ENERGY RELATIONS OF PLANTS AND ANIMALS 23 



make-up of the body and the diet. There must evidently 

 be a degree of similarity between them, inasmuch as the 

 one has been built from the other. Similar compounds 

 are met with in both, but, as we shall find, in quite unlike 

 proportions. The body is mainly water. Water makes 

 up about two-thirds oLthe_ total, weight and forms even a 

 larger percentage of the most active tissues. No material 

 reduction of its quantity can be tolerated. Even after 

 death from thirst the amount is surprisingly little dimin- 

 ished. In the diet also water occupies the first place. It 

 is likely to constitute fully five-sixths of the daily income. 

 Its most obvious services are in connection with the 

 absorption of food in solution and the removal of dissolved 

 wastes. By its evaporation from the skin and the breath- 

 ing passages it helps to keep the body temperature from 

 rising above its normal level. 



Second to water among the substances which compose 

 the body we find the group of bewilderingly complex 

 compounds known as the proteins. A protein always 

 yields the five elements carbon, oxygen, nitrogen, 

 hydrogen, and sulphur 1 when subjected to analysis. 

 Some members of the group contain phosphorus also. 

 Merely to mention these constituent elements is to give 

 no proper conception of the intricate manner in which 

 they must evidently be combined. To appreciate this 

 we need to consider the very long list of cleavage products, 

 in themselves rather complex, which can be obtained by 

 the decomposition of protein from a single source. The 

 physiologic chemist is somewhat in the position of a person 



1 The elements occur in proteins in about the following percentages: 

 C 53, O 22, N 16, H 7, S 1 per cent. Phosphorus when present 

 amounts to 1 per cent, more or less. It is quite impossible to con- 

 vey an adequate impression of the complex fashion in which the five 

 or six elements are combined. Many years ago the following formula 

 was suggested for hemoglobin, the red protein of the blood, which is 

 exceptional in containing iron: 



C 758 H 120; A 28 N 195 FeS 3 . 



It is not seriously maintained that these large numbers are precisely 

 correct, but the order of their magnitude is probably typical. 



