SYSTEM.] MAMMALIA 363 glands are members. The solitary glands are found scat tered irregularly throughout the whole intestinal tract ; the agminated, on the other hand, are always confined to the small intestine, and are most abundant in its lower part. They are subject to great variation in number and in size, and even in different individuals of the same species, and also differ in character at different periods of life, becoming atrophied in old age. The distinct glands situated outside of the walls of the intestinal canal, but which pour their secretion into it, are iver. the pancreas and the liver. The latter is the most important on account of its size, if not on account of the direct action of its secretion in the digestive process. This large gland, so complex in structure and function, is well developed in all mammals, and its secreting duct, the bile duct, always opens into the duodenum or that portion of the canal which immediately succeeds the stomach. It is situated in the right side of the abdomen in contact with the diaphragm and the stomach, but varies greatly in relative size, and also in form, in different groups of mammals. In most mammals a gall-bladder, consisting of a pyriform cliverticulum from the gall duct, is present, but in many it is wanting, and it is difficult to find the rationale of its presence or absence in relation to use or any other cir cumstance in the animal economy. The descriptions of the livers of various animals to be- met with in treatises or memoirs on comparative anatomy are very difficult to understand for want of a uniform system of nomenclature. The difficulty usually met with arises from the circumstance that this organ is divided sometimes, as in Man, Ruminants, and the Cetacea, into two main lobes, which have been always called respectively right and left, and in other cases, as in the lower Monkeys, Carnivora, Insectivora, and many other orders, into a larger number of lobes. Among the latter the primary division usually appears at first sight tripartite, the whole organ consisting of a middle, called "cystic " or "suspensory " lobe, and two lateral lobes, called respectively right and left lobes. This introduces confusion in describing livers by the same terms throughout the whole series of mammals, as the right and left lobes of the Monkey or Dog, for instance, do not correspond with parts designated by the same name in Man and the Sheep. There are, moreover, conditions in which neither the bipartite nor the tripartite system of nomenclature will answer, which we should have considerable difficulty in describing without some more general system. In order to arrive at such a system it appears desirable to consider the liver in all cases as primarily divided by the umbilical vein (seefig.14, u) into two segments, right and left. This corresponds with its development and with the condition characteristic of the organ in the inferior classes of vertebrates. The situation of this division can almost always be recognized in adult animals by the persistence of some traces of the umbilical vein in the form of the round ligament, and by the position of the suspensory ligament. When the two main parts into which the liver is thus divided are entire, as in Man, the Ruminants, and Cetacea, they may be spoken of as the right and left lobes ; when fissured, as the right and left segments of the liver, reserv ing the term lobe for the subdivisions. This will involve no ambiguity, for the terms right and left lobe will no longer be used for divisions of the more complex form of liver. In the large majority of mammals each segment is further divided by a fissure, more or less deep, extending from the free towards the attached border, which are called right and left lateral fissures (fig. 14, rlf and llf). When these are more deeply cut than the umbilical fissure (u), the organ has that tripartite or trefoil-like form just spoken of, but it is easily seen that it is really divided into four regions or lobes, those included between the lateral fissures being the right and left central (re and lc) separated by the umbilical fissure, and those beyond the lateral fissures on each side being the right and left lateral lobes (rl and II). The essentially bipartite character of the organ and its uniformity of construction throughout the class are thus not lost sight of, even in the most complex forms. The left segment of the liver is rarely complicated to any further extent, except in some cases by minor or secondary fissures marking off small lobules, generally inconstant and irregular, and never worthy of any special designation. On the other hand, the right segment is usually more complex. The gall bladder, when present, is always attached to the under surface of the right central lobe, sometimes merely applied to it, in other cases deeply embedded in its substance. In many cases the fossa in which it is sunk is continued to the free margin of the liver as an indent, or even a tolerably deep fissure (cf). The portal fissure (p), through which the portal vein and Fia. 14.- Diagrammatic Plan of the Inferior Surface of a Multilobwl Liver of a Mammal. The posterior or attached border is uppermost, , umbilical vein of the fostus, represented by the round ligament, in the adult, lying in the um bilical fissure ; dv, the ductus vcnosus ; t>c, the inferior vena ca^a ; p, the vena portoe entering the transverse fissure ; llf, the left lateral fissure ; //, the right lateral fissure ; cf, the cystic fissure ; II, the left lateral lobe ; lc, the left central lobe ; re, the right central lobe ; rl, the right lateral lobe ; s, the Spigelian lobe ; c, the caudate lobe ; g, the gall bladder. hepatic artery enter and the gall duct emerges from- the liver, crosses this lobe transversely, near the attached border of the liver. The right lateral lobe always has the great vena cava (vc) either grooving its surface or tunnel ling through its substance near the inner or left end of its attached border ; and a prolongation of the lobe to the left, between the vein and the portal fissure, sometimes a mere flat track of hepatic substance, but more often a prominent tongue-shaped process, is the so-called " Spigelian lobe "(*). From the under surface of the right lateral lobe a portion is generally partially detached by a fissure, and called the " caudate lobe " (c). In Man this is almost obsolete, but in most mammals it is of considerable magnitude, and has very constant and characteristic relations. It is connected by an isthmus at the left (narrowest or attached) end to the Spigelian lobe, behind which isthmus the vena cava is always in relation to it, channelling through or grooving its surface. It generally has a pointed apex, and is deeply hollowed to receive the right kidney, to the upper and inner side of which it is applied. Considerations derived from the comparatively small and simple condition of the liver of the Ungulates, compared with its large size and complex form in the Carnivora, have led to the perhaps too hasty generalization that the first type is related to a herbivorous and the latter to a carnivorous diet. The exceptions to such a proposition are very numerous. The fact of the great difference between the liver of the Cetacea and that of the Seals cannot be
accounted for by difference of habits of life, though it