Fig. 41.—Different forms of the stomach in Mammals. A, Dog; B, Mus decumanus; 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
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 Mitchell has brought forward some reasons for associating 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.
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
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
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[[31]] and Lankester,[[32]] in whose memoirs references to the older literature will be found. The principal 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[[33]] 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 pass 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.