352 THE ACT OF SECRETION. [BOOK n. 



between, are mucous cells secreting into the stomach generally a 

 small, but under abnormal conditions a large, amount of mucus, 

 which has its uses but is not an essential part of the gastric juice. 

 In the salivary glands we can hardly suppose that the long stretch 

 of characteristic columnar epithelium which reaches from the 

 alveoli to the mouth of the long main duct serves simply to 

 furnish a smooth lining to the conducting passages ; but we have 

 as yet no clear indications of what the function of this epithelium 

 can be. 



203. Before we leave the mechanism of secretion there are 

 one or more accessory points which deserve attention. 



In treating just now of the gastric glands we spoke as if pepsin 

 were the only important constituent of gastric juice, whereas, as we 

 have previously seen, the acid is equally essential. The formation 

 of the free acid of the gastric juice is very 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 proportionately diminished ; but nothing 

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

 decomposition. 



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 {b^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 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. 



204. Seeing the great solvent power of both gastric and 

 pancreatic juice, the question is naturally suggested, Why does 

 not the stomach digest itself ? After death, the stomach is 



