410 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



part of the peptone will be precipitated. If the alcoholic fluid be then 

 acidulated with acetic acid, boiled, and filtered, leucin and tyrosin will 

 separate when the nitrate is concentrated by evaporation. By heating 

 this deposit with water, the leucin, which is readily soluble in water, may 

 be separated from the tyrosin, which is not so soluble. 



Leucin, or amido-caproic acid (C 6 H 13 NO 2 ), belongs to the fatty bodies, and 

 is a constant decomposition product of albumen and various nitrogenous sub- 

 stances. Leucin occurs in the form of white, shining lamellae, which are insoluble 

 in ether and chloroform, readily soluble in alkalies and acids, especially when 

 hot. It readily crystallizes from its solutions in hot water in spherical masses, 

 composed of groupings of thin, white, glistening needles. When a few crystals 

 of leucin are placed on platinum-foil and evaporated gently with a drop of nitric 

 acid, if a few drops of caustic soda are added to the colorless residue a yellow or 

 brownish mass is obtained, which forms an oily drop. (Scherer's test.) 



If a dilute solution of leucin is boiled with cupric hydrate in excess, bright 

 violet scales are deposited on cooling. 



Tyrosin (CgH^NOg) is a member of the aromatic group, and may be 

 obtained from almost any proteid under the action of strong oxidizing agents. It 

 remains in the deposit of pancreatic digestion of albuminoids, prepared as above, 

 after the leucin has been removed. By dissolving this residue in hot water, and 

 rapidly crystallizing by the addition of ammonia, tyrosin may be obtained in 

 tolerable purity. It then occurs in the form of fine, white, silky needles, gener- 

 ally arranged in sheaf-like bundles, which dissolve in hot dilute ammonia, and are 

 deposited on cooling the solution in brilliant, colorless, radiating stars. It is 

 insoluble in absolute alcohol and ether, almost insoluble in cold water, and 

 slightly soluble in hot water, but readily soluble in the mineral acids and in warm 

 dilute ammonia. 



If a hot, watery solution of tyrosin is treated with a few drops of Millon's 

 reagent to boiling, a dark-red color appears, and when the solution is concentrated 

 deposits a dark-red precipitate. (Hoffman s test.) 



The application of Scherer's test, as in the case of leucin, will form a reddish- 

 yellow residue, which will become brown on the addition of caustic soda. 



In the small intestine the pancreatic juice never alone comes in 

 contact with food-stuffs, but it meets with undigested matters mixed 

 with the results of gastric digestion, and therefore with the acid gastric 

 juice and bile. 



In the animal economy, therefore, the activity of the pancreatic 

 juice must be different from that which has been stated as occurring 

 outside of the bod} r , when the operation of the pure pancreatic juice is 

 alone considered. 



When the acid chyme from the stomach reaches the small intestine, 

 the alkalinity of the bile, pancreatic and intestinal secretion, to a certain 

 extent, partially neutralizes the acid of the gastric juice. 



It has been found that when pancreatic juice is mixed with gastric 

 juice, the activity of the resulting medium will depend greatly upon the 

 relative proportions of these two fluids. As a rule, gastric juice, by 

 digesting trypsin, renders the pancreatic secretion entirely inert. The 

 process occurring in the duodenum is, however, somewhat different : 

 for we have already found that the first effect of the bile is to precipi- 

 tate pepsin, and therefore the pancreatic ferment comes into contact with 



