The digestive apparatus as a whole presents, in the first place, a segment designed to convey the food to the stomach, the œsophagus--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 œsophagus, 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.)]
A distinct digestive segment may even be entirely wanting, owing to its failure to differentiate from the œsophagus on the one hand and from the endgut on the other. In such forms the entire digestive canal appears as a tube of uniform caliber extending from mouth to anus. It is necessary to begin with these simple structural conditions in order to obtain a clear conception of the disposition of the viscera in the adult human abdomen. Such simple arrangement of the alimentary tract is found in the embryo of man and of the higher vertebrates, and similar rudimentary types are encountered, as the permanent condition, in some of the lower forms. These latter are especially valuable for purposes of study, because they afford an opportunity of examining directly, as macroscopic objects, structural conditions which are found only as temporary embryonal stages during the development of the higher mammalia (Fig. 43).