DIGESTION 



181 



FIG. 84 



ZDr. 



SDi 



layer of smooth fibers. The mucous membrane lines the entire stomach, 

 but differs somewhat in the fundus and the antrum. It contains the cells 

 which secrete the digestive juices of the organ. Judging by the varied 

 nature of these juices, peptic, rennic, acrid, etc., there must be at least 

 three sorts of cell-protoplasm present; it is not easy, however, to defi- 

 nitely discriminate more than two sorts of cells. The cells of the glands 

 of the fundus have both these sorts of cells. One sort are irregular epi- 

 thelial cells almost surrounding the capillary lumen of the straight tubular 

 glands, and known as the central (or chief or principal) cells. The other 

 sort, much less numerous, are irregular, polyhedral cells, peripheral 

 to the central cells, lying on the parietes of the gland and hence called 

 parietal cells. The latter variety are connected with the gland's lumen 

 only by very fine channels running between the central cells (Fig. 84). 

 These glands have wide mouths common 

 to several tubules, and these openings 

 give the stomach its reticulated appear- 

 ance. They are lined by columnar epithe- 

 lium and seem to be found in all parts 

 of the stomach, but much less numerously 

 in the antrum. It is by no means certain 

 what the exact functions of these two 

 kinds of cells are, but it is probable that 

 the columnar central cells secrete by way 

 of pepsinogen the enzyme pepsin, and 

 that the parietal cells produce the free 

 hydrochloric acid characteristic of the 

 stomach. 



The cells of the glands of the antrum 

 are somewhat more cuboidal than those 

 of the fundus, the lumens of the glands are 

 longer and narrower and no polyhedral 

 parietal cells are present. Toward the 

 pyloric valve these glands become larger 

 and similar in all apparent respects with 

 Brunner's glands, so called, of the duodenum. It is likely that the colum- 

 nar and cuboidal central cells all over the stomach secrete both pepsin 

 and "rennin/' but the details as to which secretes which are as yet unde- 

 veloped. Besides these three sorts of cells, small solitary lymph-follicles 

 are scattered sparingly throughout the gastric mucosa. They are similar 

 to those found in the intestine but smaller, and are like the follicles of 

 lymph-nodes. 



To summarize the functions so far as known at present of the various 

 glands of the stomach : The central cells of the glands both in the fundus 

 and in the antrum probably secrete pepsin and rennin, some of the 

 granules (zymogens) apparent in these cells after resting being precursors 

 of pepsin and some precursors of rennin. The glands of the antrum 

 secrete a form of pepsin probably less complete and less active than that 



Longitudinal section of the stomach 

 of the canary bird, to show the two 

 parts, chemical and mechanical : D, the 

 duodenum; SDr, small simple tubular 

 glands; ZDr, compound tubular 

 glands. 5/1 (Oppel.) 



