DIGESTION 325 



to show that in the fresh pancreas not trypsin, but a mother- 

 substance, which has been named trypsinogen, is present, 

 and that the latter yields trypsin, gradually when the pan- 

 creas is simply allowed to stand, more rapidly when the 

 dilute acid is used. The natural secretion of the gland is 

 active when the gland-cells contain no ferment, therefore 

 during secretion the trypsinogen must be changed into 

 trypsin. 



Similarly, a glycerine extract of a fresh gastric mucous 

 membrane is inert as regards proteids, or nearly so. But 

 if the mucous membrane has been previously treated with 

 dilute hydrochloric acid, the glycerine extract is active, as is 

 an extract made with acidulated glycerine. Here we must 

 assume the existence in the gastric glands of a mother- 

 substance, pepsinogen, from which pepsin is formed. Only 

 the chief cells of the cardiac, and the similar if not identical 

 cells of the pyloric glands, are believed to manufacture the 

 pepsin-forming substance. The ovoid cells of the former 

 are supposed to secrete the hydrochloric acid. The evidence 

 on which this belief is based is as follows : 



The pyloric glands, in which in most situations no ovoid 

 cells are to be seen, secrete pepsin, but no acid. The 

 pyloric portion of the stomach has been isolated, the con- 

 tinuity of the alimentary canal restored by sutures, and the 

 secretion of the pyloric pocket collected. It was found to 

 be alkaline, and contained pepsin. The glands of the frog's 

 oesophagus, which contain only chief cells, secrete pepsin, 

 but no acid. It seems fair to conclude that the chief cells 

 of the cardiac glands in the mammal secrete none of the 

 free hydrochloric acid, but certainly some of pepsin. But it 

 does not follow that all the pepsin is formed by these cells, 

 although it would seem that all the hydrochloric acid must 

 be secreted by the only other glandular elements present, 

 'the ovoid or * border ' cells. And, indeed, the glands in 

 the fundus of the frog's stomach, which are composed only 

 of ovoid cells, while secreting much acid, also form some 

 -pepsin, although far less than the oesophageal glands. 



Attempts made to demonstrate an acid reaction in the border cells 

 have hiiherto failed, perhaps because the acid is poured into the 



