1164 PROTEIDS. 



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, carbohydrates 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. 



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

 ment 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 deriva- 

 tives of the three chief classes ; we find many forms of proteids, 

 and bodies closely allied to proteids, in the forms of mucin, gel- 

 atin, etc. ; many varieties of fats ; and several kinds of carbo- 

 hydrates. 



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 possess or promise to possess physiologi- 

 cal 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. 



PROTEIDS. 



These form the principal solids of the muscular, nervous, 

 and glandular tissues, of the serum of blood, of serous fluids, 

 and of lymph. In a healthy condition, sweat, tears, bile, and 

 urine contain mere traces, if any, of proteids. Their general 



