10 THE HUMAN BODY 



the entire group. Of these the so-called biuret reaction is the most 

 easily and most commonly used. It consists in making the pro- 

 tein solution strongly alkaline with caustic soda or potash and 

 adding a small amount of a very dilute solution of. copper sulphate. 

 A distinct purple color is evidence of the presence of protein. The 

 common proteins of the body may also'be recognized by the follow- 

 ing characters : 



1. Boiled, either in the solid state or in solution, with strong 

 nitric acid they give a yellow liquid which becomes orange on 

 neutralization with ammonia. This is the xanthoproteic test. 



2.- Boiled" with a solution containing subnitrate and pernitrate 

 of mercury they give a pink precipitate, or, if in very small quan- 

 tity, a pink-colored solution. This is known as Millon's test. 



3. If a solution containing a protein be strongly acidulated with 

 acetic acid and be boiled after the addition of an equal bulk of a 

 saturated watery solution of sodium sulphate, the protein will be 

 precipitated. 



The simple proteins which are found in the bodies of man and 

 the lower animals fall into several groups as follows : 



1. Albumins. Several proteins of this group are found in the 

 body; serum albumin, one of the proteins of the blood, myogen, a 

 muscle protein, and cell albumin, found in the cellular tissues, are 

 examples. Egg albumin (white of egg) is perhaps the most 

 familiar example of an albumin. 



The albumins are characterized by being coagulated by heat 

 (illustrated by boiled white of egg) ; in this respect they are similar 

 to the proteins of the next group, from which they differ by being 

 soluble in pure water. 



2. Globulins. These proteins, as indicated above, do not differ 

 greatly from albumins. Like them they are coagulated by heat, 

 but unlike them, are not soluble in pure water. If a small amount 

 of an inorganic salt is added to the water they will go into solution. 

 Two blood proteins, serum globulin or paraglobulin, and fibrinogen 

 belong to this group ; also myosin, one of the muscle proteins, and 

 cell globulin, found in cellular tissues. 



3. Albuminoids. In chemical structure these simple proteins 

 are closely similar to those previously described. They are 

 found, however, exclusively in the supporting and protective 

 tissues of the body, bone, connective tissue, epidermis, and hair, 



