Digestion. 125 



stomach-fluid after feeding on different foods ; in connection 

 with the latter, it is to be remembered that the fluid con- 

 sists of the soluble, organic, and inorganic portions of the 

 food, mixed with saliva, and gastric juice : 



Stomach-fluid after feeding on 



Gastric juice is acid in reaction, clear when filtered, 

 slightly yellow in the horse, brownish in sheep, with a 

 specific gravity of 1010. A remarkable peculiarity of the 

 gastric juice of flesh feeders is the power it possesses of 

 resisting putrefaction. This is not the case with the horse. 

 Examined by the polariscope, gastric juice turns the ray of 

 polarized light to the left. The effect of reagents on the 

 secretion shows that acids produce no precipitate, whilst 

 alkalies do, and that alcohol produces a heavy precipitate, 

 which is in part due to the ferments found in the fluid. 

 The amount of salts present is about '7 per cent, or -S per 

 cent. 



The acidity of the gastric juice we have before spoken of, 

 it only remains to speak of the ferments — viz., pepsin and 

 rennin — and of the substance known as mucin. 



Pepsin is a body allied to proteids, which by appropriate 

 methods can be obtained in the form of a yellowish powder, 

 soluble in water, insoluble in alcohol ; it does not give all 

 the tests characteristic of the proteid group of bodies. The 

 ferment is secreted in the principal cells of the fundus and 

 pyloric glands, but exists in them, not in the form of pepsin, 

 but as its immediate precursor, pepsinogen. There is no 

 pepsin secreted by the cuticular or left sac of the stomach 

 of the horse, and Ellenberger considers that even the 

 pyloric region contains no ferment during the first hour 

 of digestion. 



It is found that if pepsin be obtained in a pure con- 



