessary to give more than the numbers
of the teeth on one side of the mouth, the formula may be conveniently
abbreviated into—
_i_ ³⁄₃, _c_ ¹⁄₁, _p_ ⁴⁄₄, _m_ ³⁄₃ = ¹¹⁄₁₁; total 44.
The individual teeth of each group are always enumerated from before
backwards, and by such a formula as the following—
_i_ 1, _i_ 2, _i_ 3, _c_, _p_ 1, _p_ 2, _p_ 3, _p_ 4, _m_ 1, _m_ 2, _m_ 3
-------------------------------------------------------------------------
_i_ 1, _i_ 2, _i_ 3, _c_, _p_ 1, _p_ 2, _p_ 3, _p_ 4, _m_ 1, _m_ 2, _m_ 3
or more briefly—
1,2,3 1 1,2,3,4 1,2,3
_i_ ------ _c_ -- _p_ -------- _m_ -----.
1,2,3, 1, 1,2,3,4, 1,2,3
A special numerical designation is thus given by which each one can be
indicated. In mentioning any single tooth, such a sign as _m¹_⁄ will mean
the first upper molar, ⁄_m₁_ the first lower molar, and so on. The use of
such signs saves much time and space in description.[6]
It was part of the view of the founder of this system of dental notation
that, at least throughout the group of mammals whose dentition is
derived from this general type, each tooth has its strict homologue in
all species, and that in those cases in which fewer than the typical
number are present (as in all existing mammals except the genera _Sus_,
_Gymnura_, _Talpa_, and _Myogale_), the teeth that are missing can be
accurately defined. According to this view, when the number of incisors
falls short of three it is assumed that the absent ones are missing
from the outer and posterior end of the series. Thus, when there is
but one incisor present, it is _i_ 1; when two, they are _i_ 1 and
_i_ 2. Furthermore, when the premolars and the molars are below their
typical number, the absent teeth are missing from the fore part of the
premolar series, and from the back part of the molar series. If this were
invariably so, the labours of those who describe teeth would be greatly
simplified; but there are so many exceptions that a close scrutiny into
the situation, relations, and development of a tooth is required before
its nature can be determined, and in some cases the evidence at our
disposal is scarcely sufficient for the purpose. In other instances,
however, as among the Polyprotodont Marsupials, we have decisive evidence
to show that the missing premolar teeth are not those at the extremity of
the series.
[Illustration: FIG. 3.—Milk and Permanent Dentition of Upper (I.) and
Lower (II.) Jaw of the Dog (_Canis familiaris_), with the symbols by
which the different teeth are commonly designated. The third upper molar
(_m._3) is the only tooth wanting in this animal to complete the typical
heterodont mammalian dentition.]
The milk-dentition is expressed by a similar formula, _d_ for deciduous
or _m_ for milk being commonly prefixed to the letter expressive of
the nature of the tooth. Since the three molars, and almost invariably
the first premolar of the permanent series, have no predecessors, the
typical milk-dentition would be expressed as follows—_di_ ³⁄₃, _dc_
¹⁄₁, _dm_ ³⁄₃, = ⁷⁄₇, total 28. In a few Ungulates, however, such as
the Hyrax and Tapir, and in some instances the Rhinoceros and the
extinct _Palæotherium_, the whole of the four premolars are preceded
by milk-teeth; when we have the fullest development of cheek-teeth in
the whole of the Eutheria. The teeth which precede the premolars of the
permanent series are all called molars in the milk-dentition, although
as a general rule, in form and function they represent in a condensed
form the whole premolar and molar series of the adult. When there is
a marked difference between the premolars and molars of the permanent
dentition, the first milk-molar resembles a premolar, while the last has
the characters of the posterior true molar.
The dentition of all the members of the orders Primates, Carnivora,
Insectivora, Chiroptera, and Ungulata can clearly be derived from the
above-described generalised type. The same may be said of the Rodents,
and even the Proboscidea, though at least in the existing members of the
order with greater modification. It is also apparent in certain extinct
Cetacea, as _Zeuglodon_ and _Squalodon_, but it is difficult to find
any traces of it in existing Cetacea, Sirenia, or any of the so-called
Edentata. All the Marsupials, different as they are in their general
structure and mode of life, and variously modified as is their dentition,
present in this system of organs some deep-lying common characters
which show their unity of origin. The generalised type to which their
dentition can be reduced presents considerable resemblance to that of the
placental mammals, yet differing in details. It is markedly heterodont,
and susceptible of division into incisors, canines, premolars, and molars
upon the same principles. The whole number is, however, not limited to
forty-four. The incisors may be as numerous as five on each side above,
and they are almost always different in number in the upper and the lower
jaw. The premolars and molars are commonly seven, as in the placental
mammals, but their arrangement is reversed, as there are four true molars
and three premolars.
The larger number of incisive and molar teeth among the Marsupials
suggests that their additional teeth have disappeared in the Eutheria,[7]
and Mr. O. Thomas has endeavoured to construct a generalised dental
formula from which both the Marsupial and Eutherian modifications
may have been derived by the suppression of particular teeth. Thus
the hypothetical formula _i_ ¹,²,³,⁴,⁵⁄₁,₂,₃,₄,₅, _c_ ¹⁄₁, _p_
¹,²,³,⁴⁄₁,₂,₃,₄, _m_ ¹,²,³,⁴,⁵⁄₁,₂,₃,₄,₅, by the loss of the fifth
lower incisor, and of the second premolars (which we know to be those
which disappear in the Marsupials) and the fifth molars, will give _i_
¹,²,³,⁴,⁵⁄₁,₂,₃,₄,₀, _c_ ¹⁄₁, _p_ ¹,⁰,³,⁴⁄₁,₀,₃,₄, _m_ ¹,²,³,⁴⁄₁,₂,₃,₄;
or the formula of the Opossum (_Didelphys_), usually written _i_ ⁵⁄₄, _c_
¹⁄₁, _p_ ³⁄₃, _m_ ⁴⁄₄. Again, in the same formula the loss of the fourth
and fifth incisors in both jaws, and also of the fourth molars, gives us
_i_ ¹,²,³,⁰,⁰⁄₁,₂,₃,₀,₀, _c_ ¹⁄₁, _p_ ¹,²,³,⁴⁄₁,₂,₃,₄, _m_ ¹,²,³⁄₁,₂,₃,
or the formula of a typical Eutherian, like the Pig, which we generally
write as _i_ ³⁄₃, _c_ ¹⁄₁, _p_ ⁴⁄₄, _m_ ³⁄₃. Such a generalised formula
will admit of modification into that of all existing, and a large number
of fossil Marsupials, but it is possible that some of the Mesozoic types
may have had more than four premolars, although there is no absolutely
decisive evidence that such was the case. The presence of seven or eight
true molars in some Mesozoic forms merely entails the addition of two or
three additional figures to the ideal generalised formula.
The milk-dentition of all known Marsupials, existing or extinct, is (if
not entirely absent) limited to a single tooth on either side of each
jaw, this being the predecessor of the last permanent premolar. And if
the view that the milk-dentition is an additional series grafted upon the
original permanent series be correct, it is evident that we have in this
single replacement the first stage of this additional development.
In very few mammals are teeth entirely absent. Even in the Whalebone
Whales their germs are formed in the same manner and at the same period
of life as in other mammals, and even become partially calcified, but
they never rise above the gums, and completely disappear before the birth
of the animal. In some species of the order Edentata, the true Anteaters
and the Pangolins, no traces of teeth have been found at any age. The
adult Monotremata are likewise devoid of teeth of the same structure
as those of ordinary mammals; but well-developed molars occur in the
young _Ornithorhynchus_, although no traces of teeth have hitherto been
detected in _Echidna_.
_Modifications of the Teeth in Relation to their Functions._—The
principal functional modifications noticed in the dentition of mammalia
may be roughly grouped as piscivorous, carnivorous, insectivorous,
omnivorous, and herbivorous, each having, of course, numerous variations
and transitional conditions.
The essential characters of a piscivorous dentition are best exemplified
in the Dolphins, and also (as modifications of the carnivorous type) in
the Seals. This type consists of an elongated, rather narrow mouth, wide
gape, with numerous subequal, conical, sharp-pointed, recurved teeth,
adapted simply to rapidly seize, but not to divide or masticate, active,
slippery, but not powerful prey. All animals which feed on fish as a
rule swallow and digest them entire, a process which the structure of
prey of this nature, especially the intimate interblending of delicate,
sharp-pointed bones with the muscles, renders very advantageous, and for
which the above-described type of dentition is best adapted.
The carnivorous type of dentition is shown in its most specialised
development among existing mammals in the _Felidæ_. The function being
here to seize and kill struggling animals, often of large size and great
muscular power, the canines are immensely developed, trenchant, and
piercing, and are situated wide apart, so as to give the firmest hold
when fixed in the victim’s body. The jaws are as short as is consistent
with the free action of the canines, so that no power may be lost. The
incisors are very small, so as not to interfere with the penetrating
action of the canines, and the crowns of the molar series are reduced
to scissor-like blades, with which to pare off the soft tissues from
the large bones, or to divide into small pieces the less dense portions
of the bones for the sake of nutriment afforded by the blood and marrow
they contain. The gradual modification between this and the two following
types will be noticed in their appropriate places.
In the most typical insectivorous animals, as the Hedgehogs and Shrews,
the central incisors are elongated, pointed, and project forwards, those
of the upper and lower jaw meeting like the blades of a pair of forceps,
so as readily to secure small active prey, quick to elude capture, but
powerless to resist when once seized. The crowns of the molars are
covered with numerous sharp edges and points, which, working against each
other, rapidly cut up the hard-cased insects into little pieces fit for
swallowing and digestion.
The omnivorous type, especially that adapted for the consumption of soft
vegetable substances, such as fruits of various kinds, may be exemplified
in the dentition of Man, of most Monkeys, and of the less modified Pigs.
The incisors are moderate, subequal, and cutting. If the canines are
enlarged, it is usually for other purposes than those connected with
food, and only in the male sex. The molars have their crowns broad,
flattened, and elevated into rounded tubercles. The name _Bunodont_, or
hillock-toothed, has been proposed for molars of this type, and will
frequently be found convenient.
In the most typically herbivorous forms of dentition, as seen in the
Horse and Kangaroo, the incisors are well developed, trenchant, and
adapted for cutting off the herbage on which the animals feed; the
canines are rudimentary or suppressed; the molars are large, with broad
crowns, which in the simplest forms have strong transverse ridges, but
may become variously complicated in the higher degrees of modification
which this type of tooth assumes.
Various forms of teeth of this type will be noticed among the Ungulates
and Rodents.
The natural groups of mammals, or those which in our present state of
knowledge we have reason to believe are truly related to each other, may
each contain examples of more than one of these modifications. Thus the
Primates have both omnivorous and insectivorous forms. The Carnivora show
piscivorous, carnivorous, insectivorous, and omnivorous modifications of
their common type of dentition. The Ungulata and the Rodentia have among
them the omnivorous and various modifications, both simple and complex,
of the herbivorous type. The Marsupialia exhibit examples of all forms,
except the purely piscivorous. Other orders, more restricted in number or
in habits, as the Proboscidea and Cetacea, naturally do not show so great
a variety in the dental structure of their members.
_Taxonomy._—In considering the taxonomic value to be assigned to the
modifications of teeth of mammals, two principles, often opposed to each
other, which have been at work in producing these modifications, must
be held in view:—(1) the type, or ancestral form, as we generally now
call it, characteristic of each group, which in most mammals is itself
derived from the still more generalised type described above; and (2)
variations which have taken place from this type, generally in accordance
with special functions which the teeth are called upon to fulfil in
particular cases. These variations are sometimes so great as completely
to mask the primitive type, and in this way the dentition of many animals
of widely different origin has come to present a remarkable superficial
resemblance, as in the case of the Wombat (a Marsupial), the Aye-Aye (a
Lemur), and the Rodents, or as in the case of the Thylacine and the Dog.
In all these examples indications may generally be found of the true
nature of the case by examining the earlier conditions of dentition;
for the characters of the milk-teeth or the presence of rudimentary
or deciduous members of the permanent set will generally indicate the
route by which the specialised dentition of the adult has been derived.
It is perhaps owing to the importance of the dental armature to the
well-being of the animal in procuring its sustenance, and preserving
its life from the attacks of enemies, that great changes appear to have
taken place so readily, and with such comparative rapidity, in the forms
of these organs—changes often accompanied with but little modification
in the general structure of the animal. Of this proposition the Aye-Aye
(_Chiromys_) among Lemurs, the Walrus among Seals, and the Narwhal
among Dolphins form striking examples; since in all these forms the
superficial characters of their dentition would entirely separate them
from the animals with which all other evidence (even including the mode
of development of their teeth) proves their close affinity.
[Illustration: FIG. 4.—Molar teeth of Mesozoic Mammals (enlarged).
Triconodont type—1, _Dromatherium_; 2, _Microconodon_; 3,
_Amphilestes_; 4, _Phascolotherium_; 5, _Triconodon_. Tritubercular
type—6, 7, _Spalacotherium_; 10, _Asthenodon_. Tubercular sectorial
type—8, _Amphitherium_; 9, _Peramus_; 11-13, _Amblotherium_; 14 (?)
_Amblotherium_. _pr_, Protocone; _hy_, hypocone; _pa_, paracone; _me_,
metacone, in the upper teeth; and protoconid, hypoconid, paraconid, and
metaconid in the lower. 6 and 15 are upper molars, and the rest lower
molars. (After Osborn.)]
_Trituberculism._—Recent researches, and more especially those of
Professors Cope and Osborn, tend to show that almost all of the extremely
different forms of tooth-structure found among Mammals may be traced to
one common type, in which the crown of each tooth carried three cusps,
and hence termed the _tritubercular_ type; these three cusps being
arranged in a triangle, with the apex directed inwardly in the upper
teeth (Fig. 4, ₆), and outwardly in the lower ones (Fig. 4, ₇). It is
further probable that this tritubercular type was itself derived from
a type of dentition in which the teeth were in the form of almost a
quite simple cone; such a presumably primitive type of dentition—being
apparently retained among some existing Edentates, like the Armadillos,
while it is possible that we should regard the dentition of the existing
Cetacea (Fig. 2) as a reversion to the same primitive type. None of
the Mesozoic mammals at present known exhibit this simple conical type
of teeth, although we have an approximation to it in the extremely
generalised genus _Dromatherium_. Starting then from this presumed simple
cone it appears that the teeth of _Dromatherium_ (Fig. 4, ₁) present the
first stage towards trituberculism, the crown of each tooth having one
main cone, with minute lateral cusps, and the root being grooved. In
the next or true Triconodont stage (Fig. 4, ₃₋₅) the crown has become
elongated antero-posteriorly, and consists of one central and two lateral
cones or cusps, while the root is divided. From this the transition is
easy to the tritubercular type, in which the three cusps, instead of
being placed in a line, are arranged in a triangle; the upper teeth
(Fig. 4, ₆) having one inner and two outer cusps, while the reverse
condition obtains in those of the lower jaw (Fig. 4, ₇). These three
cusps of the simple tritubercular tooth are collectively designated as
the primitive triangle; in the upper tooth the inner cusp is termed the
protocone, the antero-external one the paracone, and the postero-external
the metacone; the corresponding cusps of the lower tooth being named
protoconid, paraconid, and metaconid—the protoconid being here on the
outer side of the crown.
It is thus apparent that in the first, or haplodont type, as well as in
the triconodont type, the upper and lower molars are alike; while in
the simple tritubercular type they have a similar pattern, but with the
arrangement of the cusps reversed. This simple tritubercular type occurs
in the Mesozoic genus _Spalacotherium_ (Fig. 4, ₆ and ₇), and apparently
in the existing _Chrysochloris_; but in the majority of tritubercular
forms, while this primitive triangle forms the main portion of the
crown, other secondary cusps are added, the homologies of which in the
upper and lower teeth are somewhat doubtful. At the same time that we
have the addition of these secondary cusps we also find trituberculism
differentiating into a secodont and a bunodont series, according as to
whether the dentition becomes of a cutting or a crushing type.
Thus in the lower molars (Fig. 4, ₈ and ₉) we very frequently find the
three cusps of the primitive triangle elevated and connected by cross
crests, while there is an additional low posterior heel or talon, which
may be termed the hypoconid. This tubercular-sectorial sub-type, as it
is termed, is found in the lower molars of many Polyprotodont Marsupials
and Insectivores, and it also occurs in the lower carnassial teeth of the
true Carnivora. The presence of two cusps (inner and outer) to the talon
converts this modification into a quinquetubercular form; while, by the
suppression of one of the three primitive cusps, it develops into the
quadritubercular type of the bunodont series.
[Illustration: FIG. 5.—Diagram of two upper and two lower left
quadritubercular molars in mutual apposition. The cusps and ridges
of the upper molars in double lines, and those of the lower in black
lines. The lower molars are looked at from below, as if transparent.
_pr_, Protocone; _hy_, hypocone; _pa_, paracone; _me_, metacone; _ml_,
protoconule; _pl_, metaconule; _prd_, protoconid; _hyd_, hypoconid;
_pad_, paraconid; _med_, metaconid; _end_, entoconid. (After Osborn.)]
In the upper molars the primitive triangle in the secodont series may
remain purely tricuspid; but the addition of intermediate cusps, both in
the secodont and bunodont series, may give rise to a quinquetubercular
type; these intermediate cusps being respectively designated as the
protoconule and metaconule (Fig. 5, _ml_, _pl_). Finally, in the
bunodont series, the addition of a postero-internal cusp (Fig. 5, _hy_),
termed the hypocone, forms the sextubercular molar.
The following table exhibits, in a collective form, the names and
relations of all the above-mentioned cusps, and the letters by which they
are indicated in the figures:—
UPPER MOLARS.
Antero-internal cusp = protocone = _pr_.
Postero ” or 6th cusp = hypocone = _hy_.
Antero-external cusp = paracone = _pa_.
Postero ” ” = metacone = _me_.
Anterior intermediate cusp = protoconule = _ml_.
Posterior ” ” = metaconule = _pl_.
LOWER MOLARS.
Antero-external cusp = protoconid = _prd_.
Postero ” ” = hypoconid = _hyd_.
Antero-internal or 5th cusp = paraconid = _pad_.
Intermediate (or in quadritubercular
molars antero-internal) cusp = metaconid = _med_.
Postero-internal cusp = entaconid = _end_.
The common occurrence of trituberculism in the mammals of the earlier
geological epochs is, as remarked by Osborn, very significant of the
uniformity of molar origin. Thus, among the Mesozoic mammals (with the
exception of the group known as Multituberculata, in which the molars
are constructed on a different type), trituberculism occurs in the
great majority of the genera; while out of 82 species, belonging to
five different suborders from the Lowest or Puerco Eocene of the United
States, all but four exhibit this feature; and the same holds good for
the mammals of the corresponding European horizon. At the present day
trituberculism persists in the Lemuroidea, Insectivora, Carnivora, and
Marsupialia. In the Carnivora there is a tendency to lose the metaconid,
while in the bunodont molars of the Ungulata it is the paraconid that
disappears.
III. THE SKELETON.
_Definition._—The skeleton is a system of hard parts, forming a
framework which supports and protects the softer organs and tissues
of the body. It consists of dense fibrous and cartilaginous tissues,
portions of which remain through life in this state, but the greater
part is transformed during the growth of the animal into bone or osseous
tissue. This is characterised by a peculiar histological structure and
chemical composition, being formed mainly of a gelatinous basis, strongly
impregnated with salts of calcium, chiefly phosphate, and disposed in a
definite manner, containing numerous minute nucleated spaces or cavities
called lacunæ, connected together by delicate channels or canaliculi,
which radiate in all directions from the sides of the lacunæ. Parts
composed of bone are, next to the teeth, the most imperishable of all
the organs of the body, often retaining their exact form and internal
structure for ages after every trace of all other portions of the
organisation has completely disappeared, and thus, in the case of extinct
animals, affording the only means of attaining a knowledge of their
characters and affinities.[8]
In the Armadillos and their extinct allies alone is there an ossified
exoskeleton, or bony covering developed in the skin. In all other mammals
the skeleton is completely internal. It may be described as consisting
of an axial portion belonging to the head and trunk, and an appendicular
portion belonging to the limbs. There are also certain bones called
splanchnic, being developed within the substance of some of the viscera.
Such are the _os cordis_ and _os penis_ found in some mammals.
It is characteristic of all the larger bones of the mammalia that their
ossification takes its origin from several distinct centres. One near
the middle of the bone, and spreading throughout its greater portion,
constitutes the _diaphysis_, or “shaft,” in the case of the long
bones. Others near the extremities, or in projecting parts, form the
_epiphyses_, which remain distinct during growth, but ultimately coalesce
with the rest of the bone.
_Axial skeleton._—The axial skeleton consists of the skull, the vertebral
column (prolonged at the posterior extremity into the tail), the sternum,
and the ribs.
_Skull._—In the _skull_ of adult mammals, all the bones, except the
lower jaw, the auditory ossicles, and the bones of the hyoid arch, are
immovably articulated together, their edges being in close contact,
and often interlocking by means of fine denticulations projecting from
one bone and fitting into corresponding depressions of the other;
they are also held together by the investing periosteum, or fibrous
membrane, which passes directly from one to the other, and permits
no motion, beyond perhaps a slight yielding to external pressure.
In old animals there is a great tendency for the different bones to
become actually united by the extension of ossification from one to
the other, with consequent obliteration of the sutures. The cranium,
thus formed of numerous originally independent ossifications, which may
retain throughout life more or less of their individuality, or be all
fused together, according to the species, the age, or even individual
peculiarity, consists of a brain-case, or bony capsule for enclosing and
protecting the brain, and a face for the support of the organs of sight,
smell, and taste, and of those concerned in seizing and masticating the
food. The brain-case articulates directly with the anterior cervical
vertebra, by means of a pair of oval eminences, called condyles, placed
on each side of the large median foramen which transmits the spinal cord.
It consists of a basal axis, continuous serially with the axes or centra
of the vertebræ, and of an arch above, roofing over and enclosing the
cavity which contains the cephalic portion of the central nervous system
(see Fig. 6). The base with its arch is composed of three segments placed
one before the other, each of which is comparable to a vertebra with a
greatly expanded neural arch. The hinder or occipital segment consists
of the basioccipital, exoccipital, and supraoccipital bones; the middle
segment of the basisphenoid, alisphenoid, and parietal bones; and the
anterior segment of the presphenoid, orbitosphenoid, and frontal bones.
The axis is continued forwards into the mesethmoid, or septum of the
nose, around which the bones of the face are arranged in a manner so
extremely modified for their special purposes that anatomists who have
attempted to trace their serial homologies with the more simple portions
of the axial skeleton have arrived at very diverse interpretations.
The characteristic form and structure of the face of mammals is mainly
dependent upon the size and shape of (1) the orbits, a pair of cup-shaped
cavities for containing the eyeball and its muscles, which may be
directed forwards or laterally, placed near together or wide apart, and
may be completely or only partially encircled by bone; (2) the nasal
fossæ, or cavities on each side of the median nasal septum, forming the
passage for the air to pass between the external and the internal nares,
and containing in their upper part the organ of smell; (3) the zygomatic
arch, a bridge of bone for the purpose of muscular attachment, which
extends from the side of the face to the skull, overarching the temporal
fossa; (4) the roof of the mouth, with its alveolar margin for the
implantation of the upper teeth. The face is completed by the mandible,
or lower jaw, consisting of two lateral rami, articulated by a hinge
joint with the squamosal (a cranial bone interposed between the posterior
and penultimate segment of the brain-case, where also the bony capsule of
the organ of hearing is placed), each being composed of a single solid
piece of bone, and the two united together in the middle line in front,
at the symphysis,—which union may be permanently ligamentous or become
completely ossified. Into the upper border of the mandibular rami the
lower teeth are implanted.
[Illustration: FIG. 6.—Longitudinal and vertical section of the skull of
a Dog (_Canis familiaris_), with mandible and hyoid arch. _an_, Anterior
narial aperture; _MT_, maxillo-turbinal bone; _ET_, ethmo-turbinal;
_Na_, nasal; _ME_, ossified portion of the mesethmoid; _CE_, cribriform
plate of the ethmo-turbinal; _Fr_, frontal; _Pa_, parietal; _IP_,
interparietal; _SO_, supraoccipital; _ExO_, exoccipital; _BO_,
basioccipital; _Per_, periotic; _BS_, basisphenoid; _Pt_, pterygoid;
_AS_, alisphenoid; _OS_, orbitosphenoid; _PS_, presphenoid; _PI_,
palatine; _VO_, vomer; _Mx_, maxilla; _PMx_, premaxilla; _sh_, stylohyal;
_eh_, epihyal; _ch_, ceratohyal; _bh_, basihyal; _th_, thyrohyal; _s_,
symphysis of mandible; _cp_, coronoid process; _cd_, condyle; _a_, angle;
_id_, inferior dental canal. The mandible is displaced downwards, to show
its entire form; the * indicates the part of the cranium to which the
condyle is articulated.[9]]
In addition to the bones already mentioned as entering into the formation
of the cranium, there are many others, the most important of which may
be briefly noticed. The anterior extremity of the skull is formed by the
premaxillæ (Figs. 6, 7, _PMx_), which carry the incisors; behind them
are the maxillæ, in which all the remaining upper teeth are implanted.
Both the premaxillæ and maxillæ meet in a median suture on the palate,
where they form a floor to the nasal passage; this floor being continued
backwards by the plate-like palatines, at the hinder extremity of which
the posterior nares are usually situated. In a few instances, however,
as in certain Edentates and Cetaceans, the small pair of bones forming
the posterior continuation of the lateral borders of the palatines, and
known as the pterygoids (Fig. 6, _Pt_), likewise meet in the middle line
below the nasal passage, and thus cause the aperture of the posterior
nares to be situated near the occiput. On the upper, or frontal aspect of
the cranium the paired nasals roof over the nasal passage and fill the
interval left between the premaxilla and maxilla of either side. Behind
the nasals and maxillæ, the anterior part of the brain-case is formed
by the large paired frontals (Figs. 6, 7, _Fr_), behind which are the
parietals, which may be of still larger size, and form the greater part
of the brain-case. A median interparietal ossification (Fig. 6, _IP_)
may divide the parietals posteriorly, and is itself articulated with the
supraoccipital, to the lateral borders of which the parietals are also
joined. The squamosal (Fig. 7, _Sq_) forms the lateral wall of the hinder
part of the brain-case, and articulates superiorly with the parietal,
and posteriorly with the exoccipital. The glenoid cavity (Fig. 8), for
the reception of the articular condyle of the mandible, is formed by the
inferior portion of the squamosal, at the point where it gives off the
zygomatic process to form the hinder portion of the zygomatic arch. The
middle portion of that arch is formed by the jugal, or malar bone (Fig.
7, _Ma_), which articulates posteriorly with the zygomatic process of
the squamosal, and anteriorly with the maxilla. The jugal (as in Fig. 7)
may also articulate with a small bone situated on the anterior border
of the orbit known as the lachrymal. It is important to observe that
the zygomatic or temporal arch is a squamoso-maxillary one, and that an
arcade thus composed is found elsewhere only among the extinct Anomodont
reptiles, which have already been mentioned as showing signs of mammalian
affinity. The relative position occupied by the orbito- and alisphenoid
is sufficiently indicated in Fig. 7.
[Illustration: FIG. 7.—Side view of skull of Cape Jumping Hare (_Pedetes
caffer_). × ⅗ _PMx_, Premaxilla; _Mx_, maxilla, _Ma_, jugal or malar;
_Fr_, frontal; _L_, lachrymal; _Pa_, parietal; _Na_, nasal; _Sq_,
squamosal; _Ty_, tympanic; _ExO_, exoccipital; _AS_, alisphenoid; _OS_,
orbitosphenoid; _Per_, mastoid bulla.]
Wedged in between the squamosal and the bones of the occipital and
basisphenoidal region are the bones connected with the organ of hearing,
known as the periotic and tympanic. The position of the periotic, which
encloses the labyrinth or essential organ of hearing, is shown in Fig. 6.
The periotic is divided into a very dense antero-internal moiety known as
the petrosal, and a postero-external or mastoid portion (Fig. 8), which
appears on the outer wall of the brain-case. The tympanic is produced
horizontally outwards to form the external auditory meatus or tube of
the ear, while the inner and under surface is frequently dilated into a
shell-like auditory bulla (Fig. 8). The small bones of the internal ear
known as the malleus, incus, and stapes are contained in the membranous
_tympanic cavity_, which is situated in a space left among this group of
bones. Further mention of these bones is made below under the head of the
sense organs.
In the Carnivora and some other groups the foramina on the base of the
skull for the passage of blood-vessels and nerves are of considerable
taxonomic importance. The position of the more important of these
foramina is indicated in Fig. 8; but for details the reader may refer to
the work on the _Osteology of the Mammalia_ already mentioned. Attention
may, however, be particularly directed to the so-called alisphenoid
canal, the position of which is shown in Fig. 8, since this is a feature
of some importance in the classification of the Carnivora. This canal
is a short channel running horizontally forward from near the foramen
ovale through the alisphenoid, and opening anteriorly with the foramen
rotundum; it is traversed by the external carotid artery.
[Illustration: FIG. 8.—The right half of the hinder part of the base
of the cranium of the Wolf (_Canis lupus_). _c_, Condyloid foramen;
_l_, foramen lacerum posticum; _car_, carotid canal; _e_, eustachian
canal; _o_, foramen ovale; _a_, posterior, and _a′_, anterior aperture
of alisphenoid canal; _P_, paroccipital process of exoccipital; _m_,
mastoid process of periotic; _am_, external auditory meatus; _g_, glenoid
foramen, below which is the glenoid cavity for the condyle of the
mandible. (Flower, _Proc. Zool. Soc._, 1869, p. 25.)]
Only in those species, as Man and the smaller kinds of the Primates and
some other orders, in which the brain holds a large relative proportion
to the rest of the body, does the external form of the skull receive
much impress from the real shape of the cavity containing the brain.
The size and form of the mouth, and the modifications of the jaws for
the support of teeth of various shape and number, the ridges and crests
on the cranium for the attachment of the muscles necessary to put this
apparatus in motion, and outgrowths of bone for the enlargement of the
external surface required for the support of sense organs or of weapons,
such as horns or antlers (which outgrowths, to prevent undue increase
of weight, are filled with cells containing air), cause the principal
variations in the general configuration of the skull. These variations
are, however, only characteristically developed in perfectly adult
animals, and are in many cases more strongly marked in the male than the
female sex. Throughout all the later stages of growth up to maturity
the size and form of the brain-case remain comparatively stationary,
while the accessory parts of the skull rapidly increase and assume their
distinctive development characteristic of the species.
The hyoidean apparatus in mammals (Fig. 6) supports the tongue and
larynx, and consists of an inferior median portion termed the basihyal,
from which two pairs of half arches, or cornua, extend upwards and
outwards. The anterior is the more important, being connected with the
periotic bone of the cranium. It may be almost entirely ligamentous, but
more often has several ossifications, the largest of which is usually the
stylohyal. The posterior cornu (thyrohyal) is united at its extremity
with the thyroid cartilage of the larynx, which it suspends in position.
The median portion, or basihyal, is sometimes, as in the Howling Monkeys,
enormously enlarged and hollowed, admitting into its cavity an air-sac
connected with the organ of voice.
[Illustration: FIG. 9.—Anterior surface of Human thoracic vertebra
(fourth). _c_, Body or centrum; _nc_, neural canal; _p_, pedicle,
and _l_, lamina of the arch; _t_, transverse process; _az_, anterior
zygapophysis.]
_Vertebral Column._—The _vertebral column_ consists of a series of
distinct bones called vertebræ, arranged in close connection with each
other along the dorsal side of the neck and trunk, and in the median
line.[10] It is generally prolonged posteriorly beyond the trunk, to
form the axial support of the appendage called the tail. Anteriorly it
is articulated with the occipital region of the skull. The number of
distinct bones composing the vertebral column varies greatly among the
Mammalia, the main variation being due to the degree of elongation of
the tail. Apart from this, in most mammals the number is not far from
thirty, though it may fall as low as twenty-six (as in some Bats), or
rise as high as forty (_Hyrax_ and _Cholœpus_). The different vertebræ,
with some exceptions, remain through life quite distinct from each other,
though closely connected by means of fibrous structures which allow of
a certain, but limited, amount of motion between them. The exceptions
are the following:—(1) near the posterior part of the trunk, in nearly
all mammals which possess completely developed hinder limbs, two or more
vertebræ become ankylosed together to form the “sacrum,” or portion of
the vertebral column to which the pelvic girdle is attached; (2) in some
species of Whales and Armadillos there are constant ossific unions of
certain vertebræ of the cervical region.
[Illustration: FIG. 10.—Side view of the first lumbar vertebra of a Dog
(_Canis familiaris_). _s_, Spinous process; _az_, anterior zygapophysis;
_pz_, posterior zygapophysis; _m_, metapophysis; _a_, anapophysis; _t_,
transverse process.]
Although the vertebræ of different regions of the column of the same
animal or of different animals present great diversities of form, yet
there is a certain general resemblance among them, or a common plan on
which they are constructed, which is more or less modified by alteration
of form or proportions, or by the addition or suppression of parts to
fit them to fulfil their special purpose in the economy. An ordinary or
typical vertebra consists, in the first place, of a solid piece of bone,
termed the body or centrum (Fig. 9, _c_), of the form of a disk or short
cylinder. The bodies of contiguous vertebræ are connected together by
a very dense, tough, and elastic material called the “intervertebral
substance,” of peculiar and complex arrangement. This substance forms
the main, and in some cases the only, union between the vertebræ. Its
elasticity provides for the vertebræ always returning to their normal
relation to each other and to the column generally, when they have been
disturbed therefrom by muscular action. A process (_p_) arises on each
side from the dorsal surface of the body. These processes, meeting
in the middle line above, form an arch, surmounting a space or short
canal (_nc_). Since it contains the posterior prolongation of the great
cerebro-spinal nervous axis, or spinal cord, this space is called the
neural canal, and the arch the neural arch, in contradistinction to
another arch on the ventral surface of the body of the vertebræ, called
the hæmal arch. The latter is, however, never formed in mammals by
any part of the vertebra itself, but by certain distinct bones placed
more or less in apposition to it, namely the ribs in the thoracic, and
the “chevron bones” in the caudal region. In most cases the arch of
one vertebra is articulated with that of the next by distinct surfaces
with synovial joints, placed one on each side, called “zygapophyses”
(_az_, _pz_), but these are often entirely wanting when flexibility is
more needed than strength, as in the greater part of the caudal region
of long-tailed animals. In addition to the body and the arch, there
are certain projecting parts called processes, chiefly serving for the
attachment of the numerous muscles which move the vertebral column. Of
these two are single and median, viz. the spinous process, neural spine,
or neurapophysis (_s_), arising from the middle of the upper part of
the arch, and the hypapophysis from the under surface of the body. The
latter, however, is as frequently absent as the former is constant. The
other processes are paired and lateral. They are the transverse processes
(_t_), of which there may be two, an upper and a lower, in which case the
former is called, in the language of Owen (to whom we are indebted for
the terminology of the parts of vertebræ in common use), “diapophysis,”
and the latter “parapophysis.” Other processes less constantly present
are called respectively “metapophyses” (_m_) and “anapophyses” (_a_).
The vertebral column is divided for convenience of description into five
regions—the cervical, thoracic or dorsal, lumbar, sacral, and caudal.
This division is useful, especially as it is not entirely arbitrary,
and in most cases is capable of ready definition; but at the contiguous
extremities of the regions the characters of the vertebræ of one are
apt to blend into those of the next region, either normally or as
peculiarities of individual skeletons.
[Illustration: FIG. 11.—Anterior surface of sixth cervical vertebra
of Dog. _s_, Spinous process; _az_, anterior zygapophysis; _v_,
vertebrarterial canal; _t_, transverse process; _t′_, its inferior
lamella.]
_Cervical Vertebræ._—The _cervical_ region constitutes the most anterior
portion of the column, or that which joins the cranium. The vertebræ
which belong to it are either entirely destitute of movable ribs, or
if they have any these are small, and do not join the sternum. As a
general rule they have a considerable perforation through the base
of the transverse process (the vertebrarterial canal, Fig. 11, _v_);
or, as it is sometimes described, they have two transverse processes,
superior and inferior, which meet at their extremities to enclose a
canal. This, however, rarely applies to the last vertebra of the region,
in which only the upper transverse process is usually developed. The
transverse process, moreover, very often sends down near its extremity
a more or less compressed plate (inferior lamella), which, being
considered serially homologous with the ribs of the thoracic vertebræ
(though not developed autogenously), is often called the “costal” or
“pleurapophysial” plate. This is usually largest on the sixth, and
altogether wanting on the seventh vertebra. The first and second cervical
vertebræ, called respectively “atlas” and “axis,” are specially modified
for the function of supporting and permitting the free movements of the
head. They are not united together by the intervertebral substance, but
connected only by ordinary ligaments and synovial joints.
The cervical region in mammals presents the remarkable peculiarity that,
whatever the length or flexibility of the neck, the number of vertebræ
is the same, viz. seven, with the exception of the Manatee and Hoffman’s
Two-toed Sloth (_Cholœpus hoffmanni_), which both have but six, and
the Three-toed Sloth (_Bradypus tridactylus_), which has nine, though
in this case the last two usually support movable ribs, which are not
sufficiently developed to reach the sternum.
According to Parker there may occasionally be eight cervicals in the
Pangolins (_Manis_).
_Dorsal Vertebræ._—The _dorsal_ (or, as it would be more correctly
termed, _thoracic_) region consists of the vertebræ succeeding those of
the neck, which have ribs movably articulated to them. These ribs arch
round the thorax—the anterior one, and usually the greater number of
those that follow, being attached below to the sternum.
_Lumbar Vertebræ._—The _lumbar_ region consists of those vertebræ of
the trunk in front of the sacrum which bear no movable ribs. It may
happen that, as the ribs decrease in size posteriorly (the last being
sometimes more or less rudimentary), the step from the thoracic to
the lumbar region may be gradual and rather undetermined in a given
species; but most commonly this is not the case, and the distinction is
as well defined here as in any other region. As a general rule there
is a certain relation between the number of the thoracic and lumbar
vertebræ, the whole number being tolerably constant in a given group
of animals, and any increase of the one being at the expense of the
other. Thus in all known Artiodactyle Ungulata there are 19 dorso-lumbar
vertebræ; but these may consist of 12 dorsal and 7 lumbar vertebræ, or
13 dorsal and 6 lumbar, or 14 dorsal and 5 lumbar. The smallest number
of dorso-lumbar vertebræ in mammals occurs in some Armadillos, which
have but 14. The number found in Man, the higher Apes, and most Bats,
viz. 17, is exceptionally low; 19 prevails in the Artiodactyla, nearly
all Marsupials, and very many Rodents; 20 or 21 in Carnivora and most
Insectivora; and 23 in Perissodactyla. The highest and quite exceptional
numbers are in the Two-toed Sloth (_Cholœpus_) 27, and the Hyrax 30. The
prevailing number of rib-bearing vertebræ is 12 or 13, any variation
being generally in excess of these numbers.
_Sacral Vertebræ._—The _sacral_ region offers more difficulties of
definition. Taking the human “os sacrum” as a guide for comparison,
it is generally defined as consisting of those vertebræ between the
lumbar and caudal regions which are ankylosed together to form a single
bone. It happens, however, that the number of such vertebræ varies in
different individuals of the same or nearly allied species, especially
as age advances, when a certain number of the tail vertebræ generally
become incorporated with the true sacrum. Other suggested tests—as those
vertebræ which have a distinct additional (pleurapophysial) centre of
ossification between the body and the ilium, those to which the ilium
is directly articulated, or those in front of the insertion of the
ischiosacral ligaments—being equally unsatisfactory or unpractical, the
old one of ankylosis, as it is found to prevail in the average condition
of adults in each species, is used in the enumeration of the vertebræ in
the following pages. The Cetacea, having no iliac bones, have no part of
the vertebral column modified into a sacrum.
[Illustration: FIG. 12.—Anterior surface of fourth caudal vertebræ of
Porpoise (_Phocæna communis_). _s_, Spinous process; _m_, metapophysis;
_t_, transverse process; _h_, chevron bone.]
_Caudal Vertebræ._—The _caudal_ vertebræ are those placed behind the
sacrum, and terminating the vertebral column. They vary in number
greatly—being reduced to 5, 4, or even 3, in a most rudimentary
condition, in Man and in some Apes and Bats, and being numerous and
powerfully developed, with strong and complex processes, in many mammals,
especially among the Edentata, Cetacea, and Marsupialia. The highest
known number, 46, is possessed by the African Long-tailed Pangolin.
Connected with the under surface of the caudal vertebræ of many mammals
which have the tail well developed are certain bones formed more or less
like an inverted arch, called chevron bones, or by the French _os en V_.
These are always situated nearly opposite to an intervertebral space, and
are generally articulated both to the vertebra in front and the vertebra
behind, but sometimes chiefly or entirely either to one or the other.
In some of the Anomodont Reptiles and Labyrinthodont Amphibians these
chevrons are attached to the intercentra—or imperfect disks alternating
with the true centra—which suggests that they are primarily intercentral
elements which have been transferred to the edges of the centra by the
disappearance of the intercentra.
_Sternum._—The _sternum_ of mammals is a bone, or generally a series
of bones, placed longitudinally in the mesial line, on the inferior
or ventral aspect of the thorax, and connected on each side with the
vertebral column by a series of more or less ossified bars called “ribs.”
It is present in all mammals, but varies much in character in the
different groups. It usually consists of a series of distinct segments
placed one before the other, the anterior being called the presternum or
“manubrium sterni” of human anatomy, and the posterior the xiphisternum,
or xiphoid or ensiform process, while the intermediate segments, whatever
their number, constitute the mesosternum or “body.” In the Whalebone
Whales the presternum alone is developed, and but a single pair of ribs
is attached to it.
[Illustration: FIG. 13.—Human sternum and sternal ribs. _ps_, Presternum;
_ms_, mesosternum; _xs_, xiphisternum; _c_, point of attachment of
clavicle; 1 to 10, the cartilaginous sternal ribs.]
_Ribs._—The _ribs_ form a series of long, narrow, and more or less
flattened bones, extending laterally from the sides of the vertebral
column, curving downwards towards the median line of the body below, and
mostly joining the sides of the sternum. The posterior ribs, however,
do not directly articulate with that bone, but are either attached by
their extremities to the edges of each rib in front of them, and thus
only indirectly join the sternum, or else they are quite free below,
meeting no part of the skeleton. These differences have given rise to the
division into “true” and “false” ribs (by no means good expressions),
signifying those that join the sternum directly and those that do not;
and of the latter, those that are free below, are called “floating” ribs.
The portion of each rib nearest the vertebral column and that nearest
the sternum differ in their characters, the latter being usually but
imperfectly ossified, or remaining permanently cartilaginous. These are
called “costal cartilages,” or when ossified “sternal ribs.”
[Illustration: FIG. 14.—Sternum and strongly ossified sternal ribs of
Great Armadillo (_Priodon gigas_). _ps_, Presternum; _xs_, xiphisternum.]
In the anterior part of the thorax the vertebral extremity of each rib is
divided into two parts, “head” or “capitulum,” and “tubercle”; the former
is attached to the side of the body of the vertebra, the latter to its
transverse process; the former attachment corresponds to the interspace
between the vertebræ, the head of the rib commonly articulating partly
with the hinder edge of the body of the vertebra antecedent to that which
bears its tubercle. Hence the body of the last cervical vertebra usually
supports part of the head of the first rib. In the posterior part of the
series the capitular and tubercular attachments commonly coalesce, and
the rib is attached solely to its corresponding vertebra. The number of
pairs of ribs is of course the same as that of the thoracic vertebræ.
The circumstance that in some of the Anomodont reptiles and
Labyrinthodonts the capitula of the ribs articulate with the intercentral
elements of the vertebral column has suggested, as in the instance of
the chevron bones, that the intercentral capitular articulation of the
ribs of mammals is a feature directly inherited from those extinct types
by the gradual disappearance of the intercentra.
_Appendicular Skeleton._—The appendicular portion of the framework
consists, when completely developed, of two pairs of limbs, anterior and
posterior (Fig. 15).
[Illustration: FIG. 15.—Skeleton of Lion (_Felis leo_). _cd_, Caudal
vertebræ; _cp_, carpus; _cr_, coracoid process of scapula, _cv_, cervical
vertebræ; _d_, dorsal vertebræ; _fb_, fibula; _fm_, femur; _h_, humerus;
_il_, ilium; _isch_, ischium; _l_, lumbar vertebræ; _m_, metatarsus;
_mc_, metacarpus; _p_, patella; _pb_, pubis; _ph_, phalanges; _pv_,
pelvis; _r_, radius; _s_, sacral vertebræ; _sc_, scapula; _sk_, skull;
_tb_, tibia; _ts_, tarsus; _u_, ulna; _zy_, zygomatic arch.]
_Anterior Limb._—The anterior limb is present and fully developed in all
mammals, being composed of a shoulder girdle and three segments belonging
to the limb proper; viz. the upper arm or brachium, the forearm or
antebrachium, and the hand or manus.
_Shoulder-girdle._—The _shoulder_ or _pectoral girdle_ in the large
majority of mammals is in a rudimentary or rather modified condition,
compared with that in which it exists in other vertebrates. In the
Monotremata (_Ornithorhynchus_ and _Echidna_) alone is the ventral
portion, or coracoid, complete and articulated with the sternum below,
as in the Sauropsida; and in this group alone do we find an anterior
ventral element, apparently corresponding with the pre-coracoid of the
Anomodont reptiles, although generally known as the epi-coracoid. In
all other mammals the coracoid, though ossified from a distinct centre,
forms only a process, sometimes a scarcely distinct tubercle, projecting
from the anterior border of the glenoid cavity of the scapula. The
last-named cavity, which in the Monotremes is formed jointly by the
scapula and coracoid, receives the head of the humerus, or arm-bone. The
scapula is always well developed, and generally broad and flat (whence
its vernacular name “blade bone”), with a ridge called the “spine” on
its outer surface, which usually ends in a free curved process, the
“acromion.” As the scapula affords attachment to many of the muscles
which act upon the anterior limb, its form and the development of its
processes are greatly modified according to the uses to which the
member is put. Thus it is most reduced and simple in character in those
animals whose limbs are mere organs of support, as the Ungulates; and
most complex when the limbs are also used for grasping, climbing, or
digging. The development or absence of the clavicle or “collar-bone,”
an accessory bar which connects the sternum with the scapula and
steadies the shoulder-joint, has a somewhat similar relation, though
its complete absence in the Bears shows that this is not an invariable
rule. A complete clavicle is found in Man and all the Primates, in
Chiroptera, all Insectivora (except _Potamogale_), in many Rodents, in
most Edentates, and in all Marsupials, except _Perameles_. More or less
rudimentary clavicles (generally suspended freely in the muscles) are
found in the Cat, Dog, and most Carnivora, _Myrmecophaga_, and some
Rodents. Clavicles are altogether absent in most of the _Ursidæ_, all the
Pinnipedia, _Manis_ among Edentates, the Cetacea, Sirenia, Ungulates, and
some Rodents.
The Monotremes are peculiar in possessing a T-shaped interclavicle
like that of many reptiles, lying upon the sternum, and articulating
superiorly with the clavicles.
_Brachium and Antebrachium._—The proximal segment of the anterior or
pectoral limb proper contains a single bone, the humerus, and the second
segment two bones, the radius and the ulna, placed side by side, and
articulating with the humerus at their proximal, and with the carpus
at their distal extremity (Fig. 15). In their primitive and unmodified
condition these bones may be considered as placed one on each border of
the limb, the radius being preaxial or anterior, and the ulna postaxial
or posterior, when the distal or free end of the limb is directed
outwards, or away from the trunk. This is their position in the earliest
embryonic condition, and is best illustrated among adult mammals in the
Cetacea, where the two bones are fixed side by side and parallel to each
other. In the greater number of mammals the bones assume a very modified
and adaptive position, usually crossing each other in the forearm, the
radius in front of the ulna, so that the preaxial bone (radius), though
external (in the ordinary position of the limb) at the upper end, is
internal at the lower end; and the hand, being mainly fixed to the
radius, also has its preaxial border internal. In the large majority of
mammals the bones are fixed in this position, but in some few, as in Man,
a free movement of crossing and uncrossing—or pronation and supination,
as it is termed—is allowed between them, so that they can be placed in
their primitive parallel condition, when the hand (which moves with the
radius) is said to be supine, or they may be crossed, when the hand is
said to be prone.
The humerus frequently has a foramen piercing the inner border of the
distal extremity, known as the entepicondylar foramen, which corresponds
with a similar one found in the Anomodont reptiles. The hollow in the
head of the ulna for the reception of the head of the humerus is known as
the greater sigmoid cavity, and that for the head of the radius as the
lesser sigmoid cavity (Fig. 16). The term olecranon is applied to that
process of the ulna which forms the prominence of the elbow.
[Illustration: FIG. 16.—Outer aspect of the proximal extremity of the
right ulna of a Bear (_Ursus_). _a_, Anterior tubercle; _ol_, olecranon;
_b_, greater sigmoid cavity; _c_, lesser do.]
In most mammals walking on four limbs, in which the hand is permanently
prone, the ulna is much reduced in size, and the radius increased,
especially at the upper end; so that the articular surface of the latter,
instead of being confined to the external side of the trochlea of the
humerus, extends all across its anterior surface, and the two bones,
instead of being external and internal, are anterior and posterior. In
many hoofed or Ungulate mammals, and in Bats, the ulna is reduced to
little more than its upper articular extremity, and firmly ankylosed to
the radius—stability of these parts being more essential than mobility.
_Manus._—The terminal segment of the anterior limb is the hand or
manus. Its skeleton consists of three divisions: (1) the “carpus,” a
group of small, more or less rounded or angular bones with flattened
surfaces applied to one another, and, though articulating by synovial
joints, having scarcely any motion between them; (2) the “metacarpus,”
a series of elongated bones placed side by side, with their proximal
ends articulating by almost immovable joints with the carpus; (3) the
“phalanges” or bones of the digits, usually three in number to each,
articulating to one another by freely movable hinge-joints, the first
being connected in like manner to the distal end of the metacarpal bone
to which it corresponds.
[Illustration: FIG. 17.—Dorsal surface of the right manus of a Water
Tortoise (_Chelydra serpentina_). After Gegenbaur. U, Ulna; R, radius;
_u_, ulnare; _i_, intermedium; _r_, radiale; _c_, centrale; 1-5, the five
bones of the distal row of the carpus; _m¹_-_m⁵_, the five metacarpals.]
_Carpus._—To understand thoroughly the arrangement of the bones of the
carpus in mammals, it is necessary to study their condition in some of
the lower vertebrates. Fig. 17 represents the manus in one of its fullest
and at the same time most generalised forms, as seen in one of the Water
Tortoises (_Chelydra serpentina_). The carpus consists of two principal
rows of bones. The upper or proximal row contains three bones, to which
Gegenbaur has applied the terms _radiale_ (_r_), _intermedium_ (_i_), and
_ulnare_ (_u_), the first being on the radial or preaxial side of the
limb.[11] The lower or distal row contains five bones, called _carpale_
1, 2, 3, 4, and 5 respectively, commencing on the radial side. Between
these two rows, in the middle of the carpus, is a single bone, the
_centrale_ (_c_). In this very symmetrical carpus it will be observed
that the _radiale_ supports on its distal side two bones, _carpale_ 1
and 2; the _intermedium_ is in a line with the _centrale_ and _carpale_
3, which together form a median axis of the hand, while the _ulnare_ has
also two bones articulating with its distal end, viz. _carpale_ 4 and 5.
Each of the carpals of the distal row supports a metacarpal.
In the carpus of the Mammalia there are usually two additional bones
developed in the tendons of the flexor muscles, one on each side of the
carpus, which may be called the radial and ulnar sesamoid bones; the
latter, which is the more constant and generally larger, is commonly
known as the pisiform bone. The fourth and fifth carpals of the distal
row are always united into a single bone, and the centrale is very often
absent. As a general rule all the other bones are present and distinct,
though it not unfrequently happens that two may have coalesced to form a
single bone, or one or more may be altogether suppressed.
The following table shows the principal names in use for the various
carpal bones,—those in the second column being the terms generally
employed by English anatomists:—
_Radiale_ = Scaphoid = _Naviculare_.
_Intermedium_ = Lunar = _Semilunare_, _Lunatum_.
_Ulnare_ = Cuneiform = _Triquetrum_, _Pyramidale_.
_Centrale_ = Central = _Intermedium_ (Cuvier).
_Carpale_ 1 = Trapezium = _Multangulum majus_.
_Carpale_ 2 = Trapezoid = _Multangulum minus_.
_Carpale_ 3 = Magnum = _Capitatum_.
_Carpale_ 4 } = Uneiform = _Hamatum_, _Uncinatum_.
_Carpale_ 5 }
The radial and ulnar sesamoids are regarded by Bardeleben[12] as the
rudiments of a prepollex and a postminimus digit; the primitive number
of digits being thus supposed to have been seven. These bones have
been observed in all orders of mammals having five complete digits.
Occasionally, as in _Pedetes caffer_, the so-called prepollex consists
of two bones, of which the distal one bears a distinct nail-like horny
covering. In _Bathyergus maritimus_ the pisiform, or postminimus, is
likewise double; the two elements being regarded by their describer as
representing the carpal and metacarpal of the presumed seventh digit.
Similarly in the posterior limb the tibial sesamoid, and a fibular
ossification corresponding to the pisiform, are regarded as representing
a prehallux and a postminimus.
_Metacarpus and Phalanges._—The metacarpal bones, with the digits which
they support, are never more than five in number, and are described
numerically—first, second, etc., counting from the radial towards the
ulnar side. The digits are also sometimes named (1) the pollex, (2)
index, (3) medius, (4) annularis, (5) minimus. One or more may be in
a rudimentary condition, or altogether suppressed. If one is absent,
it is most commonly the first. Excepting the Cetacea, no mammals have
more than three phalanges to each digit, but they may occasionally have
fewer by suppression or ankylosis. The first or radial digit is an
exception to the usual rule, one of its parts being constantly absent,
since, while each of the other digits has commonly a metacarpal and
three phalanges, it has only three bones altogether; whether the missing
one is a metacarpal or one of the phalanges is a subject which has
occasioned much discussion, and has not yet been satisfactorily decided.
The terminal phalanges of the digits are usually specially modified to
support the nail, claw, or hoof, and are called “ungual phalanges.” In
walking, some mammals (as the Bears) apply the whole of the lower surface
of the carpus, metacarpus, and phalanges to the ground; to these the
term “plantigrade” is applied. Many others (as nearly all the existing
Ungulata) only rest on the last one or two phalanges of the toes, the
first phalanx and the metacarpals being vertical and in a line with the
forearm. These are called “digitigrade.” Intermediate conditions exist
between these two forms, to which the terms “phalangigrade” (as the
Camel) and “subplantigrade” (as in most Carnivora), are applied. When the
weight is borne entirely on the distal surface of the ungual phalanx,
and the horny structures growing around it, as in the Horse, the mode of
progression is called “unguligrade.”
In the Chiroptera the digits are enormously elongated, and support a
cutaneous expansion constituting the organ of flight. In the Cetacea the
manus is formed into a paddle, being covered by continuous integument,
which conceals all trace of division into separate digits, and shows no
sign of nails or claws. In the Sloths the manus is long and very narrow,
habitually curved, and terminating in two or three pointed curved claws
in close apposition with each other, and incapable, in fact, of being
divaricated; so that it is reduced to the condition of a hook, by which
the animal suspends itself to the boughs of the trees among which it
lives. These are only examples of the endless modifications to which the
distal extremity of the limb is subjected in adaptation to the various
purposes to which it is applied.
_Posterior Limb._—The posterior limb is constructed upon a plan very
similar to that of the anterior extremity. It consists of a pelvic girdle
and three segments belonging to the limb proper, viz. the thigh, the leg,
and the foot or pes (Fig. 15).
_Pelvic Girdle._—The pelvic girdle is present in some form in all
mammals, though in the Cetacea and the Sirenia it is in an exceedingly
rudimentary condition. In all mammals except those belonging to the
two orders just named, each lateral half of the pelvic girdle consists
essentially, like the corresponding part of the anterior limb, of a
flattened rod of bone crossing the long axis of the trunk, having an
upper or dorsal and a lower or ventral end. The upper end diverges from
that of the opposite side, but the lower end approaches, and, in most
cases, meets it, forming a symphysis, without the intervention of any
bone corresponding to the sternum. The pelvic girdle differs from the
shoulder girdle in being firmly articulated to the vertebral column, thus
giving greater power to the hinder limb in its function of supporting
and propelling the body. Like the shoulder girdle, it bears on its outer
side, near the middle, a cup-shaped articular cavity (“acetabulum”),
into which the proximal end of the first bone of the limb proper is
received. Each lateral half of the girdle is called the “os innominatum,”
or innominate bone, and consists originally of three bones which unite
at the acetabulum. The “ilium” or upper bone is that which articulates
with the sacral vertebræ. Of the two lower bones the anterior or “pubis”
unites with its fellow of the other side at the symphysis; the posterior
is the “ischium.” These lower elements form two bars of bone, united
above and below, but leaving a space between them in the middle, filled
only by membrane, and called the “thyroid” or “obturator” foramen. The
whole circle of bone formed by the two innominate bones and the sacrum
is called the pelvis. In the Monotremata and Marsupialia, a pair of
thin, flat, elongated ossifications called epipubic or marsupial bones
are attached to the fore part of the pubis, and project forward into the
muscular wall of the abdomen.