CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 419 



and this is supported by the fact that when the secretion of gastric 

 juice is actively going on, the amount of chlorides leaving the 

 blood by the kidney is proportionately diminished ; but nothing 

 definite can at present be stated as to the mechanism of that 

 decomposition. And even admitting that the sodium chloride of 

 the body at large is the ultimate source of the chlorine element of 

 the acid, it appears more likely that that element should be set 

 free in the stomach by the decomposition of some highly complex 

 and unstable chlorine compound previously generated, than that 

 it should arise by the direct splitting-up of so stable a body as 

 sodium chloride at the very time when the acid is secreted. 



In the frog, while pepsin free from acid is secreted by the 

 glands in the lower portion of the oesophagus, an acid juice is 

 afforded by glands in the stomach itself, which have accordingly 

 been called oxyntic (c^vveuv to sharpen, acidulate) glands; but 

 these oxyntic glands appear also to secrete pepsin. In the 

 mammal the isolated pylorus secretes an alkaline juice ; in fact, 

 the appearance of an acid juice is limited to those portions of the 

 stomach in which the glands contain both ' chief ' or ' central/ and 

 ' ovoid ' or 'border' cells. Now from what has been previously said 

 there can be no doubt that the chief cells do secrete pepsin. On 

 the other hand there is no evidence whatever of the formation of 

 pepsin by the ' border ' or ' ovoid ' cells, though this was once 

 supposed to be the case and these cells were unfortunately 

 formerly called 'peptic' cells. Hence it has been inferred that the 

 border cells secrete acid ; but the argument is at present one of 

 exclusion only, there being no direct proof that these cells actually 

 manufacture the acid. 



The rennin appears to be formed by the same cells which 

 manufacture the pepsin, that is, by the chief cells of the fundus 

 generally and to some extent by the cells of the pyloric glands. 

 We may add that we have evidence of the existence of a zymogen 

 of rennin analogous to the zymogen of pepsin or of trypsin. 



The mucus which is present as a thin layer over the surface 

 of the fasting stomach, and which especially in herbivorous animals 

 is increased during digestion, comes as we have said from the 

 mucous cells which line the mouths of the several glands and 

 cover the intervening surfaces. 



242. We previously called attention to the fact that in the 

 case of the stomach the absorption of the products of digestion 

 largely increased the activity of the secreting cells. This has led 

 to the idea that one effect of food is to 'charge' the gastric cells with 

 pepsinogen, and that certain articles of food might be considered 

 as especially peptogenous, i.e. conducive to the formation of pepsin. 

 Such a view is tempting, but needs as yet to be more fully sup- 

 ported by facts. 



243. Seeing the great solvent power of both gastric and 

 pancreatic juice, the question is naturally suggested, Why does 



272 



