1. Stomach Forms Depending on the Influence exerted by the Habitual Amount of Food required by the Animal.—The greater the activity of tissue changes is, the greater will be the amount of food required and the more pronounced will be the gastric dilatation of the alimentary canal. Hence in the higher vertebrates generally the stomach appears as a large and more sac-like dilatation than in lower forms, such as fishes and amphibia and some reptilia, in which the stomach is usually smaller and fœtal in shape, forming a slight longitudinal dilatation situated in the long axis of the body. An example is seen in the stomach of Coluber natrix (Fig. 52). Frequently this slight dilatation is scarcely differentiated from the œsophagus at the cephalic and from the small intestine at the caudal end. Many batrachians and perennibranchiates possess this form among the amphibia. It is also encountered in the pickerels, the Cyprini, and in Labrus among fishes, and in some saurians and ophidia among reptiles. It constitutes a slight advance in development over the earliest stage represented, as we have seen, by the nearly uniform and undifferentiated alimentary tube of amphioxus and the cyclostomata.

Fig. 53.—Human adult. Mucous surface of œsophageo-gastricjunction. (Columbia University Museum, No. 1842.)

 

Fig. 55.—Series of sections showing humanpyloric valve and gastro-duodenal junction:

1. Stomach of fœtus at term in section.

2. Adult pyloric valve, gastric surface.

3. Adult pyloric valve and gastro-duodenaljunction in section.

4. Fœtal gastro-duodenal junc­tion in section.Entrance of biliary and pancreatic ducts onsummit of papilla of duo­denum. (ColumbiaUni­ver­sity Mu­seum, No. 1851.)

Fig. 54.—Human adult. Pyloro-duodenal junction andpyloric valve in section. (Columbia University Museum, No.1842.)

 

This transition of the fœtal form to the more advanced secondary types of the stomach is marked by the development of two important structural features:

(a) The separation in the interior of the canal of the stomach from the intestine by the appearance of a ring-shaped valve, the pyloric valve. This is produced by an aggregation of the circular muscular fibers of the intestine at this point, and causes a projection of the mucous membrane into the lumen of the canal. It begins to appear in the fishes (pickerel, sturgeon, etc.), is found in most amphibia and is regularly present in the stomach of the higher vertebrates. (Figs. 54 and 55.) A good example of the ring-shaped plate of the pylorus with central circular opening produced by the aggregation of the circular muscular fibers is afforded by the view of the interior of the cormorant’s stomach given in [Fig. 69]. The opposite or œsophageal extremity of the stomach is less well differentiated from the afferent tube of the œsophagus.

There is no aggregation of muscular circular fibers in this situation and no valve. Superficially the external longitudinal muscular fibers of the œsophagus pass continuously and without demarcation into the superficial gastric muscular layer. The separation between œsophagus and stomach is, however, marked on the mucous surface by a well-defined line along which the flat, smooth and glistening œsophageal tesselated epithelium passes into the granular cuboidal epithelium of the gastric mucous membrane. The œsophageo-gastric junction in the adult human subject is shown in Fig. 53.

(b) The pyloric end of the stomach makes an angular bend, while the rest of the organ remains in the original vertical position in the long axis of the body. An example of this condition is presented by the stomach of Scincus ocellatus ([Fig. 56]; cf. also [Fig. 202]).

The purpose of both of these provisions is to retain the gastric contents for a longer time within the stomach. Hence this form is encountered especially in those fishes and amphibians in which the nutritive demands require a more complete digestion of the food taken. This is the case, for example, in Gobius ([Fig. 57]), the plagiostomata ([Fig. 58]), and many saurians. The same transitory stomach form is even found in some mammals, as the seals. [Fig. 59] shows the stomach in Phoca vitulina, the harbor seal. With the further increase in the demand for complete digestion of the food the entire stomach assumes a transverse position to the long axis of the body. This may occur while the stomach still retains its primitive tubular form, as in most chelonians (Fig. 60). In others the change in position occurs after the gastric dilatation has assumed the sac-like form, as in many land-turtles, crocodiles, some batrachians and all higher vertebrates ([Figs. 61] and [62]). This transverse position, at right angles to the long axis of the body, forms the starting point for the derivation of all secondary types of stomach.

Fig. 56.—Alimentary canal of Scincus ocellatus. Pyloric extremity of the slightly marked gastric dilatation presents an angular bend. (Nuhn.)   Fig. 57.—Alimentary canal of Gobius niger. (Nuhn.)   Fig. 58.—Alimentary canal of shark. (Nuhn.)
Fig. 59.—Stomach of Phoca vitulina, harbor seal. (Columbia University Museum, No. 600.)   Fig. 60.—Stomach of Pseudemys elegans, pond turtle. (Columbia University Museum, No. 1710.)
Fig. 61.—Stomach of Chelydra serpentina, snapping turtle. (Columbia University Museum, No. 1852.)   Fig. 62.—Same in section.