278 THEORY OF SECRETION. [BOOK n. 



obscure and many ingenious but unsatisfactory views have been 

 put forward to explain it. It seems natural to suppose that 

 it arises in some way from the decomposition of sodium chloride 

 drawn from the blood; 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 pro- 

 portionately diminished; but nothing definite can at present be 

 stated as to the mechanism of that decomposition, though an 

 organic acid such as lactic, which as we have seen appears in the 

 juice, might under certain conditions succeed in decomposing 

 chlorides. 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 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 (6t;vveiv 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 there can be no 

 doubt that the chief cells do secrete pepsin. During life the 

 granules visible in the living chief cells abound or are scanty 

 according as pepsin is about to be or has been secreted, and 

 after death they contain pepsin (or pepsinogen), and that in 

 proportion to their richness in granules. No such correspondence 

 can be seen in the ' border ' or ' ovoid ' cells. Hence it has been 

 inferred that the border cells secrete acid; but the argument is one 

 of exclusion only, there being no direct proofs of these cells actually 

 manufacturing the acid. 



The rennet ferment 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 the rennet ferment analogous to the zymogen of pepsin 

 or 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 from the mucous cells which line 

 the mouths of the several glands and cover the intervening surfaces. 

 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 



