700 CHEMICAL BASIS [Apr. 



representatives of carbohydrates and fats as well as of proteids. We 

 might perhaps even go as far as to say, that in all forms of living proto- 

 plasm, the proteid basis is found upon analysis to have some carbohydrate 

 and some kind of fat associated with it. Further, not only does the 

 normal food which is eventually built up into protoplasm consist of all 

 three classes, but, as we have seen in the sections on nutrition, proto- 

 plasm gives rise by metabolism to members of the same three classes ; 

 and as far as we know at present, carbohydrates and fats, when formed 

 in the body out of proteid food, are so formed by the agency of living 

 protoplasm, by some living tissue. Hence there is at least some 

 reason for thinking it probable that the molecule of protoplasm, if 

 we may use such a phrase, is far more complex than a molecule of 

 proteid matter, that it contains in itself residues so to speak not only of 

 proteid but also of carbohydrate and fatty material. 



Be this as it may, for no dogmatic statement can at present be made, 

 when we examine the various tissues and fluids of the animal body from 

 a chemical point of view we find present in different places, or at different 

 times, several varieties and derivatives of the three chief classes ; we 

 find many forms of proteids, and bodies closely allied to proteids, in the 

 forms of mucin, gelatine, &c. ; many varieties of fats ; and several kinds 

 of carbohydrates. 



We find moreover many other bodies which we may regard as 

 stages in the constructive or destructive metabolism of both native and 

 differentiated protoplasm, and which are important not so much from 

 the quantity in which they occur in the animal body at any one time as 

 from their throwing light on the nature of animal metabolism ; these 

 are such bodies as urea, other organic crystalline bodies, and the ex- 

 tractives in general. 



In the following pages the chemical features of the more important 

 of these various substances which are known to occur in the animal 

 body will be briefly considered, such characters only being described as 

 possess or promise to possess physiological interest. The physiological 

 function of any substance must depend ultimately on its molecular 

 (including its chemical) nature; and though at present our chemical 

 knowledge of the constituents of an animal body gives us but little 

 insight into their physiological properties, it cannot be- doubted that 

 such chemical information as is attainable is a necessary preliminary to 

 all physiological study. 



