The caudal vertebrae complete the series. They begin in as fully developed a condition as the lumbars, with well-marked transverse processes, etc.; but they end as no more than centra, from which sometimes tiny outgrowths represent in a rudimentary way the neural arches, etc. Very often the caudal vertebrae are furnished with ventral, generally [12]-shaped, appendages, the chevron bones or intercentra.[13] These are {25} particularly conspicuous in the Whales and in the Edentates. In the former group the occurrence of the first intercentrum serves to mark the separation of the caudal from the lumbar series. The number of caudals varies from three in Man--and those quite rudimentary--to nearly fifty in _Manis macrura_ and _Microgale longicaudata_.

[Ill.u.s.tration]

FIG. 14.--Lateral view of skull of a Dog. _C.occ_, Occipital condyle; _F_, frontal; _F.inf_, infra-orbital foramen; _Jg_, jugal; _Jm_, premaxilla; _L_, lachrymal; _M_, maxilla; _Maud_, external auditory meatus; _Md_, mandible; _N_, nasal; _P_, parietal; _Pal_, palatine; _Pjt_, process of squamosal; _Pt_, pterygoid; _Sph_, alisphenoid; _Sq_, squamosal; _Sq.occ_, supraoccipital; _T_, tympanic. (From Wiedersheim's _Comparative Anatomy_.)

THE SKULL.--The skull in the Mammalia differs from that of the lower Vertebrata in a number of important features, which will be enumerated in the following brief sketch of its structure. In the first place, the skull is a more consolidated whole than in reptiles; the number of elements entering into its formation is less, and they are on the whole more firmly welded together than in Vertebrates standing below the Mammalia in the series. Thus in the cranial region the post- and pre-frontals, the post-orbitals and the supra-orbitals have disappeared, though now and again we are reminded of their occurrence in the ancestors of the Mammalia by a separate ossification corresponding to some of the bones. Nowhere is this consolidation seen with greater clearness than in the lower jaw. That bone, or rather each half of it, is in mammals formed of one bone, the dentary (to which occasionally, as it appears, a separate mento-Meckelian {26} ossification may be added). The angular, splenial, and all the other elements of the reptilian jaw have vanished, though the numerous points from which the mammalian dentary ossifies is a reminiscence of a former state of affairs; and here again an occasional continuance of the separation is preserved, as the case observed by Professor Albrecht of a separate supra-angular bone in a Rorqual attests. Among other reptilian bones that are not to be found in the mammalian skull are the basipterygoids, quadrato-jugal, and supratemporal. A few of these bones, however, though no longer traceable in the adult skull save in cases of what we term abnormalities, do find their representatives in the foetal skull. Professor Parker, for example, has described a supra-orbital in the embryo Hedgehog; a supratemporal also appears to be occasionally independent.

[Ill.u.s.tration]

FIG. 15.--Head of a Human embryo of the fourth month. Dissected to show the auditory ossicles, tympanic ring, and Meckel's cartilage, with the hyoid and thyroid apparatus. All these parts are delineated on a larger scale than the rest of the skull. _an_, Tympanic ring; _b.hy_, basihyal element; _hy_, so-called hyoid bone; _in_, incus; _md_, bony mandible; _ml_, malleus; _st_, stapes; _tp_, tympanum; _tr_, trachea; I. (_mk_), first skeletal (mandibular) arch (Meckel's cartilage); II. second skeletal (hyoid) arch; III. third (first branchial) arch; IV. V. fourth and fifth arches (thyroid cartilage). (From Wiedersheim's _Structure of Man_.)

In the mode of the articulation of the lower jaw to the skull the Mammalia apparently, perhaps really, differ from other Vertebrates. In the Amphibia and Reptilia, with which groups alone any comparisons are profitable, the lower jaw articulates by means of a quadrate bone, which may be movably or firmly attached to the skull. In the mammals the articulation of the lower jaw is with the squamosal. The nature of this articulation is one of the most debated points in comparative anatomy. Seeing that Professor Kingsley[14] in the most recent contribution to the subject quotes no less than fifty-two different views, many of which are more or less convergent, it will be obvious that in a work like the present the matter cannot be treated exhaustively. As, however, Professor Kingsley justly says that "no single bone occupies a more important position in the discussion of the origin of the Mammalia than does the quadrate," and with equal justice adds that "upon the answer given as to its fate in this group depends, in large measure, the broader problem of the phylogeny of the Mammalia," it becomes, or indeed has long been, a matter which cannot be ignored in any work dealing with the mammals. A simple view, due to the late Dr. Baur and to Professor Dollo, commends itself at first sight as meeting the case. The last-named author holds, or held, that in all the higher Vertebrates it is at least on _a priori_ grounds likely that two such characteristically vertebrate features as the lower jaw and the chain of bones bringing the outer world {27} into communication with the internal organ of hearing would be h.o.m.ologous throughout the series. He believed, therefore, that the entire chain of ossicula auditus in the mammal is equal to the columella of the reptile, since their relations are the same to the tympanum on the one hand and to the foramen ovale on the other; and that the lower jaw articulates in the same way in both. It follows, therefore, that the glenoid part of the squamosal must be the quadrate which has become ankylosed with it after the fashion of concentration in the mammalian skull that has already been referred to. The fact that occasionally the glenoid part of the squamosal is a separate bone[15] appeared to confirm this way of looking at the {28} matter. But the hall-mark of truth is not always simplicity; indeed the converse appears to be frequently the case. And on the whole this view does not commend itself to zoologists at present. For it must be borne in mind that the lower jaw of the mammal is not the precise equivalent of that of the reptiles. Apart from the membrane bones, which may be collectively the equivalents of the dentary of the mammal, there is the cartilaginous articular bone to be considered, which forms the connexion between the rest of the jaw and the quadrate in reptiles. Even in the Anomodontia, whose relations to the Mammalia are considered elsewhere, there is this bone. But in these reptiles the articular bone articulates not only with the quadrate, but also to a large extent with the squamosal, the quadrate shrinking in size and developing processes which give to it very much the look of either the incus or the malleus of the mammalian ear.

In fact it seems on the whole to fit in with the views of the majority, as well as with a fair interpretation of the facts of embryology, to consider that the chain of ear bones in the mammal is not the equivalent of the columella of the reptile, but that the stapes of the mammal is the columella, and that the articulare is represented by the malleus and the quadrate by the incus. It is very interesting to note this entire change of function in the bones in question. Bones which in the reptile serve as a means of attachment of the lower jaw to the skull are used in the mammal to convey the waves of sound from the tympanum of the ear to the internal organ of hearing.

Another important and diagnostic feature in the mammalian skull is that the first vertebra of the vertebral column always articulates with two separate occipital condyles, which are borne by the exoccipital bones and formed mainly though not entirely by them. Certain Anomodontia form the nearest approach to the mammals in this particular. The two condyles of Amphibia are purely exoccipital in origin.

In the Mammalia, unlike what is found in lower Vertebrates (but here again the Anomodontia form at least a partial exception), the jugal arch does not connect the face with the quadrate, for, as already said, that bone does not exist, in the Sauropsidan form, in mammals. This arch pa.s.ses from the squamosal to the maxillary, and has but one separate bone in addition to those two, viz. the jugal or malar. {29}

[Ill.u.s.tration]

FIG. 16.--Under surface of the cranium of a Dog. . _apf_, Anterior palatine foramen; _as_, posterior opening of alisphenoid ca.n.a.l; _AS_, alisphenoid; _BO_, basioccipital; _BS_, basisphenoid; _cf_, condylar foramen; _eam_, external auditory meatus; _Ex.O_, exoccipital; _flm_, foramen lacerum medium; _flp_, foramen lacerum posterius; _fm_, foramen magnum; _fo_, foramen ovale; _fr_, foramen rotundum; _Fr_, frontal; _gf_, glenoid fossa; _gp_, post-glenoid process; _Ma_, malar; _Mx_, maxilla; _oc_, occipital condyle; _op_, optic foramen; _Per_, mastoid portion of periotic; _pgf_, post-glenoid fossa; _Pl_, palatine; _PMx_, premaxilla; _pp_, paroccipital process; _ppf_, posterior palatine foramen; _PS_, presphenoid; _Pt_, pterygoid; _sf_, sphenoidal fissure or foramen lacerum anterius; _sm_, stylomastoid foramen; _SO_, supraoccipital; _Sq_, zygomatic process of squamosal; _Ty_, tympanic bulla; _Vo_, vomer. (From Flower's _Osteology_.)

In connexion with the elaboration of the chain of auditory ossicles it is very usual for mammals to possess a thin inflated bone, sometimes partly or entirely formed out of the tympanic bone, and known as the tympanic bulla.

Whether this structure is thin and inflated or thick and depressed in form it is characteristic of the mammals, and does not occur below them in the series. But it is not present in all mammals. It is absent, for example, in the Monotremes. When it is present it is sometimes formed from other bones, as, for instance, from the alisphenoids. The tympanic ring has been held to be the equivalent of the quadrate. It is more probably the quadrato-jugal.[16]

[Ill.u.s.tration]

FIG. 17.--A, First thoracic skeletal segment for comparison with B, fifth cervical vertebra (Man), _b.v._ Body of vertebra; _c_, first thoracic rib; _c_', cervical rib (which has become united with the transverse process, _tr_), the two enclosing the costo-transverse foramen (_f.c.t_); _st_, sternum; _zy_, articular process of the arch (zygapophysis). (From Wiedersheim's _Structure of Man_.)

RIBS.--All mammals are furnished with ribs, of which the number of pairs differs considerably from group to group, or it may be even from species to species. The ribs are attached as a rule by two heads, of which one, the capitulum, arises as a rule between two centra of successive vertebrae. The other, the tuberculum, springs from the transverse process. Only in the Monotremes {30} are there ribs with but one, the capitular, head. In the posterior part of the series the two heads often gradually coalesce, so that there comes to be but one, the capitular, head. The Whales also, at least the Whalebone Whales, are exceptional in possessing but one head to the ribs, which is the capitular. The first rib joins the sternum below, and a variable number after this have the same attachment. There are always a number of ribs, sometimes called floating ribs, which have no sternal attachment. In the Whalebone Whales it is the first rib alone which is so attached. As a rule, to which the Whales mentioned are again an exception, the rib is divided into at least two regions--the vertebral portion which is always ossified, and the sternal moiety which is usually cartilaginous.

This is, however, often very short in the first rib. They are, however, ossified in the Armadillos and in some other animals. Between the vertebral and sternal portions an intermediate tract is separated off and ossified in the Monotremata. The ribs of existing mammals belong only to the dorsal region of the vertebral column, but there are traces of lumbar ribs and also of cervical ribs. In the Monotremata, indeed, these latter {31} are persistently free for a very long period, and in some cases never become ankylosed with their vertebrae. But it should be noted that in this group there is no approximation to the state of affairs which exists in many lower Vertebrates, where there is a gradual transition between the ribs of the cervical and those of the dorsal region of the vertebral column; for that of the seventh ribs in Monotremes is smaller than those which precede it.

[Ill.u.s.tration]

[Ill.u.s.tration]

FIG. 18.--Sternum and sternal ribs of the Common Mole (_Talpa europaea_), with the clavicles (_cl_) and humeri (_H_); _M_, manubrium sterni. Nat.

size. (From Flower's _Osteology_.)

FIG. 19.--Sternum of the Pig (_Sus scrofa_). . _ms_, Mesosternum; _ps_, presternum; _xs_, xiphisternum. (From Flower's _Osteology_.)

THE STERNUM.--All the Mammalia so far as is known possess a sternum. This is the bone, or series of bones (sternebrae), which lies upon the ventral surface of the chest, and to which the ribs are attached below. The development of the sternum has been shown to take place from the fusion of the ribs below into two lateral bands, one on each side; the approximation of these bands forms the single and unpaired sternum of most mammals. Very considerable traces, however, of the paired state of the sternal bones often exist; thus in the Sperm Whale the first piece of the sternum is divided into two by a longitudinal division, and the second piece is longitudinally grooved. The development of the sternum out of the fused ends of ribs is shown in a more complete condition in some species of _Manis_ than in many other mammals. Thus in _M. tricuspis_ the last ribs of those which are attached to the sternum are completely fused together into a single piece on each side.[17] As a general rule the last ribs which come into relation with the sternum do so only in an imperfect way, being simply firmly attached at their sides to, but not fused with, the last ribs which are definitely articulated with the sternum. Contrary to what is found in lower {32} Vertebrates, the sternum of the Mammalia consists of a series of pieces, as many as eight or nine or even sixteen in _Choloepus_, of which the first is called the manubrium sterni, and the last the ensiform cartilage, xiphisternum, or xiphoid process. The latter often remains largely cartilaginous throughout life; in fact this is generally but not universally the case with that part of the breastbone. The most extraordinary modification of the xiphoid process is seen in the African species of the genus _Manis_, where it diverges into two long cartilages, which run back to the pelvis and then, curving round, run forwards and fuse together in the middle line anteriorly. These processes serve for the attachment of certain tongue-muscles. They were looked upon by Professor Parker as the equivalents of the "abdominal ribs" of reptiles elsewhere non-existent among mammals. This view is not, however, usually held. The manubrium sterni is often keeled in the middle line below; this is so with the Bats, which thus approach the birds, and probably for the same reason, i.e. the need of an enlarged origin for the pectoral muscle, which is concerned in the movements of flight. In many forms this part of the sternum is much broader than the pieces which follow; this is so with the Viscacha. In the Pig the precise reverse is seen, the manubrium being narrower than the rest of the sternal bonelets. It will be noticed, however, that in this and similar cases there are no clavicles. Ribs are attached between the successive pieces of the sternum. When the sternum is reduced, as it is in the Cetacea and in the Sirenia, it is the intermediate part of the series of bones which becomes abbreviated or vanishes. The Sperm Whale has only a manubrium sterni and a following piece belonging to the mesosternum. It is fair to say that the xiphoid process and the rest of the sternum have disappeared, since among the Toothed Whales a progressive shortening of the sternum can be seen. In the Whalebone Whales the sternum is still further reduced; the manubrium is alone left, and to it are attached but a single pair of ribs. In _Balaena_, however, a rudimentary {33} piece, apparently comparable to a xiphoid process, has been detected.

[Ill.u.s.tration]

[Ill.u.s.tration]

FIG. 20.--Sternum of Rudolphi's Whale (_Balaenoptera borealis_), showing its relation to the inferior extremities of the first pair of ribs. 1/10.

(From Flower's _Osteology_.)

FIG. 21.--Sternum of a young Dugong (_Halicore indicus_). . From a specimen in the Leyden Museum, _ps_, Presternum; _xs_, xiphisternum. (From Flower's _Osteology_).

From the instances which have been described, as well as from the mode of development of the sternum and from the number of free ribs, _i.e._ ribs which are not attached to it, it would seem that the sternum has undergone a considerable reduction in its size. This reduction may be possibly accounted for by the need for respiratory activity, which is clearly increased by a less-marked fixity of the walls of the thoracic cavity. In the case of the Whales one can hardly help coming to that conclusion. The arrangement in the Monotremata does not, however, point in the same direction; for these animals are precisely like the higher Mammalia in the reduction of the sternum and of the number of ribs which reach it.

[Ill.u.s.tration]

FIG. 22.--Shoulder girdle of Ornithorhynchus. _c_^1, _c_^2, _c_^3, First, second, third ribs; _cl_, clavicle; _e.c_, epicoracoid; _es_' and _es_", interclavicle (episternum); _m.c_, metacoracoid; _m.s_, manubrium sterni; _sc_, scapula; _st_, sternebra. (From Wiedersheim's _Structure of Man_.)

[Ill.u.s.tration]

FIG. 23.--Episternum of an embryo Mole. (After A. Gotte.) _cl_, Clavicle; _es_', central portion of the episternum; _es_", lateral portion of the same; _r.c_, costal ribs; _st_, sternum. (The figure was constructed from two consecutive horizontal sections.) (From Wiedersheim's _Structure of Man_.)

THE EPISTERNUM.--The Mammalia are as a rule to be distinguished from lower Vertebrates by the absence of an episternum, or interclavicle as it is also called. In the Monotremata, however, there is a large [18]-shaped bone which does not overlie the sternum as in reptiles, but is anterior to it.

The relations of this bone to the clavicles seem to leave no doubt that it is the equivalent of the Lacertilian interclavicle or episternum. The Monotremata are not, however, the only mammals in which this structure is to be seen. The Mole in the embryonic condition is {34} provided with pieces of bone which overlie the manubrium sterni and are attached to the clavicles, and are no doubt to be regarded as the same structure. Probably in many mammals the manubrium will be found to be partly made up of corresponding rudiments. In any case, vestiges of an episternum in the shape of two minute ossicles have been discovered in Man, lying in front of the manubrium. They have been termed ossa suprasternalia. In Man and in the Mole the paired nature of the episternum is clearly apparent. It has been suggested that this structure in its entirety belongs to the clavicles, just as the sternum belongs to the ribs; _i.e._ that it formed out of the approximated and fused ends of the clavicles. Dr. Mivart[19] figured a good many years since a pair of ossicles in _Mycetes_, lying in one case between the ends of the clavicles and the manubrium sterni, and in another example anterior to the ventral ends of the clavicles. Gegenbaur has figured a {35} pair of similar bones in the Hamster.[20] It is possible that these are to be referred to the same category. It has also been suggested that these supposed episternal rudiments are the vestiges of a pair of cervical ribs.

[Ill.u.s.tration]

FIG. 24.--Episternal vestiges in Man. _cl_, Clavicle, sawn through; _es_, "episternum" (sternoclavicular cartilage); _l_', interclavicular ligament; _l_", costoclavicular ligament; _m.s_, manubrium sterni; _o.s_, ossa suprasternalia; _r.c_, first rib; _st_, sternum. (From Wiedersheim's _Structure of Man_.)

THE PECTORAL GIRDLE.--The skeleton by which the fore-limb is connected with the trunk is known as the Pectoral Girdle. The main part of this girdle is formed by the large scapula, or blade-bone as it is often termed. The coracoidal elements will be dealt with later. The scapula is not firmly connected with the backbone; it is attached merely by muscles, thus presenting a great difference from the corresponding pelvic girdle. The reason for this difference is not easy to understand. On the one hand it may be pointed out that in all running animals at any rate there is a greater need for the fixation in a particularly firm way of the hind-limbs; but, again, in the climbing creatures both limbs would, one might suppose, be bettered by a firm fixation. It must be remembered, however, that in the latter case the same result is at least partly brought about by a well-developed clavicle, which fixes the girdle to the sternum and so to the vertebral column by means of the ribs.

Broadly speaking, too, the fore-limbs require a greater freedom and variety of movement than the hind-limbs, which are supports {36} for or serve to push along the rapidly-moving body. Stronger fixation is therefore a greater necessity posteriorly than anteriorly. In any case, whatever the explanation, this important difference exists.

[Ill.u.s.tration]

FIG. 25.--Right scapula of Dog (_Canis familiaris_). . _a_, Acromion; _af_, prescapular fossa; _c_, coracoid; _cb_, coracoid or anterior border; _css_, indicates the position of the coraco-scapular suture, obliterated in adult animals by the complete ankylosis of the two bones; _gb_, glenoid or posterior border; _gc_, glenoid cavity; _pf_, postscapular fossa; _s_, spine; _ss_, suprascapular border. (From Flower's _Osteology_.)

[Ill.u.s.tration]

FIG. 26.--Right scapula of Red Deer (Cervus elaphus). . _a_, Acromion; _af_, anterior or prescapular fossa; _c_, coracoid; _gc_, glenoid cavity; _pf_, postscapular fossa; _ss_, partially ossified suprascapular border.

(From Flower's _Osteology_.)

The shoulder-blade of mammals is as a rule a much-flattened bone with a ridge on the outer surface known as the spine; this ridge ends in a freely-projecting process, the acromion, from which a branch often arises known as the metacromion. This gives a bifurcate appearance to the end of the ridge. The spine is less developed and the scapula is narrower in such animals as the Dog and the Deer which simply run, and whose fore-limbs therefore are not endowed with the complexity of movement seen, for instance, in the Apes.

{37} [Ill.u.s.tration]

FIG. 27.--Right scapula of Dolphin (_Tursiops tursio_). . _a_, Acromion; _af_, prescapular fossa; _c_, coracoid; _gc_, glenoid cavity; _pf_, postscapular fossa. (From Flower's _Osteology_.)

[Ill.u.s.tration]

FIG. 28.--Side view of right half of shoulder girdle of a young Echidna (_Echidna hystrix_). 2/3. _a_, Acromion; _c_, coracoid; _cb_, coracoid border; _cl_, clavicle; _css_, coraco-scapular suture; _ec_, epicoracoid; gb, glenoid border; _gc_, glenoid cavity; _ic_, interclavicle; _pf_, postscapular fossa; _ps_, presternum; _s_, spine; _ss_, suprascapular epiphysis; _ssf_, subscapular fossa. (From Flower's _Osteology_.)

It has been pointed out that the area which lies in front of the spine, the prescapular lamina, is most extensively developed in such animals as perform complex movements with the fore-limbs. The Sea Lion and the Great Anteater are cited by Professor G. B. Howes as examples of this preponderance of the anterior portion of the scapula over that which lies behind the spine. The general shape of the scapula varies considerably among the different orders of mammals; but it always presents the characters mentioned, which are nowhere seen among the Sauropsida except among certain Anomodonts, which will be duly referred to (see p. 90). The most conspicuous divergences from the normal are to be found in the Cetacea and the Monotremata. In the former the acromion is approximated so nearly to the anterior border of the blade-bone that the prescapular fossa is reduced to a very small area; and in _Platanista_ the acromion actually coincides with the anterior border, so that that fossa actually disappears.

In the Whales, too, the scapula is as a rule very broad, especially above; it has frequently a fan-like contour. In the Monotremata the acromion also coincides with the anterior border of the scapula; but the sameness of appearance which it thus presents (in this feature) to the Cetacean scapula is {38} apparently not due to real resemblance. What has happened in the Monotremata is, that the prescapular fossa is so enormously expanded that it occupies the whole of the inner side of the blade-bone, while the subscapular fossa which, so to speak, should occupy that situation, has been thus pushed round to the front, where it is divided from the postscapular fossa by a slight ridge only.

The clavicle is a bone which varies much in mammals. It is sometimes indeed, as in the Ungulata, entirely absent; in other forms it shows varying degrees of retrocession in importance; it is only in climbing, burrowing, digging, and flying mammals that it is really well developed.

[Ill.u.s.tration]