268 DIGESTION 



tion can be demonstrated in the mucous membrane only after the latter appear. 

 (3) If different parts of the stomach mucosa be extracted, it is found that the 

 quantity of pepsin shows no dependence upon the number of parietal cells 

 in the different parts, but varies in direct proportion to the number of 

 chief cells. 



How far the parietal cells participate in the formation of pepsin must be 

 regarded as still an open question. In various lower vertebrates whose gastric 

 glands possess cells of only one kind, it has been observed that both pepsin and 

 hydrochloric acid are produced. But we cannot draw any positive conclusion 

 with respect to the more differentiated gland of the higher vertebrates from 

 this discovery. 



Weight for weight the pyloric mucosa produces much less pepsin than does 

 that of the fundus which is quite intelligible when we consider that the fundic 

 glands are much more thickly set than the pyloric glands, and also that the 

 length of the former is considerably greater than that of the latter. 



The amount of rennin of the gastric juice during the different stages of 

 digestion always runs parallel to the amount of pepsin. From this and other 

 facts it appears permissible to conclude that although rennin is not identical 

 with pepsin (page 250), it is formed in the pyloric glands as well as in the 

 chief cells of the fundus glands. Whether it originates in the parietal cells 

 also cannot yet be decided. 



Views are widely divergent as to the seat of hydrochloric acid production. 

 While some assume that it is produced in all the gland cells of the stomach, 

 others suppose that it originates only in the parietal cells of the fundic glands. 



As a matter of fact it appears to be shown with a fair degree of certainty 

 that the pyloric glands do not produce hydrochloric acid, for in the secretion 

 of the isolated pyloric sac one finds in exceptional cases an alkaline reaction 

 and, what is important also, the mucous membrane of the blind sac exhibits 

 at such times perfectly normal properties throughout. Moreover, it is stated 

 that the free surface of the mucosa gives an acid reaction only in places 

 where glands with two kinds of cells are found; at other places it reacts 

 alkaline. 



After considering all the facts obtainable bearing on this subject, Heiden- 

 hain came to the conclusion that hydrochloric acid is formed in the parietal 

 cells of the fundic glands. It must be stated, however, that he reached this 

 conclusion by the process of exclusion and not by direct evidence. 



The cells of the gastric glands in the process of secretion undergo morpho- 

 logical changes which have been followed by Langley on fresh preparations. In 

 the fasting condition the chief cells are strongly granular, but during digestion 

 are clear i. e., the outer zone (about one- third to one-half of the entire 

 cell) shows no granules; they occur only along the luminal border of the 

 cell. Extracts from different parts of the mucosa yield pepsin in greater 

 abundance, the richer the glands in granules. 



We find therefore the same state of affairs in the cells of the gastric glands 

 as in the salivary glands. During the fasting period a substance is laid down 

 in the chief cells, which in fresh preparations appears in the form of granules. 

 During the act of secretion this substance is gradually used up ; at the same 



