PROTEIDS. PEPTONES. 11 



Among the more important proteids obtained from the 

 human Body are the following: 



Serum albumin. This exists in solution in the blood and 

 is very like egg albumin in its properties. It is coagulated 

 (like the white of an egg) when boiled, and then passes 

 into the state of coagulated proteid which is, unlike the 

 original serum albumin, insoluble in dilute acids or alka- 

 lies or in water containing neutral salts in solution. All 

 other proteids can by appropriate treatment be turned into 

 coagulated proteid. 



Fibrin. This forms in blood when it " clots," either iiir 

 side or outside of the body. . It is made by the interaction 

 of two other proteids known &sfibrinogen an&fibrinoplastin. 

 It is insoluble in water. 



Myosin. This is derived from the muscles, in which it 

 develops and solidifies after death, causing the "death- 

 stiffening." 



Globulin exists in the red globules of the blood and dis- 

 solved in some other liquids of the body. In the blood 

 corpuscles it is combined with a colored substance to form 

 hcemoglobin, which is crystallizable. An allied substance, 

 paraylobulin, is found dissolved in the blood liquid. 



Caxein exists in milk. It is insoluble in water but sol- 

 uble in dilute acids and alkalies. Its solutions do not 

 coagulate spontaneously, or like that of serum albumin on 

 boiling. In the milk it is held in solution by the free- 

 alkali present; when milk becomes sour the casein is pre- 

 cipitated as the "curd." Cheese consists mainly of casein. 



Peptones. These are formed in the alimentary canal 

 by the action of some of the digestive liquids upon the 

 proteids swallowed as food. They contain the same elements 

 as the proteids and give the xantho-proteic and Millon's 

 reactions, but are not precipitated by boiling with acetic 

 acid and sodium sulphate. Their great distinctive charac- 

 ter is however their diffusibility. The proteids proper will 

 not dialyze (see Physics), but the peptones in solution pass 

 readily through moist animal membranes. 



Albuminoids. These contain carbon, hydrogen, oxy- 

 gen and nitrogen, but rarely any sulphur. Like the 



