SECRETION OF SALIVA AND GASTRIC JUICE. 277 



the alveolus, or what corresponds to the alveolus, is wrapped round with 

 plain muscular fibres, the contraction of which appears to force the secretion 

 outward, the idea has been suggested that in glands, such as we are now 

 considering, the cell-substance, making use of " protoplasmic " contraction 

 instead of actual muscular contraction, may force part of the cell contents 

 into the lumen of the alveolus. Such a mode of secretion would be com- 

 parable to the ejection of undigested material, or "excretion," by an amoeba. 

 But we have no satisfactory evidence in favor of this view. 



210. Throughout the above we have spoken as if the secretion were 

 furnished exclusively by the cells of the alveoli or secreting portion of the 

 gland, as if the epithelial cells lining the ducts, or conducting portion of 

 the gland, contributed nothing to the act. In the gastric glands the slender 

 cells lining the mouths of the glands (which correspond to ducts) and cover- 

 ing the ridges between, are raucous cells secreting into the stomach gener- 

 ally 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 character- 

 istic columnar epithelium which reaches from the alveoli to the mouth of 

 the long main duct serves simply to furnish a smooth lining to the conduct- 

 ing passages ; but we have as yet no clear indications of what the function 

 of this epithelium can be. 



211. 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 definite 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 of the 

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

 stomach itself, which have accordingly been called oxyntic (dzui>iv,t<j 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 "bor- 

 der" 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 ex- 

 clusion 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 evi- 

 dence of the existence of a'zymogen of rennin analogous to the zymogen of 

 pepsin or of trypsin. 



212. Seeing the great solvent power of both gastric and pancreatic juice 

 the question is naturally suggested, Why does not the stomach digest itself? 



