CHEMIC COMPOSITION OF THE HUMAN BODY. 1 9 



or ten minutes. On careful neutralization with dilute hydrochloric 

 acid, it is precipitated. It is also insoluble in distilled water and in 

 alkaline solutions; it is not coagulable by heat. 



COAGULATED PROTEINS. 



Although these proteins are not found as constituents of the animal organ- 

 ism, they possess much interest on account of their relation to prepared foods 

 and to the digestive process. They are produced when solutions of egg- 

 albumin, serum-albumin, or globulins are subjected to a temperature of 

 100 C. or to the prolonged action of alcohol. They are insoluble in water, 

 in dilute acids, and in neutral saline solutions. In this same group may be 

 included also those coagulated proteins which are produced by the action of 

 animal ferments on soluble proteins e. g., fibrin, myosin, casein. 



(a) Fibrin. Fibrin is derived from one of the blood proteins, fibrin- 

 ogen. It arises from the combination of fibrinogen and thrombin. 

 It is not present under normal circumstances in the circulating blood, 

 but makes its appearance after the blood is withdrawn from the 

 vessels and at the time of coagulation. It can also be obtained by 

 whipping the blood with a bundle of twigs, on which it accumulates. 

 When freed from blood by washing under water, it is seen to consist 

 of bundles of white elastic fibers or threads. It is insoluble in water, 

 in alcohol, and ether. In dilute acids it swells, becomes transparent, 

 and finally is converted into acid-albumin. In dilute alkalies a 

 similar change takes place, but the resulting product is an alkali- 

 albumin. Fibrin possesses the property of decomposing hydrogen 

 dioxid, H 2 O 2 i. e., liberating oxygen, which accumulates in the 

 form of bubbles on the fibrin. On incineration fibrin yields an ash 

 which contains calcium phosphate and magnesium phosphate. 

 (6) Myosin. Myosin or myogen develops in muscles after death and 

 is the cause of the stiffening of the muscles. It has been regarded 

 as a derivative of the soluble protein myosinogen alone, but there is 

 evidence that in its formation both paramyosinogen and myosinogen 

 take part. It is not definitely known whether this is the result of the 

 action of a special ferment or not. 



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

 inogen by the action of a special ferment known as rennin or chy- 

 mosin. This ferment is a constituent of gastric juice. 



PROTEOSES AND PEPTONES. 



During the progress of the digestive process, as it takes place in the stomach 

 and intestines, there is produced by the action of the gastric and pancreatic 



