The Cambridge Natural History Part 4

In a few mammals there is a kind of dent.i.tion known as the monophyodont, in which only one series of teeth reaches maturity; where in fact there is no replacement of a milk series by a permanent dent.i.tion. Of the monophyodont dent.i.tion Whales form an example. The Marsupials are very nearly an instance of the {54} same phenomenon; for Sir W. Flower showed, and Mr.

Thomas confirmed his discovery, that only one tooth, according to Mr.

Thomas the fourth premolar, is replaced in that group. But even the purely monophyodont dent.i.tion of the Toothed Whales is a more apparent than real contrast to the diphyodont dent.i.tion elsewhere prevalent. An investigation of the embryos of various Toothed Whales by Dr. Kukenthal and by Dr. Leche has brought to light the highly important fact that two dent.i.tions are present, but that one only comes to maturity; from this fact obviously follows the interesting question:--To which of the two dent.i.tions of more normal Mammalia does the monophyodont dent.i.tion of the Whales and Marsupials belong? To this question a clear answer is fortunately possible.

As has been pointed out in the foregoing sketch of tooth development, and has been ill.u.s.trated in the figures, the milk teeth develop as lateral outgrowths of the common enamel germ, while the permanent teeth arise from the end of the same band of tissue. This fact enables it to be stated apparently beyond a doubt that in the Whales and in the Marsupials it is the milk dent.i.tion which is the only one to arrive at maturity. Thus the earlier theoretical conclusion that the Marsupial dent.i.tion "is a secondary dent.i.tion with only one tooth of the primary set left," is proved on embryological grounds to be untrue. But there are other monophyodont animals than those already mentioned.[27] _Orycteropus_, the Cape Anteater, is an example. Mr. Thomas has lately discovered that in this Edentate there is a set of minute though calcified milk teeth which probably never cut the gum; here we have a different sort of monophyodontism, in which the teeth belong to the second and not to the first set. Between the latter condition and the diphyodont state are intermediate stages. Thus in the Sea Lions the milk teeth are developed but disappear early, probably before the animal is born.

In the typical diphyodont dent.i.tion, such as is exhibited for example in Man and the vast majority of mammals, the milk teeth eventually completely disappear and are entirely replaced by the permanent set of teeth, with the exception, of course, of the molars, which though they are developed late belong to the milk series. {55}

Their correspondence with the milk series is shown in an interesting way by the close resemblance which the last milk premolar often bears to the first molar. These two extremes of dent.i.tion, _i.e._ purely monophyodont and, excepting for the molars, purely diphyodont, are however connected by an intermediate state of affairs, which is represented by more than one stage.

In _Borhyaena_ (probably a Spara.s.sodont) the incisors and the canines and two out of the four premolars belong to the permanent dent.i.tion, while the two remaining premolars and of course the three molars are of the milk series. _Prothylacinus_, a genus belonging to the same group, has a dent.i.tion which is a step or two further advanced in the direction of the recent Marsupials. We find, according to Ameghino,[28] whose conclusions are accepted by Mr. Lydekker, that the incisors, canines, and two premolars belong to the milk series, while the permanent series is represented only by the two remaining premolars. We can tabulate this series as follows:--

(1) Purely monophyodont, with teeth only of the first set--Toothed Whales.

(2) Incompletely monophyodont, as in the Marsupials, where there is a milk dent.i.tion with only one tooth replaced.[29]

(3) Incompletely diphyodont, with the dent.i.tion made up partly of milk, partly of permanent teeth, as in _Borhyaena_.

(4) Diphyodont, where all the teeth except the molars are of the second set; this characterises nearly all the mammals.

As we pa.s.s from older forms to their more recent representatives there is as a rule a progressive development of the form of the teeth. This is especially marked among the Ungulata. The extremely complicated type of tooth found in such a form as the existing Horse can be traced back through a series of stages to a tooth in which the crown is marked by a few separated tubercles or cusps. Arrived at this point, the differences between the teeth of ancestral Horses and ancestral Rhinoceroses and Tapirs are hard to distinguish with accuracy; and the same difficulty is experienced in attempting to give a definition of other large orders by the characters of the teeth, such as will apply to the Eocene or {56} even earlier representatives of these families. Fig. 36 (p. 51) ill.u.s.trating a series of mammalian teeth will ill.u.s.trate the above remarks. That there is such a convergence in tooth structure shows that it is, theoretically at least, possible to determine the ancestral form of the mammalian tooth.

Practically, however, the difficulties which beset such theorising are great; that there are such divergent and such strongly-held ant.i.thetical views is sufficient proof of this. Two main views hold the field: one, which has found most favour in America, and is due chiefly to the labours and persuasiveness of Professors Cope, Scott, Osborn, and others, is known as "trituberculy."[30] The alternative view, as urged by Forsyth Major, Woodward, and Goodrich, attempts to show that the dent.i.tion of the original mammal included grinding teeth which were multicuspidate or "mult.i.tubercular." There is much to be said for both views, and something to be said against both.

[Ill.u.s.tration]

FIG. 38.--Molar teeth of A, _Phenacodus_, and B, the Creodont _Palaeonictis_. _End_, endoconid; _hld_, hypoconulid; _hyd_, hypoconid; _med_, metaconid; _prd_, protoconid. (After Osborn and Wortman.)

This question is, however, wrapped up in a wider one. Its solution depends upon the ancestry of mammals. If the Mammalia are to be derived from reptiles with simple conical teeth, then the first stage in the development of trituberculy is proved. On the other hand, however, the evidence is gradually growing that the Theromorpha represent more nearly than any non-mammalian group with which we are acquainted the probable ancestral form of the mammals. These animals offer some support to both the leading views. _Cynognathus_ had triconodont teeth which, as will be pointed out later, are a theoretically intermediate stage in the evolution of tritubercular teeth; on the other hand, the teeth of _Diademodon_ and some others are mult.i.tuberculate, and have been very properly compared to the mult.i.tubercular teeth of such primitive mammalia as the Ornithorhynchus.

Professor Osborn is no doubt correct in italicising a remark of an anonymous writer in _Science_ to the effect that in _Diademodon_ the teeth, though mult.i.tubercular, show the prevalence of three cusps arranged in the tritubercular fashion. {57} But this may be only a proof that the mult.i.tubercular antedates the tritubercular. It may be, indeed, that the mammalian tooth was already differentiated among the mammal-like Saurians and that from such a form as _Cynognathus_ the Eutheria and other forms in which a tritubercular arrangement can be detected were evolved, and from such form as _Tritylodon_ the Monotrematous branch of the mammals. This way of looking at the matter harmonises a much-disputed question, but involves a diphyletic origin of the mammals--an origin which for other reasons is not without its supporters.

We shall now attempt to give a general idea of the facts and arguments which support or tend to support "trituberculy." As a matter of fact the name is inaccurate; for the holders of this view do not derive the mammalian molar from a trituberculate condition, but in the first place from a simple cone such as that of a crocodile!

To this main and at first only cusp came as a reinforcement an additional cusp at each side, or rather at each end, having regard to their position with reference to the long axis of the jaw. This stage is the "triconodont"

stage, and teeth exist among living as well as extinct mammals which show this early form of tooth. We have, indeed, the genus _Triconodon_, so named on that very account. Among living mammals the Seals and the Thylacine all show some triconodont teeth. A Toothed Whale, it may be remarked, is a living example of a mammal with monoconodont teeth. The three primary cusps, as the supporters of Cope's theory of trituberculism denominate them, are termed respectively the protocone, paracone, and metacone, or, if they are in the teeth of the lower jaw, protoconid, paraconid, and metaconid. At a slightly later stage, or coincidently, a rim partly surrounded the crown of the tooth; the rim is known as the cingulum, and from a prominent elevation of this rim a fourth cusp, the hypocone, was developed. The three main cones then moved, or rather two of them moved, so as to form a triangle; this is the tritubercular stage. Teeth of this pattern are common, and occur in such ancient forms as Insectivora and Lemurs, besides numerous extinct groups. An amendment has been suggested, and that is to term the teeth with the simple primitive triangle "trigonodont," and to reserve the term tritubercular for those teeth in which the hypocone has appeared. The platform bearing the hypocone widened into the {58} "talon"; and this ledge became produced into two additional cusps, the hypoconule or hypoconulid, and the ectocone or ectoconid. Thus the typical s.e.xtuberculate tooth of the primitive Ungulate, and indeed of many primitive Eutherians, is arrived at. From this the still further complicated teeth of modern Ungulates can be derived by further additions or fusions, etc.[31] On the other hand, the development of the Primate molar stops short at the stage of four cusps.

[Ill.u.s.tration]

FIG. 39.--Epitome of the evolution of a cusped tooth. 1, Reptile; 2, _Dromatherium_; 3, _Microconodon_; 4, _Spalacotherium_; _me_, metaconid; _pa_, paraconid; _pr_, protoconid; 5, _Amphitherium_. (After Osborn.)

That such a series can be traced is an undoubted fact. Every stage exists, or has existed. But whether the stages can be connected or not is quite another question. It is by three main lines of argument that the view here sketched out in brief is supported. In the first place, the tracing of the pedigrees of many groups of mammals has met with very considerable success; and it is clear that as we pa.s.s from the living Horse and Rhinoceros, with their complicated molars, to their forerunners, we find that both can be referred to a primitive Ungulate molar with but six cusps. Going still further back to the lowest Eocene and ancestral type as it appears, _Euprotogonia_, we still find in the molar tooth the s.e.xtubercular plan of structure. We can hardly get further back in the evolution of the Perissodactyles with any probability of security. On the other hand, many facts point to a fundamental relationship between the primitive Ungulates and the early Creodonts. The latter frequently show plainly tritubercular molars. Such Ungulates as _Euprotogonia_ and _Protogonodon_, though s.e.x- or quinque-tubercular as to their molars, have a distinctly prevailing trituberculism, when the _size_ and importance of three of the cusps is taken into account. But this {59} lacks finality as a convincing proof of the tritubercular tooth as a primitive Ungulate tooth.

Professor Osborn has ingeniously utilised certain deviations from the normal type of tooth structure (for the group) in favour of his strongly-urged opinions. If the stages of development have been as he suggests, a retrogression would naturally be in the inverse order; thus the "apparently 'triconodont' lower molar of _Thylacinus_" may be interpreted as a retrogression from a tritubercular tooth. In the same way may be explained the triconodont teeth of Seals and of the Cetacean _Zeuglodon_.

Finally, the modern Toothed Whales have retrograded into "haplodonty."

Embryological evidence has also been called in, and with some success, to contribute towards the proof of the tritubercular theory of teeth. Taeker has shown that in the Horse and the Pig, and some other Ungulates, there is first of all a single hillock or cusp, and that later the additional cones arise separately. An apparent stumbling-block raised by these investigations is that it is not always the protocone or its equivalent in the upper jaw which arises first, as it obviously ought to do phylogenetically. This, however, is not a final argument in either direction. We know from plenty of examples that ontogenetic processes sometimes do not correspond in their order with phylogenetic changes. Thus in the mammalian heart the ventricle divides before the auricle; and of coa.r.s.e, phylogenetically, the reverse ought to occur, since a divided auricle precedes a divided ventricle. This method of development has, moreover, been interpreted otherwise. It has been held to signify that the complex teeth of mammals are indeed derived from simple cones but by the fusion of a number of those cones.

On the other hand there are the claims of the mult.i.tubercular theory of the origin of mammalian teeth to be considered. The palaeontological evidence has been already, to some extent, utilised. The occurrence of such teeth among the possible forerunners of mammals, and in some of the most primitive types of Mammalia, has been referred to. Senor Ameghino dwells upon the s.e.xtubercular condition of many primitive mammals even belonging to the Eutheria. In a recent communication[32] he attempts to identify six tubercles in the molars of types belonging to a {60} variety of Orders. The same condition, as has been noted, characterises that ancient Ungulate form _Euprotogonia_. Even where the teeth seem at first sight to be tritubercular a detailed study shows traces of otherwise vanished cusps.

It must be remembered in basing arguments upon the early Jura.s.sic and Cretaceous mammals, that our knowledge of them mainly depends upon lower jaws, the teeth of which are usually simpler in pattern than those of the upper jaws. Moreover, another fact, not always insisted upon, must not be lost sight of. In many of those creatures the jaws were of small size, and yet accommodated a large series of molar teeth. _Amphitherium_, for example, had six molar teeth, and five is a number frequently met with. As the teeth are so numerous and the jaws so small it seems reasonable to connect the simplicity of the structure of the teeth with the need for crowding a number together. The same argument may partly account for the superabundant teeth of many Toothed Whales. It is true that the Manatee has very numerous grinders which are yet complex; but then in this animal there is a succession, and the jaw does not hold at a given time the entire series, with which it is provided in relays. On the other hand, where there are few molars they are often of the mult.i.tubercular type, or at least approach it; of this the Mult.i.tuberculate _Polymastodon_ is a good example; so, too, the molars of _Hydroch.o.e.rus_, and of many other Rodents.

It is well known that the fourth deciduous molar of the upper jaw, which is replaced by a permanent premolar in the fully adult animal, is of a more complex structure than its successor. This may indeed be extended to premolars earlier in the series. In the Dog "the second and first milk molars closely resemble the third and second premolars"; now the milk premolars belong evidently to the same dent.i.tion as the permanent molars, and they are earlier teeth than the later-developed replacing teeth. It is therefore significant that these earlier teeth should be more cuspidate than the later teeth. It tells distinctly in favour of the simplification as opposed to the complication of teeth in time, in the groups concerned.

These facts may possibly be applied in explanation of the simple teeth of some of the Jura.s.sic and Cretaceous mammals. It has been mentioned that absolute trituberculy is exceedingly rare among those ancient creatures; more generally there are to {61} be found at least traces of more cusps.

Now in some of them we may be dealing with instances of a complete tooth change; the suppression, save for one tooth, which is found in Marsupials, was probably not developed in at least some of these early mammals. The simplicity may therefore have been preceded by complexity, and may have been merely an adaptation to an insectivorous diet.

ALIMENTARY Ca.n.a.l.--The _mouth_ of the Mammalia is remarkable for the fact that with a few exceptions, such as the Whales, there are thick and fleshy lips. The office of these is to seize the food. The roof of the mouth is formed by the "hard palate" in front, which covers over the maxillary and palatine regions. This region is often covered with raised ridges, which have a symmetrical disposition, and are particularly strong in Ruminant animals. They are much reduced in the Rodents, where the anterior part of the palate is ill-defined owing to the way in which its sides fade into the lateral surface of the face. It has been shown that these ridges, in the Cat at least, develop as separate papilliform outgrowths, and it has been suggested that these papillae, which later become united to form the ridges, are the last remnant of palatine teeth such as occur in lower vertebrates.

[Ill.u.s.tration]

FIG. 40.--Palatal folds of the Racc.o.o.n (_Procyon lotor_). _p.p_, Papilla palatina; _r.p_, palatal folds. (From Wiedersheim's _Structure of Man_.)

The _tongue_ is a well-developed organ, usually playing a double part. It acts as an organ of prehension, especially in such animals as the Giraffe and the Anteater, where it is long and protrusible beyond the mouth for a considerable distance. It also carries gustatory organs, which serve for the discrimination of the nature of the food. Beneath the tongue there may be a hardish plate, known as the sublingua. This is especially prominent in the Lemurs, where it projects as a h.o.r.n.y structure below the tongue, and has an independent and free tip. It is supported in some of these animals by a cartilaginous {62} structure. It is held by Gegenbaur that this organ is the equivalent of the reptilian tongue, and that in the skeletal vestiges which it contains are to be found the equivalents of the hyoid skeletal cartilages which support the tongue in lizards. In this case the tongue of mammals is a subsequently added structure.

The _oesophagus_ leads from the mouth cavity to the _stomach_. The latter organ has commonly a distinctive shape in mammals. This is well shown in Man. The orifices of the oesophagus and intestine are somewhat approximated; and this causes a bulging of the lower border of the organ, usually spoken of as the greater curvature. A stomach of this typical form is found in many orders of mammals, and is unlike the stomach in any of the groups of lower vertebrates in shape. Sometimes the shape of the organ is greatly altered: it may be drawn out, sacculated, or divided, as in the Ruminants and Whales, into a series of differentiated chambers, each of which plays some special part in the phenomena of digestion.

The _intestine_ of mammals is always long and much coiled, though the length and consequent degree of coiling naturally varies. On the whole it is perhaps safe to say that it is shorter in carnivorous than in vegetable-feeding beasts. Thus the Paca has an intestine of 39 inches total length, while the Cat, an animal of about the same size, has an intestine which is only 36 inches long. A fish diet, however, to judge from the Seals, is a.s.sociated with a long intestinal tract. The intestine is divisible in the vast majority of mammals into a small and a large intestine. The two are separated by a valvular constriction save in certain Carnivores; and in the majority of cases the distinction is also emphasised by the presence at the junction of a blindly-ending diverticulum, the _caec.u.m_. This latter organ varies greatly in length, being very short in the Cat-tribe and exceedingly long in Rodents. Its size is, to some extent, dependent upon the flesh-eating or gra.s.s-eating propensities of the animal in which it occurs. One of the longest caeca is possessed by the Vulpine Phalanger, in which the organ is one-fifth of the length of the small intestine; while the opposite extremity is reached by _Felis macroscelis_, which has a small intestine one hundred times the length of the caec.u.m.

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

FIG. 41.--Different forms of the stomach in Mammals. A, Dog; B, _Mus dec.u.ma.n.u.s_; C, _Mus musculus_; D, Weasel; E, scheme of the Ruminant stomach, the arrow with the dotted line showing the course taken by the food; F, Human stomach. _a_, Minor curvature; _b_, major curvature; _c_, cardiac end. G, Camel; H, _Echidna aculeata_. _Cma_, Major curvature; _Cmi_, minor curvature. I, _Bradypus tridactylus_. _Du_, Duodenum; _MB_, coecal diverticulum; **, outgrowths of duodenum; +, reticulum; ++, rumen.

_A_ (in E and G), Abomasum; _Ca_, cardiac division; _O_, psalterium; _Oe_, oesophagus; _P_, pylorus; _R_ (to the right in E and to the left in G), rumen; _R_ (to the left in E and to the right in G), reticulum; _Sc_, cardiac division; _Sp_, pyloric division; _WZ_, water-cells. (From Wiedersheim's _Comparative Anatomy_.)

An interesting point in connexion with the gut of mammals is the varying proportion of the small to the large intestine. As a general rule the former is very considerably longer than the latter; in _Paradoxurus_, for instance, the small intestine may be fifteen times the length of the large.

The excess of length of one section over the other is not generally so marked {64} as this. In _Phalanger maculatus_ the two sections of the gut are as nearly as possible equal in length, while in _Phaseolarctos_ the large intestine is considerably longer than the small, the lengths being respectively 160 inches and 111 inches. It is common among the Marsupials and also among the Rodents for these proportions to exist, _i.e._ for the large intestine to be as long as, or longer than, the small. But there are so many exceptions that no general statements can be extracted from the facts.

Some few details will be found in the systematic part of this book. Mr.

Chalmers Mitch.e.l.l has brought forward some reasons for a.s.sociating a great length of large intestine with an archaic systematic position, in the birds at any rate. The facts here briefly touched upon are not at variance with the extension of such a view to the mammals.

[Ill.u.s.tration]

FIG. 42.--Diagrammatic plan of the liver of a Mammal (posterior surface).

_c_, Caudate lobe; _cf_, cystic fissure; _dv_, ductus venosus; _g_, gall-bladder; _lc_, left central lobe; _ll_, left lateral lobe; _llf_, left lateral fissure; _p_, portal vein entering transverse fissure; _rc_, right central lobe; _rl_, right lateral lobe; _rlf_, right lateral fissure; _s_, Spigelian lobe; _u_, umbilical vein; _vc_, post-caval vein. (After Flower and Lydekker.)

Appended to the alimentary tract are three glands or sets of glands.

Opening into the mouth cavity are the _salivary glands_, which are of enormous size in Anteaters, and small or absent in Whales. In their number and position these glands are characteristic of mammals. Into the intestine open the ducts of the pancreas and liver, two glands which the mammals share with lower vertebrates. The form of the _liver_ is, however, generally characteristic of mammals. It is divided as a rule into a right and a left half, the line of division being marked by the insertion of the umbilical ligament, a vestige of the primitive ventral mesentery. Each half is again commonly subdivided into central and lateral lobes. In addition to these, two other divisions are often to be seen--the Spigelian and the caudate lobe. The liver is less divided in Cetacea and {65} some others, very much subdivided in Rodents and other groups. The degree of subdivision and the proportions of the several lobes frequently offer valuable systematic characters. The gall-bladder may be present or absent; it is always a diverticulum of the hepatic duct. The two are never separate, as in birds, for instance.

ORGANS OF CIRCULATION.--The heart of all mammals is a completely four-chambered organ. In the adult heart there is no communication between the right and left halves. The auricles are comparatively thin-walled, the ventricles thick-walled, in relation to the amount of work that they have severally to perform. The right ventricle, moreover, which has only to drive the blood into the lungs, is much thinner-walled than the left ventricle, which is concerned with the entire systemic circulation. The exits of the arteries and the auriculo-ventricular orifices are guarded by valves, which are so arranged as only to permit the blood to flow in the proper direction. But these valves have a morphological as well as a physiological interest. At the origin of each artery, the aorta and the pulmonary, there is a row of three watch-pocket valves, as they have been generally termed on account of their form. These three valves meet accurately in the middle of the lumen of the arterial tube when liquid is poured into them from above, and thus completely occlude the orifice. The auriculo-ventricular valves differ in structure in the two ventricles. That of the left ventricle has only two flaps, and is therefore often spoken of as the bicuspid or mitral valve. Both these flaps are membranous, and together they completely surround the exit from the auricle into the ventricle. The edges of the valve are bound down to the parietes of the heart by numerous branching tendinous threads, the chordae tendineae, which often take their origin from pillar-like muscles arising from the walls of the heart, the so-called musculi papillares. The valve of the right ventricle is composed of three flaps, and is therefore often spoken of as the tricuspid valve; it is in the same way membranous, and has chordae tendineae and musculi papillares connected with it. The disposition of the musculi papillares and their number differ in different mammals, but no exhaustive study has as yet been made of the arrangements in different groups; the amount of individual variation even is not known, though it is certainly considerable in some cases, for {66} instance in the heart of the Rabbit. The heart of the Monotremata presents differences of some importance from those of other Mammalia; the modern knowledge of the Monotrematous heart is mainly due to Gegenbaur[33] and Lankester,[34] in whose memoirs references to the older literature will be found. The princ.i.p.al features of interest in which the heart of the Monotremata differs from that of the higher Mammalia are these. When the two ventricles are cut across transversely, the cavity of the right is seen to be wrapped round that of the left in a fashion precisely like that of the bird's heart; on the other hand in the higher mammal the two cavities lie side by side. The main difference between Monotremes and other Mammals concerns the right auriculo-ventricular valve. The differences which it presents from the corresponding structure of the rest of the Mammalia are two: in the first place, the valve itself does not completely surround the ostium; it is only developed on one side; the septal half (_i.e._ that turned towards the interventricular septum) is either entirely absent or more generally represented by a small bit of membrane; nevertheless I found[35] recently in an _Ornithorhynchus_ heart a complete septal half to the right auriculo-ventricular valve. The second point of interest in connexion with this valve is, that the musculi papillares instead of ending in chordae tendineae attached to the free edge of the valve are directly attached to the valve, and in some cases pa.s.s through its membranous flap, to be attached to its origin at the boundary of the auricle and of the ventricle.

The invading of the valve-flap by muscle in this way is highly interesting, as it recalls the heart of the bird and of the crocodile. The imperfect condition of the valve (from which, as has already been stated, the septal half is as a rule nearly absent) is a point of resemblance to the heart of the bird; the corresponding valve of the crocodile's heart being complete.

{67}

[Ill.u.s.tration]

FIG. 43.--_Lepus cuniculus._ Ventral view of the vascular system. The heart is somewhat displaced towards the left of the subject; the arteries of the right and the veins of the left side are in great measure removed. _a.epg_, internal mammary artery; _a.f_, anterior facial vein; _a.m_, anterior mesenteric artery; _a.ph_, anterior phrenic vein; _az.v_, azygos vein; _br_, brachial artery; _c.il.a_, common iliac artery; _c.il.v_, common iliac vein; _coe_, coeliac artery; _d.ao_, dorsal aorta: _e.c_, external carotid artery; _e.il.a_, external iliac artery; _e.il.v_, external iliac vein; _e.ju_, external jugular vein; _fm.a_, femoral artery; _fm.v_, femoral vein; _h.v_, hepatic veins; _i.c_, internal carotid artery; _i.cs_, intercostal vessels; _i.il.a_, internal iliac artery; _i.il.v_, internal iliac vein; _i.ju_, internal jugular vein; _i.l_, iliolumbar artery and vein; _in_, innominate artery; _l.au_, left auricle; _l.c.c_; left common carotid artery; _l.pr.c_, left pre-caval vein; _l.v_, left ventricle; _m.sc_, median sacral artery; _p.a_, pulmonary artery; _p.epg_, epigastric artery and vein; _p.f_, posterior facial vein; _p.m_, posterior mesenteric artery; _p.ph_, posterior phrenic veins; _pt.c_, post-caval vein; _p.v_, pulmonary vein; _r_, renal artery and vein; _r.au_, right auricle; _r.c.c_, right common carotid artery; _r.prc_, right pre-caval vein; _r.v_, right ventricle; _s.cl.a_, right subclavian artery; _s.cl.v_, subclavian vein; _spm_, spermatic artery; _s.vs_, vesical artery; _ut_, uterine artery and vein; _vr_, vertebral artery. (From Parker's _Zootomy_.)

There are also features in the system of arteries and veins which are eminently distinctive of mammals. In the first place, the aorta leaving the heart and conveying blood to the body is only a half arch, and bends to the left side as seen in Fig. 43. The right and left halves are present in reptiles, and meet behind the heart. In the bird the right half alone has remained. This fact, therefore, shows that the mammal cannot have been derived from a bird-like ancestor, but that both must have independently come from an ancestor with both halves of the aortic arch present, of which one half has disappeared in one group, and the other half in the other. It is an interesting fact, too, to notice that the four {68} cavities of the mammal's heart, which fourfold division it shares with birds alone, do not exactly correspond compartment for compartment with those of the bird's heart, at least in so far as concerns the ventricles. For the reptilian heart is provided with only one ventricle, and therefore the division of that cavity must have been independently accomplished in mammals and in birds.

There are two features in the venous system which distinguish all the Mammalia (with the exception of _Echidna_ in one of these points) from vertebrates standing lower in the series. The hepatic portal system is limited to a vein which conveys to the liver blood derived from the alimentary tract; in no mammal except in _Echidna_ is there any representative of the anterior abdominal vein of lower vertebrates. In that animal there is such a vein, which apparently arises from a capillary network upon the bladder and pa.s.ses up, supported by a membrane, along the ventral wall of the abdomen to the liver, thus emptying blood into that organ exactly as does the anterior abdominal vein of the frog. In no mammal is there any trace of a renal portal system. The kidneys derive their blood from the renal arteries only.

Many mammals have two superior venae cavae; this is the case, for instance, in the Elephant and the Rodents and other types lying comparatively far down in the series. In most if not in all mammals there are considerable remains of one of the posterior cardinals, in the form of the azygos vein, which opens into the vena cava superior or pre-caval vein, _i.e._ the superior cardinal just before the latter debouches into the heart. This one posterior cardinal is usually on the right side; but it may be on the left side, for instance in _Trichosurus vulpecula_. In _Halmaturus bennettii_ there are two azygos veins, one left and one right, of which the left is rather the larger.[36]