346 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



cent. The usual salts found in animal fluids are also here present 

 chlorides of sodium, potassium, calcium, and ammonium being in excess. 

 Phosphatic earths with some iron occupy the next place, while the 

 sulphates are either absent or present only in minute traces. The con- 

 stituents, therefore, of the gastric juice consist, in the first place, of two 

 soluble ferments, pepsin and the milk-curdling ferment, which represent 

 the organic constituents of the gastric secretion ; second, a free acid, 

 which is, in all probability, hydrochloric; and, third, the mineral salts. 



(a) Pepsin. Pepsin belongs to the category of soluble ferments. 

 As yet it has been impossible to obtain it in a state of absolute purity. 



The procedure which gives the best results is that of Briicke. The mucous 

 membrane of the stomach is digested at 40 C. with dilute hydrochloric acid. It 

 is then neutralized with lime, which is thus precipitated, and carries down 

 mechanically with it the ferment, pepsin. This precipitate is washed and dissolved 

 in dilute hydrochloric acid, and to this is added a solution of cholesterin in four 

 parts of alcohol and one part of ether. The cholesterin throws the pepsin out of 

 solution. It is then washed with water and with ether. The ethereal layer is 

 poured off, and pepsin remains in watery solution, from which it may be 'obtained 

 by evaporation. Von Wittich treats the mucous membrane with glycerin, after 

 having allowed it to remain for twenty-four hours in alcohol, so as to precipitate 

 the proteids in the tissue of the stomach, and at the end of a week or two the 

 glycerin is filtered off, and pepsin may be obtained by precipitating the glycerin 

 solution of pepsin with absolute alcohol. 



Obtained by either of these processes, pepsin is a yellowish powder, 

 which is soluble in water and glycerin and insoluble in alcohol. When 

 precipitated by alcohol from its aqueous or glycerin solutions it does 

 not lose its solubility in water, thus differing from the proteids; it is 

 not diffusible. When dried it may be warmed up to 110 C. without 

 losing its activity. While in solution it may be transformed into a sub- 

 stance which is less active, and which has been termed isopepsin by 

 Finckler. At 80 it becomes entirely inactive. Pepsin is also soluble 

 in dilute acids. If pure, pepsin should not give proteid reactions. It 

 should 3 7 ield no precipitate with nitric acid, tannic acid, iodine, or 

 mercuric chloride. It is precipitated from its solutions by acetate of 

 lead and platinum chloride. 



The proportion of pepsin in the gastric juice varies at different 

 periods of digestion. At the commencement of digestion it is present 

 in the smallest amount, and acquires its maximum between the fourth 

 and fifth hours of digestion. In man it is said to be present in amounts 

 varying from 0.41 to 1.17 per cent. 



Without the addition of dilute acid pepsin manifests no specific 

 action, and the characteristic test of the presence of this ferment is 

 known as the pepsin test with fibrin. If a little fibrin, obtained by 

 whipping the blood as it flows from a divided vessel, is washed until 

 perfectly white and placed in a test-tube with a little gastric juice, and 

 warmed up to 35 C., the fibrin will entirely disappear. There will be 



