350 NATURE OF THE ACT OF SECRETION. [Boon n. 



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 undeniably proteid body, the name of zymogen has been 

 applied. But it is better to reserve the term zymogen as a gene- 

 ric 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 particu- 

 lar 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. 



201. The act of secretion itself. The above discussion pre- 

 pares us at once for the statement that the old view of secretion 

 according to which the gland picks out, separates, secretes (hence 

 the name secretion) and so filters as it were from the common 

 store of the blood the several constituents of the juice, is unten- 

 able. According to that view the specific activity of any one gland 

 was confined to the task of letting certain constituents of the blood 

 pass from the capillaries surrounding the alveolus through the 

 cells to the channels of tie ducts, while refusing a passage to 

 others. We now know that certain important constituents of each 

 juice, the pepsin of gastric juice, the mucin of saliva and the like 

 are formed in the cell, and not obtained ready made from the 

 blood. A minute quantity of pepsin does exist it is true in the 

 blood, but there are reasons for thinking that this has made its way 

 back into the blood, either being absorbed from the interior of the 

 stomach or, as seems more probable, picked up directly from the 

 gastric glands ; and so with some of the other constituents of other 

 juices. The chief or specific constituents of each juice are formed 

 in the cell itself. 



But the juice secreted by any gland consists not only of the 

 specific substances such as mucin, pepsin or other ferment, or other 

 bodies, found in it alone, but also of a large quantity of water, and 

 of various other substances, chiefly salines, common to it, to other 

 juices and to the blood. And the question arises, Is the water, 

 are the salts and other common substances furnished by the same 

 act as that which supplies the specific constituents ? 



Certain facts suggest that they are not. For instance, as 

 mentioned some time ago, in the submaxillary gland of the dog, 

 stimulation of the chorda tympani produces a copious flow of 

 saliva, which is usually thin and limpid, while stimulation of the 

 cervical sympathetic produces a scanty flow of thick viscid saliva. 

 That is to say, stimulation of the chorda has a marked effect in 



