(iv) Mucosa: It is the innermost layer which is comparatively thick and secretory in function. The epithelial lining is sunken to form y-shaped gastric pits into which the compound gastric glands open. Each gastric gland is a compound tubular gland differentiated into the basal glandular part and a short narrow neck opening into the gastric pit. The glandular part consists mainly of three types of cells, the central or chief cells or zymogen cells which secrete pepsinogen, the parietal or oxyntic cells that secrete hydrochloric acid, and the cells that secrete mucus. The gastric glands of the fundus part are well developed and secrete enzymes, while those of the cardiac and pyloric stomach are short with long ducts and secrete mucus.

Histology of the intestine : The wall of the intestine shows the following four layers (going from outside towards the inner side). (Fig. 16.11)

(i) Serosa is made up of a single outer layer of flat epithelial cells.