THE SCOPE OF BIOLOGY 7 



proportions of the chief chemical elements in the animal body. 

 Oxygen, carbon, hydrogen, and nitrogen constitute about 

 98 per cent of the animal body and not far from the same pro- 

 portion of the composition of the body of most plants. These 

 four elements also constitute by far the largest proportion of 

 the material present in the earth's crust; so that the living 

 body is made of the same materials that are most abundantly 

 present in the inanimate world around us. 



Chemical Compounds in Living Tissues. It is perfectly 

 evident that the elements enumerated do not exist in the 

 living body as uncombined elements. Two or more of them 

 are always united as chemical compounds to form a substance 

 different from either of them. The chemical compounds that 

 are present in the bodies of animals and plants are of an endless 

 variety; but a few general types are most widely present and 

 may be regarded as the fundamental compounds of living 

 things. These compounds are important, since they enter 

 into the food of all animals. They are as follows: proteids, 

 carbohydrates, fats. 



Proteids. Proteids are extremely complex substances, com- 

 posed chiefly of the elements: carbon, oxygen, hydrogen, and 

 nitrogen, but containing also in small proportions sulphur 

 and the other elements that have been enumerated above. 

 They are by far the most complex substances in living things; 

 that is, in a proteid molecule, there are present more chemical 

 atoms than are found in a molecule of any other substance 

 existing in the animal body. The exact chemical composition 

 of proteids is not known and it suffices for our purpose to 

 state, that they are composed of a highly complex combina- 

 tion of the elements we have mentioned, so united that hun- 

 dreds of atoms are probably always combined to make a mole- 

 cule. Some idea of their complexity may be obtained from 

 the fact that one chemist gave as a formula for egg-albumen, 

 C204H322N 52 O66S2 (a formula too complicated to have any real 

 meaning); and indeed, no two chemists agree upon the chem- 



