CHAPTER V. 



THE CHIEF CHEMICAL CONSTITUENTS OF THE 

 ANIMAL BODY. 



THE term biological chemistry is a comprehensive one, 

 and embraces the consideration of all substances which 

 form the constituents of living organisms. The number 

 would naturally be very large, as the constituents of both 

 plants and animals would enter the category of sub- 

 stances to be included in a complete treatise on biological 

 chemistry. It is, therefore, necessary to limit at the outset 

 the number of products of which the chemical reactions 

 can be studied in a detailed manner. There are various 

 reasons for selecting for the more complete study the 

 constituents of the animal body. In the first place, man 

 himself belongs to the animal kingdom, and the chief 

 physiological studies have naturally been those of the 

 human body and the mammalia generally. In the second 

 place, paradoxical as it may at first sight seem, the study 

 of the chemistry of higher organisms is in many respects 

 simpler than that of the lower organisms and of plants. 

 This is due to the fact that, owing to the greater morpho- 

 logical differentiation of the tissues of the higher organisms, 

 the functions of the various organs are more highly 

 specialized. In the higher animals certain chemical 

 changes will be limited to certain definite organs, such as 

 the liver or the pancreas, whereas in the lower organisms, 

 or in plants, where the morphological differentiation is less 



