4 CHEMICAL BASIS OF THE ANIMAL BODY. 



may fairly consider as the nearest approach to native protoplasm, when 

 they can be obtained in sufficient quantity for chemical analysis, are 

 found to contain some 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 substance, 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 living substance consist of all three classes, but, as we have seen 

 in the sections on nutrition, gives rise by metabolism to members of 

 the same three classes; and as far as we know at present, carbohy- 

 drates and fats, when formed in the body out of proteid food, are so 

 formed by the agency of living substance, by the action of some 

 living tissue. Hence there is at least some reason for thinking it 

 probable that the molecule of living substance, 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. 



The plasmodium of 2Ethalium septicum, a myxomycastous fungus, presents a 

 convenient source of extremely primitive protoplasm which may be obtained in 

 large quantities. It occurs as an extended, yellow, gelatinous mass, frequently of 

 considerable thickness, on the surface of heaps of spent tan or other similar 

 decaying vegetable matter, and exhibits very active movements both internally 

 and more particularly at its edges, of an essentially amoeboid nature. It has been 

 carefully analysed by Keinke, Studien uber das Protoplasma, Berlin, 1881. See 

 also Krukenberg, Unters. a. d. physiol Inst. Heidelb., Bd. n. 1882, S. 273. 



Whether this be so or not, for at present no dogmatic statement 

 can be made, there is no doubt that 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 in the same tissue 

 or fluid, 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 substances which we may regard as 

 stages in the constructive or destructive metabolism of the various 

 forms and phases of living matter, 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 substances as urea, uric acid, other organic 

 crystalline bodies, and the extractives 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 



