376 DIGESTION 



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 eve'ry 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 pre- 

 cursors, the mother-substances, of important constituents of the 

 secretion. These granules are sharply marked off from the proto- 

 plasm' 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 



Fig. 158. Mucous Cells (from Submaxillary of Dog) in Rest and Activity (Langley). 

 A, B, fresh; A', B', after treatment with dilute acetic acid; A", B", alveoli hard- 



ened in alcohol and stained with carmine. A, A', and A" represent the loaded; 

 B, B', and B", the discharged condition. 



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 fer- 

 ments are not present in the secreting cells at all. 



We have already seen that the pancreas and even the fresh pan- 

 creatic 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 pre- 

 viously 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-sub- 

 stance, 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. 347), must be 

 pepsinogen. The proteolytic power of an extract of the pancreas, 



