CHAP. II.] DIGESTION IN THE LIVING STOMACH. 151 



mined, and also its power of coagulating casein. Assuming all the dis- 

 solved substance to consist of, pure ferment, it would curdle from 

 400,000 to 800,000 times its weight of casein. 



The relative activity of solutions of milk-curdling ferment of 

 unknown strength is determined by observing, caeteris paribus, the 

 relative rapidity with which they induce the curdling of milk. 



The nature of the process which goes on in the curdling of milk 

 under the influence of the milk-curdling ferment will be treated of 

 at length under Milk. 



SECT. 12. ASSUMED EXISTENCE OF A LACTIC ACID FERMENT IN 



THE STOMACH. 



Hammarsten made the observation that pure solutions of the 

 milk-curdling ferment, just as pure solutions of pepsin, exert no action 

 on milk-sugar or on proteid solutions containing milk-sugar, whilst 

 gastric mucus or the neutralized infusion of stomach possess the pro- 

 perty of engendering the lactic acid fermentation. Pepsin and milk- 

 curdling ferment can be destroyed by the action of dilute solutions of 

 caustic soda, and the liquid thus freed from these two ferments still 

 possesses the power of converting milk-sugar into lactic acid. Ham- 

 marsten thence conceives that there exists in the mucous membrane 

 of the stomach a third lactic-acid-forming ferment. It appears to 

 the author that before assuming the presence of an unformed fer- 

 ment exerting this action, the presence of lactic-acid-generating 

 micro-organisms would have to be more particularly disproved. 



SECT. 13. THE PROCESS OF DIGESTION IN THE LIVING 

 STOMACH. 



Having now studied the chemical composition of the gastric juice, 

 the character of its separate constituents, and the action which they 

 exert upon the particular class of proximate principles which are 

 acted upon in the stomach, it remains to consider the actual process 

 of digestion as it occurs in the living organ, and in doing so we shall 

 be brought face to face with certain questions which have not been 

 discussed in the preceding sections. Although experiments on artifi- 

 cial digestion teach us the nature of the process which occurs in the 

 stomach, we cannot pretend that such experiments will furnish us 

 with data which will apply exactly to the stomach, for in this organ 

 we have conditions which are very different from those which exist 

 in vitro. In the stomach, we have no ordinary receptacle, into which 

 artificial gastric juice is poured, so as to be mixed with food, but 

 a receptacle kept constantly at a temperature most favourable to 

 digestion, provided with arrangements which enable it continually to 

 alter its capacity according to the mass which it contains, which it 



