THE SECRETION OF THE DIGESTIVE JUICES 



377 



the clear outer zone. After prolonged secretion the zymogen granules 

 may be entirely absent from the cells, and only a narrow rim of 

 prozymogen granules can be seen around the lumen. 



In one respect the pancreas differs remarkably from the salivary 

 glands namely, in the presence of the islets of Langerhans 

 characteristic groups of small 

 polygonal cells, richly sup- 

 plied with bloodvessels, but 

 not arranged in the form of 

 alveoli. Some observers state 

 that they are remarkably in- 

 creased in size, and even in 

 number when the pancreas is 

 caused to secrete actively by 

 repeated injections of secretin, 

 and also in starvation. But 

 it has been shown that this 

 conclusion was based upon 

 faulty methods of counting 

 the islets, and even of identi- 

 fying the islet cells. There 

 appears to be no foundation 

 for the view that they are 

 derived from the ordinary 

 secreting cells, and that they 

 can, in turn, give rise to new 

 alveoli by a process of pro- 

 liferation. It is far more 

 probable that they are inde- 

 pendent structures, with a 

 different function from the 

 pancreatic alveoli (p. 6^,8). 



Changes in the Glands of 

 the Stomach during Secre- 

 tion. The mucous membrane 

 of the stomach is covered 

 with a single layer of colum- 

 nar epithelium, largely con- Fig. 156. A Fundus Gland prepared by 

 sisting of mucigenoUS goblet- Gol S i>s Method, showing the Moda of Com- 



,, J T j A , . , munication of the Parietal Cells with the 



IS Studded Wltn Gland-Lumen (Schafer, after E. Muller). 



minutt pits, into which open 



the ducts of the peptic and pyloric glands, the ducts being lined 

 with cells just like those of the general gastric surface. Three 

 varieties of gastric glands have been distinguished: (i) The 

 glands of the cardia. In man these occupy a small portion of 

 the mucous membrane at the cardiac end, near the orifice of 



Fig. 155. Fig. 156. 



Fig. 155. A Fundus Gland of Simple Form 

 from the Bat's Stomach (Osmic Acid Pre- 

 paration) (Langley). c, Columnar epithe- 

 lium of the surface; n, neck of the gland 

 with chief or central and parietal cells; 

 /, base, occupied only by chief cells, which 

 show the granules accumulated towards 

 the lumen of the gland. 



