SECRETION OF SALIVA AND GASTRIC JUICE. 275 



during full digestion, contains but little ready-made ferment, though there is 

 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 favored by the presence of a small quan- 

 tity 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 undeniably 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 fer- 

 ments, 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 shows that the gastric glands, both the car- 

 diac and the pyloric glands, while they contain comparatively little actual 

 pepsin, contain a considerable quantity of zymogen of pepsin, or pepsinogen; 

 and there can be little doubt but that this pepsinogen is lodged in the cen- 

 tral cells of the cardiac glands and in the somewhat similar cells which line 

 the whole of the pyloric glands. 



209. The act of secretion itself. The above discussion prepares 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 untenable. 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 the 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 sub- 

 stances 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, fur- 

 nished 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, 

 whilst 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 promoting the discharge of water, while stimulation of the sympathetic 

 has a marked effect in promoting the discharge of mucin. To this we may 

 add the case of the parotid of the dog. In this gland stimulation of a 

 cerebro-spinal nerve, the auriculo-temporal, produces a copious flow of limpid 

 saliva, while stimulation of the sympathetic produces itself little or no secre- 

 tion at all ; but when the sympathetic and the cerebro-spinal nerves are 

 stimulated at the same time, the saliva which flows is much richer in solid 



