DIGESTION 353 



Everything, therefore, points to the granules in what we may 

 now call the mucin-forming cells as being in some way or other 

 precursors of the fully-formed mucin ; manufactured during 

 ' rest ' by the protoplasm and partly at its expense, moved 

 towards the lumen in activity, discharged as mucin in the secre- 

 tion. It has been asserted that not only is the protoplasm 

 lessened in the loaded cell and renewed after activity, but that 

 many of the mucigenous cells may be altogether broken down 

 and discharged, their place being supplied by proliferation of 

 the small cells of the demilunes. This conclusion, however, is 

 not supported by sufficient evidence. The cells of the crescents 

 contain fine granules, but none which can be changed into mucin. 

 They are of serous and not of mucous type. But the fact on 

 which we would specially insist is that the granules of the resting 

 mucigenous cell may be looked upon as a mother-substance from 

 which the mucin of the secretion is derived ; they are not actual, 

 but potential, mucin. 



So in the pancreas, the serous or albuminous salivary glands, 

 and the glands of the stomach, there is every reason to believe 

 that the granules which appear in the intervals of rest, and are 

 moved towards the lumen and discharged during activity, are 

 the precursors, the mother-substances, of important constituents 

 of the secretion. These granules are sharply marked off from the 

 protoplasm in which they lie and by which they are built up. 

 By every mark, by their reaction to stains, for instance, they are 

 non-living substance, formed in the bosom of the living cell from 

 the raw material which it culls from the blood, or, what is more 

 likely, formed from its own protoplasm, then shed out in granular 

 form and secluded from further change. The granules in the 

 ferment-forming glands are not in general composed of the actual 

 ferments, and, indeed, in several instances it has been shown that 

 the actual ferments are not present in the secreting cells at all. 



We have already seen that the pancreas and even the fresh 

 pancreatic juice are devoid of active trypsin. Similarly, a 

 glycerin extract of a fresh gastric mucous membrane is inert as 

 regards proteins, or nearly so. But if the mucous membrane has 

 been previously treated with dilute hydrochloric acid, the 

 glycerin extract is active, as is an extract made with acidulated 

 glycerin. Here we must assume the existence in the gastric 

 glands of a mother-substance, pepsinogen, from which pepsin is 

 formed. The rennin of the gastric juice, which is formed in the 

 chief cells, also has a precursor, which, if the ferment is identical 

 with pepsin (p. 326), must be pepsinogen. The proteolytic power 

 of an extract of the pancreas, when the trypsinogen has been 

 activated into trypsin, or of the gastric mucous membrane, when 

 the pepsinogen has "been changed into pepsin, seems to be, roughly 



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