ELEMENTARY PHYSIOLOGICAL CHEMISTRY 225 



ferment rennin. It will be remembered that caseinogen is precipitated 

 by weak acids ; it might be naturally supposed, therefore, that the curd 

 which forms when milk enters the stomach was due to precipitation by 

 acid, and not to coagulation by the ferment. That it is the rennin 

 which acts, however, is proved by neutralising the gastric juice before 

 adding the milk, when curdling will occur as usual, or by treating some 

 of the curd with weak alkali in which it will not dissolve, whereas a 

 precipitate of caseinogen would dissolve with ease. 



The gastric juice scarcely affects other food-stuffs. In the case of 

 fat, however, it dissolves the proteid envelope of the fat cell, and 

 liberates the contents, which now float in the chyme as oil globules. It 

 inverts disaccharides, but has no action on polysaccharides. 



The conditions which influence the activity of gastric digestion are 

 discussed in the Advanced Course. 



CHAPTER XIII. 

 DIGESTION IN THE INTESTINE. 



IN about twenty minutes to half an hour after the food enters the 

 stomach, small portions of it begin to pass through the pyloric sphincter 

 into the duodenum. These have undergone gastric digestion and 

 constitute chyme. This leakage goes on till the stomach has completely 

 emptied itself, the length of time necessary for this (3-10 hours) varying 

 with the quantity and quality of the food, and with the activity of the 

 gastric juice. 



The chyme, as it leaves the stomach, is strongly acid in reaction, of a 

 dirty yellow colour, with no characteristic smell, and has floating in it 

 unemulsified globules of oil. In the duodenum it becomes mixed with 

 the secretions of the pancreas and liver, which are poured into that 

 portion of the intestine by one common duct, and as it travels on to 

 the jejunum it also becomes gradually mixed with the intestinal juice, 

 secreted from Lieberkiihn's follicles. These three secretions are alkaline 

 in reaction, in consequence of which the acid of the chyme is neutralised, 

 so that the contents of the lower portion of the duodenum and of the 

 upper portion of the jejunum become alkaline in reaction. Now, 

 although the acidity of the gastric juice prevents the growth of 

 organisms in it, it does not kill their spores, and these are carried into 

 the intestine along with the chyme. When this latter becomes alkaline, 

 however, the conditions are very favourable for bacterial growth, and 



P 



