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 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. [Illustration] 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.