. CHEMICAL CONSTITUENTS OF BODY AND FOOD. 35 



Classification of Proteids or Proteins. 



For the sake of convenience and study the proteids have been 

 divided into various groups and classes by different authorities. They 

 are almost universally divided into the two main groups of animal 

 and vegetable origin. The amount of proteid matter in plants, par- 

 ticularly the full-grown ones, is less than in animals. It is found dis- 

 solved in their juices, in the protoplasm, or deposited in the form of 

 grains called aleuron granules. Vegetable proteids are divisible into 

 the same classes as the animal, but, since human physiology deals 

 with animal proteids, the vegetables are disregarded. 



A convenient classification is: (1) native albumins, (2) 

 derived albumins, or albuminates, (3) compound proteids, (4) globu- 

 lins, (5) peptones, (6) albuminoids, (7) Jiistons, and' (8) protamins. 



i. Native Albumins. 



The proteids of this class are those that are found in an unal- 

 tered, natural state or condition in the solids of the body. They are 

 soluble in water and are not precipitated by the dilute acids. The 

 two main forms are egg-albumin and serum-albumin. The egg- 

 albumin occurs in the part of the egg known as the white. The 

 serum-albumin is found not only in the blood-serum, but also in the 

 lymph as it is found in its proper lymphatic channels and diffused 

 throughout the tissues, in the chyle, milk, and transudations. 



2. Derived Albumins, or Albuminates, or Meta=proteins. 



To this class belong two divisions: acid-albumin and alkali- 

 albumin. 



The derived albumins are formed from the native albumins by 

 the action of weak alkalies or acids. Thus, when a native albumin, 

 such as serum-albumin, is treated for a while with dilute hydro- 

 chloric acid its properties become entirely changed. The solution is 

 no longer able to be coagulated by heat, and when the solution is care- 

 fully neutralized the whole of the proteid is thrown down as a pre- 

 cipitate. The substance into which the native albumin was changed 

 by the action of an acid is called an acid-albumin, or syntonin. This 

 acid-albumin is insoluble in distilled water and neutral saline solu- 

 tions, but readily soluble in dilute acids and alkalies. This is the 

 process through which albumins pass when undergoing gastric diges- 

 tion and when acted upon by the HC1 of the gastric juice. 



