CHAP. L] TISSUES AND MECHANISMS OF DIGESTION. 415 



present in it a body which, by some kind of decomposition, gives 

 birth to the ferment. We may remark incidentally that though the 

 presence of an alkali is essential to the proteolytic action of the 

 actual ferment, the formation of the ferment out of its forerunner 

 is favoured by the presence of a small quantity of acid ; the acid 

 must be used with care, since the trypsin, once formed, is destroyed 

 by acids. To this body, this mother of the ferment, which has not 

 at present been satisfactorily isolated, but which appears to be a 

 complex body, splitting up into the ferment, which as we have 

 seen is at all events not certainly a proteid body, and into an un- 

 deniably proteid body, the name of zymogen has been applied. 

 But it is better to reserve the term zymogen as a generic name 

 for all such bodies as not being themselves actual ferments, may by 

 internal changes give rise to ferments, for all ' mothers of ferment ' 

 in fact ; and to give to the particular mother of the pancreatic 

 proteolytic ferment, the name trypsinogen. 



Evidence of a similar kind shews that the gastric glands, both 

 the cardiac and the pyloric glands, while they contain compara- 

 tively little actual pepsin, contain a considerable quantity of a 

 zymogen of pepsin, or pepsinogen ; and there can be little doubt 

 but that this pepsinogen is lodged in the central cells of the 

 cardiac glands and in the somewhat similar cells which line the 

 whole of the pyloric glands. 



It is further interesting to observe that, as a general rule, the 

 amount of trypsinogen in a pancreas at any given time rises and 

 sinks pari passu with the granular inner zone, i.e. with the amount 

 of granules in the cell. The wider the inner zone and the more 

 abundant the granules the larger the amount, the narrower the 

 zone and the fewer the granules the smaller the amount, of 

 trypsinogen ; and in the cases of old-established fistulas, where the 

 secretion is wholly inert on proteids, the inner granular zone is 

 absent from the cells. And the same parallelism has been 

 observed between the abundance of granules in the central cells 

 and the quantity of pepsinogen present in the gastric glands. 



The parallelism however, at all events in the case of the 

 pancreas, appears not to be absolute, for it is stated that in the 

 pancreas of dogs after long starvation there is little or no 

 trypsinogen in the gland and yet the cells exhibit a marked inner 

 zone of granules. Moreover we should not, in any case, be justified 

 in concluding that the granules of the pancreatic cell are wholly 

 composed of trypsinogen ; for, as we shall presently see, the pan- 

 creatic juice contains besides trypsin not only other important fer- 

 ments but also certain proteid constituents ; and the granules, 

 which are of a proteid nature, probably supply these proteids of 

 the juice. Hence the parallelism between granules and trypsinogen 

 is at best an incomplete one. But even such an incomplete paral- 

 lelism is of value. The granules whatever their nature are pro- 

 ducts of the metabolism of the cell, lodged for a while in the 



