THE ALIMENTARY CANAL 173 



which are larger than the <'hicf cells. These cells are oval or 

 polyji;onal, their finely irraniilar cytoplasm has an affinity for acid 

 (lyes, and the large spherical nucleus is centrallx located. Parietal 

 cells are associated with the production of the hydrochloric acid 

 present in the gastric juice, and are often called oxyntic or acid 

 cells for this reason. 



The pyloric glands intermingle with the fundic type in that 

 portion of the stomach near the small intestine. The transition 

 is gradual; parietal cells become less and less numerous and finally 

 no longer appear in the more typical pyloric glands. The gastric 

 pits become longer, and the secreting tubules become relatively 

 shorter imtil they are about as long as the excretory portions but 

 are more twisted than in the fundic region. The cells of the pyloric 

 glands are distinctly mucous in appearance. 



The tunica ])r()i)ria of the stomach extends in between and around 

 the secreting tubules of the gastric glands, and the muscularis 

 mucosa lies just below the deepest ends of the secreting tubules. 

 Scattered diffusely throughout the tunica proi)ria are lymphocytes, 

 but in some regions solitary lymph nodules occur. The submucosa 

 is typical, being composed of loose fibroelastic connective tissue 

 whose longitudinal ridge-like extensions form rugte. The muscu- 

 laris may have three layers in some regions, an inner oblique, a 

 middle circular, and an outer longitudinal layer. In the j)yloric 

 region the two inner layers are thickened to form a sphincter muscle. 

 The adventitia is composed of a coat of loose fibroelastic connective 

 tissue enclosed by a single layer of mesothelium. 



Although the stomach functions mainly as a temporary ])lace 

 for storing food, some digestion occurs as a result of glandular action. 

 Proteins may be converted into proteoses and peptones by the action 

 of pepsin, while another enzyme, rennin, if present in the gastric 

 juice, plays a role in the digestion of milk. 



The Small Intestine. — Continuing from the stomach, the digestive 

 tract becomes a smaller tube whose internal surface is much increased 

 b}' numerous folds. The muscularis coats, both the circular and 

 longitudinal, tend to encircle the tract in a gentle spiral. In general, 

 with the exception of the birds and mammals, the glands that are a 

 prominent feature in the stomach mucosa are lacking in the intestine 

 where the main function is absorption of the material digested by 

 the secretions passing into it from the stomach and from the pan- 

 creas and liver. The epithelium of the mucosa may contain numer- 

 ous mucous secreting cells which cover the membrane with their 

 lubricating secretion. 



