24 NUTRITION OF FARM ANIMALS 



4. THE PROTEINS 



40. Importance. By far the larger share of the organic 

 matter of the animal body, aside from fat, consists of sub- 

 stances belonging to the well-defined group of the proteins, 

 these compounds, according to the results of analyses recorded 

 on subsequent pages (98), making up from 17.5 to 21 per cent 

 of the fat-free body. These substances are characteristic of 

 the animal body, as the carbohydrates are of plants. Biologi- 

 cally, they are of prime importance to both plants and animals, 

 since they form the basis of the cytoplasm and nucleus of 

 every living cell. 



41. Nomenclature. The chemical structure of the pro- 

 tein molecule has until quite recently been almost entirely un- 

 known and even yet has been but very partially unraveled. 

 Accordingly, the basis for a scientific classification of these 

 substances has been lacking. As a matter of necessity, there- 

 fore, the nomenclature hitherto followed has been based chiefly 

 on their physical properties, more particularly their solubilities 

 and coagulation temperatures. Naturally, such a classification 

 has been far from satisfactory and this has been the more true 

 on account of the difficulty of accurately separating the differ- 

 ent proteins either by precipitation or crystallization. 



Accordingly, there has existed a great and confusing diversity 

 in the nomenclature of the proteins, and uniformity is still far 

 from having been reached. For the present, it seems desirable 

 to follow the classification and nomenclature which has been 

 adopted provisionally by the American Physiological Society 1 

 and the American Society of Biological Chemists. 2 This 

 nomenclature rejects entirely the term proteid as ambiguous 

 on account of the wide diversity in its use, and employs protein 

 as a general term to signify the group of substances which, 

 according to the nomenclature adopted by the Association of 

 American Agricultural Colleges and Experiment Stations in 

 1 8g8, 3 was called proteids. In other words, protein under the 

 new plan excludes altogether the non-protein nitrogenous 

 substances of plants and animals. 



1 Proceedings, Amer. Physiol. Soc., Amer. Jour. Physiol., 21 (1908), xxvii. 



2 Proceedings, Amer. Soc. Biol. Chemists, 1, 142. 



3 U. S. Dept. Agr., Office of Expt. Stas., Bui. 65, pp. 117-123. 



