For the purpose of studying the adult human peritoneum it is in the first place absolutely necessary to obtain a correct appreciation of the disposition of the chief viscera within the abdominal cavity and of their mutual relations. In the second place the visceral vascular supply of the abdomen must be carefully considered in order to correctly appreciate certain important relations of the peritoneal membrane. A review of the visceral contents of the abdomen shows that we have to deal chiefly with the divisions of the alimentary tract below the oesophagus and the structures directly derived from the same, as liver and pancreas, or associated topographically with the alimentary canal, as the spleen. Portions of the urinary and reproductive systems situated within the abdominal and pelvic cavities will also require consideration. The digestive apparatus as a whole presents, in the first place, a segment designed to convey the food to the stomach, the oesophagus--supplemented in mammalia by the special apparatus of the mouth and pharynx, in which the food is mechanically prepared for digestion by chewing and mixed with the secretion of the salivary glands. The _digestive apparatus proper_, succeeding to the oesophagus, is usually divisible into two sections differing in function and structure. 1. The STOMACH, a short sac-like dilatation, in which chiefly nitrogenous material is digested. 2. The SMALL INTESTINE, a long and usually much convoluted narrow tube, chiefly devoted to the digestion of starches, fats and sugars, and to the absorption of the digested matters. In some of the lower vertebrates, as the _Cyclostomata_ (Fig. 43), _Esox_, _Belone_, etc., among fishes (Fig. 48), _Necturus_ and _Proteus_ among amphibians (Figs. 50 and 51), the separation of the digestive portion of the alimentary tract into stomach and small intestine is not clearly defined (vide infra, p. 43). [Illustration: FIG. 43.--Entire alimentary canal of the lamprey, _Petromyzon marinus_, below the pericardium. (Columbia University Museum, No. 1575.)] [Illustration: FIG. 48.--Alimentary canal of _Belone_, pickerel. (Nuhn.)] [Illustration: FIG. 50.--_Necturus maculatus_, mud-puppy. Alimentary canal and appendages. (Columbia University Museum, No. 1454.)] [Illustration: FIG. 51.--Alimentary canal of _Proteus anguineus_. (Nuhn.)]