This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
* Sir E. Home, Lectures on Comparative Anatomy, vol. i. p. 225.
(2312). The rest of the alimentary canal in most quadrupeds, like that of Man, is divisible into the small and the large intestines, - the division between the two being marked by one or even two appendages, called respectively the caecum and the appendix vermiformis.
(2313). The small intestines require no particular description, as in all minor circumstances, such as their proportionate length and diameter, or in the number and arrangement of the valvulce conniventes, they do not differ from the human. The large intestines, however, offer very great variations of structure, and will therefore merit our more attentive consideration; we shall accordingly lay before the reader the following resume of the principal facts connected with this subject, as given by the indefatigable Cuvier *.
(2314). In Man, the Orang (Simia), and the Wombat (Phascolomys), both caecum and vermiform appendage are met with.
(2315). In the other Qjjadrumana, the Digitigrade Carnivora, the Marsupialia, the Rodentta, the Pachydermata, the Ruminantia, the Solipeds, and the Amphibious Mammals, there is a caecum without any vermiform appendage.
(2316). Neither caecum nor appendix vermiformis is found in the Edentata, the Plantigrade Carnivora, nor in the Cetacea.
(2317). Numerous exceptions, of course, occur to the above summary; but it would be useless to notice them in a survey so general as the present.
(2318). Even where no caecum exists, the separation between the large and small intestines is generally indicated by a valve (ilio-colic) formed by the lining membrane of the bowel: this, for example, is the case in the Sloths and Armadillos.
(2319). In all the Mammalia that possess a caecum, this organ appears to be a prolongation of the colon beyond the point at which the small intestine enters its cavity. The caecum thus formed varies materially, both as relates to its size, shape, and structure: in animals that live upon vegetables, and even in some that are omnivorous, it is generally very large, gathered into sacculi, and often distinctly glandular; but in such as live upon flesh it is always small, and its cavity smooth, resembling a small intestine.
(2320). The assistant chylopoietic viscera, namely, the liver, the pancreas, and the spleen, are constructed upon the same principles as in the human subject, and, except in a few minor circumstances, offer little to arrest our particular notice.
(2321). The liver occupies the same position as in Man, being principally situated in the right hypochondrium, where it is securely suspended by broad folds of peritoneum connecting it to the abdominal surface of the diaphragm and to the circumjacent parts. It is most frequently, especially in the more active carnivorous families, divided by deep fissures into several lobes, a disposition whereby the free movement of this part of the body is evidently facilitated. The gall-bladder, when present, which is not invariably the case, receives the bile indirectly through a cystic duct derived from the hepatic; so that the biliary fluid, poured into the duodenum through a ductus communis choledochus, is derived either immediately from the liver, or is regurgitated from the gall-bladder as occasion requires.
* Lecjons d'Anat. Comp. iii. p. 465.
(2322). The pancreas resembles the human in every particular; and its secretion enters the duodenum at the same point as that of the liver.
(2323). The spleen is always attached to the stomach by a dupli-cature of the peritoneal lining of the abdomen, and is organized in the same manner as that of Man, except in the Cetacea, where this viscus is divided into several small portions quite distinct from each other.
(2324). The system of the vena portae is made up of the venous trunks derived from the spleen, the stomach, the pancreas, and the intestinal canal: these all unite to form one large central trunk, which, after entering the liver, again divides and subdivides minutely in that viscus, and furnishes the venous blood, from which the bile is principally, if not entirely, elaborated.
(2325). The peritoneum, or the serous membrane lining the abdominal cavity, forms in the Mammalia a shut sac, and by its numerous inflexions invests all the chylopoietic viscera, forming broad mesenteric folds to support the intestines; it thus encloses between its laminae the entire system of mesenteric vessels, and also the lacteals derived from the alimentary canal: as to the rest, its structure and disposition, even to the formation of the omental sacs, differ in no important respect from what is found in the human body.
(2326). The chyle, the result of the digestive process, is taken up from the mucous lining of the intestinal canal by innumerable microscopic orifices that form the commencement of the lacteal system, which in the Mammalia seems to assume its most perfect development. This important system of absorbent vessels consists of slender canals enclosed between the two layers of the mesentery, to the root of which they converge from all the tract of the intestine. The valves formed by the lining membrane of these tubes are in Mammals so numerous and perfect that it is no longer possible to inject them from trunk to branch. Before terminating in the thoracic duct, these vessels permeate numerous "mesenteric glands," as they are called, by means whereof they appear to communicate freely with the venous system; but the bulk of the matter absorbed enters a kind of reservoir called the "receptaculum chyli," whence, by means of the thoracic duct, the chyle is conveyed to be mixed up with the mass of circulating fluid, and is ultimately poured into the vena innominata at the junction of the jugular and subclavian veins of the left side of the body.
(2327). The lymphatic system of Mammals, as far as it has been studied, conforms in its arrangement to that of Man.