CHEMIC COMPOSITION OF THE HUMAN BODY. 19 



Two views are held as to the origin of fibrin : first that it is the result 

 of the action of a special enzyme, termed thrombin on fibrinogen, 

 though the nature of the action is not very clear; second that it is 

 the result of a definite combination, physio-chemic in character, of 

 fibrinogen with thrombin which, however, is not regarded as an enzyme, 

 inasmuch as it is not destroyed by boiling, but a definite compound 

 partaking of the nature of an organic colloid. The amount of fibrin 

 formed from fibrinogen will be proportional to the amount of thrombin 

 present (Howell). 



(b) Myosin fibrin andMyogen fibrin are two insoluble proteins developed 

 out of the two chief proteins of muscle plasma. Their develop- 

 ment after death is believed to be the cause of the stiffening of the 

 muscles. It is not definitely known whether this is the result of the 

 action of a special enzyme or not. 



(c) Casein. Casein is derived from the chief protein of milk caseinogen 



by the action of a special ferment known as rennin or chymosin. 

 This ferment is a constituent of gastric juice. 



The Color Reactions of Proteins. When proteins are present in 

 solution, they may be detected by the following color reactions viz.: 



1. Xanthoproteic. The solution is boiled with nitric acid for several 



minutes, when the protein assumes a light yellow color. After 

 the solution has cooled, the addition of ammonia changes the color 

 to an orange or amber-red, due to the presence of phenylalanin and 

 ty rosin. 



2. The rose-red reaction. The solution is boiled with acid nitrate of 



mercury (Millon's reagent) for a few minutes, when the coagulated 

 protein turns a purple-red color. This color is attributed to the 

 presence of tyrosin. 



3. The blue-violet reaction. A few drops of copper sulphate solution are 



first added to the protein solution, and then an excess of sodium 

 hydroxid. A blue-violet color is produced, which deepens somewhat 

 on heating, but no further change ensues. This is also known as 

 Piotrowski's test: As this same color is developed with the substance 

 biuret, it is also known as the biuret reaction. Biuret is formed by 

 heating urea to 180 C and driving off ammonia. 



Precipitation Tests. Proteins in solution may be precipitated by 

 nitric acid, acetic acid and potassium ferrocyanid, picric acid, copper 

 sulphate, tannin, alcohol, etc. As stated in a foregoing paragraph, certain 

 of the proteins, e.g.j fibrinogen, caseinogen and myosinogen, will undergo, 

 by the action of an animal ferment a change of state by virtue of which 

 they become solid. To this process the term ferment coagulation is applied. 

 The solidification of proteins by the action of heat is designated heat 

 coagulation. 



INORGANIC CONSTITUENTS. 



The inorganic compounds and mineral constituents obtained from the 

 solids and fluids of the body are very numerous, and, in some instances, 

 quite abundant. Though many of the compounds thus obtained are 

 undoubtedly derivatives of the tissues and necessary to their physical and 



