THE FUNCTIONS OF THE DIGESTIVE ORGANS. 497 



dogs there is from 0.46 to 0.6 per cent acid present, while in man the 

 values vary greatly. Thus the amount present has been variously stated 

 in the literature as from 0.05 to 0.57 per cent. We have already said 

 that it now seems impossible to regard this free acid as taken directly 

 from the blood, for the blood may be considered as of neutral reaction. 

 A great number of experiments have been undertaken without definitely 

 settling this question of the acid formation. We must rest content with 

 the hypothesis that it is due to a specific activity on the part of the cells, 

 and will state once more that it does not imply a process any more com- 

 plicated than that of the formation of other substances which result from 

 the specific activity of the cells in every gland. There is no justifiable 

 ground for giving a peculiar position to the acid secretion. 



The individual ferments are not found in a finished state in the mucous 

 coat of the stomach; i.e., they are not given up by it in an active condi- 

 tion. This preliminary condition of the ferment is spoken of in general 

 as that of a zymogen, and in the case of pepsin itself we have pepsinogen. 

 The presence of such a substance may be shown by the following experi- 

 ment: Pepsin itself is extremely sensitive to a solution of soda, by 

 means of which it is soon destroyed. Pepsinogen, on the other hand, 

 resists the action of soda much more strongly. If the mucous membrane 

 of the stomach is cut up into small pieces and extracted with a dilute 

 soda solution, there is obtained after filtering a liquid which of itself exerts 

 no digestive action upon albumin, but does so on being acidified with 

 hydrochloric acid. The acid serves to converts pepsinogen into pepsin. 

 If, now, after the digestion has proceeded for a short time, the liquid is 

 again made alkaline, a further addition of acid will no longer bring forth 

 the digestive action of pepsin. The pepsin has been destroyed by the 

 soda. We are at present unable to explain how the acid activates the 

 zymogen, and, in fact, such processes will remain obscure until we know 

 more about the chemical nature of the ferments themselves. We can 

 indeed assume that the hydrochloric acid in some way causes a rearrange- 

 ment of the atoms in the ferment molecule, possibly by the formation of 

 an anhydride or something similar, and that in the new state the ferment 

 is capable of exerting its characteristic action. 



Rennin likewise is not present in the mucous membrane as such, but in 

 the form of a zymogen. It is activated in precisely the same manner as 

 pepsinogen, and in fact these two ferments are very similar in their entire 

 behavior. 



It is highly significant that the function of the mucous membrane and 

 its glands is dependent upon definite kinds of stimulation. These can be ef- 

 fected in part in the stomach itself, or the stimuli may be transmitted to the 

 stomach from other organs. The secretion of the gastric juice is brought 

 about by reflex action. This may be demonstrated very prettily by making 



