56 The Feeding of Animals 



consists more largely of non-pro teids. They are also 

 the chief constituents of muscular tissue. The chemical 

 constitution of the proteids is not definitely known. 

 No investigator has yet been wise enough to search out 

 their manner of combination, but it is generally con- 

 sidered to be very complex. It is believed that a cer- 

 tain one of these compounds holds in a single molecule 

 no less than 5,000 atoms. These bodies are con- 

 structed from the simpler ones of the inorganic world 

 through the vital energies of plants, and they appar- 

 ently must come to the animal fully organized. 



The ultimate composition of proteids, that is, the 

 proportions of the elements which they contain, has 

 been carefully studied, and while there are material 

 differences among them in this respect, the limits of 

 variation are not especially wide, as can be seen from 

 the following figures taken from Neumeister: 



Elementary composition of the proteids 



Per cent Per cent Average 



Carbon 50. to 55. 52. 



Hydrogen 6.5 to 7.3 7. 



Nitrogen 15. to 17.6 16. 



Oxygen 19. to 24. 23. 



Sulphur 3 to 2.4 2. 



We see that the number of elements ordinarily found 

 in the proteids is five, nitrogen and sulphur being 

 those that chiefly distinguish these bodies from all 

 others which make up the mass of combustible matter. 

 Two other elements are occasionally involved, as, for 

 instance, the phosphorus of casein and the iron of 

 blood. 



