i8 TEXT-BOOK OF PHYSIOLOGY. 



and in the presence of acids and alkalies. The number of derivatives 

 obtained between the first cleavage of the protein molecule and its final 

 cleavage to ammo-acids is large and will be presented at length in the para- 

 graph relating to protein digestion. The chief derivatives are as follows: 



INFRA-PROTEINS. 



(a) Acid-albumin. This is formed when a native albumin is digested 

 with dilute hydrochloric acid (0.2 per cent.) or dilute sulphuric acid 

 for some minutes. It is precipitated by neutralization with sodium 

 hydroxid (o.i per cent, solution). After the precipitate is washed, 

 it is found to be insoluble in distilled water and in neutral saline solu- 

 tions. In acid solutions it is not coagulated by heat. 



(b) Alkali-albumin. This is formed when a native albumin is treated 

 with a dilute alkali e.g., o.i per cent, of sodium hydroxid for five 

 or ten minutes. On careful neutralization, with dilute hydrochloric 

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

 saline solutions; it is not coagulable by heat. 



PROTEOSES, PEPTONES AND POLYPEPTIDS. 



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

 ach and intestines, there is produced by the action of the gastric and pan- 

 creatic juices, out of the proteins of the food, a series of new proteins, 

 known as proteoses, peptones and polypeptids. The chemic properties of 

 these substances will be considered in connection with the process of digestion. 



COAGULATED PROTEINS. 



Although these proteins are not found as constituents of the animal 

 organism, 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 viz., 

 fibrinogen. It is not present under normal circumstances in the 

 circulating blood, but makes its appearance after the blood is with- 

 drawn 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 in- 

 soluble in water, in alcohol, and ether. In dilute acids it swells, be- 

 comes transparent, and finally is converted into acid albumin. In 

 dilute alkalies a similar change takes place, but the resulting pro- 

 duct is an alkali-albumin. Fibrin possesses the property of decom- 

 posing hydrogen dioxid, H 2 O 2 i.e., liberating oxygen, which accu- 

 mulates in the form of bubbles on the fibrin. On incineration fi- 

 brin yields an ash which contains calcium phosphate and magnesium 

 phosphate. 



