[Illustration: FIG. 54.--Human adult. Pyloro-duodenal junction and pyloric valve in section. (Columbia University Museum, No. 1842.)] [Illustration: FIG. 55.--Series of sections showing human pyloric valve and gastro-duodenal junction: 1. Stomach of foetus at term in section. 2. Adult pyloric valve, gastric surface. 3. Adult pyloric valve and gastro-duodenal junction in section. 4. Foetal gastro-duodenal junction in section. Entrance of biliary and pancreatic ducts on summit of papilla of duodenum. (Columbia University Museum, No. 1851.)] There is no aggregation of muscular circular fibers in this situation and no valve. Superficially the external longitudinal muscular fibers of the oesophagus pass continuously and without demarcation into the superficial gastric muscular layer. The separation between oesophagus and stomach is, however, marked on the mucous surface by a well-defined line along which the flat, smooth and glistening oesophageal tesselated epithelium passes into the granular cuboidal epithelium of the gastric mucous membrane. The oesophageo-gastric junction in the adult human subject is shown in Fig. 53. [Illustration: FIG. 53.--Human adult. Mucous surface of oesophageo-gastric junction. (Columbia University Museum, No. 1842.)] (_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). [Illustration: FIG. 56.--Alimentary canal of _Scincus ocellatus_. Pyloric extremity of the slightly marked gastric dilatation presents an angular bend. (Nuhn.)] 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. [Illustration: FIG. 57.--Alimentary canal of _Gobius niger_. (Nuhn.)] [Illustration: FIG. 58.--Alimentary canal of shark. (Nuhn.)]