APPENDIX. 



ON THE CHEMICAL BASIS OF THE ANIMAL BODY. 



THE animal body, /from a chemical point of view, may be regarded as a mixture 

 of various representatives of three large classes of chemical substances, viz., pro- 

 teids, carbohydrates, and fats, in association with smaller quantities of various saline 

 and other crystalline bodies. By proteids are meant bodies containing carbon, oxy- 

 gen, hydrogen, and nitrogen in a certain proportion, varying within narrow limits, 

 and having certain general features ; they are frequently spoken of as albuminoids. 

 By carbohydrates are meant starches and sugars and their allies. We have also 

 seen that the animal body may be considered as an assemblage of protoplasm under 

 various modifications and of numerous products of protoplasmic activity. We do 

 not at present know anything definite about the molecular composition of active 

 living protoplasm ; but when we submit protoplasm to chemical analysis, in which 

 act it is killed, we always obtain from it a considerable quantity of the material 

 spoken of as proteid. And many authors go so far as to speak of protoplasm as 

 being purely proteid in nature ; they regard the living protoplasm as proteid mate- 

 rial, which, in passing from death to life, has assumed certain characters and pre-" 

 sumably has been changed in construction, but still is proteid matter ; they some- 

 times speak of protoplasm as "living proteid " or "living albumin." It is worthy 

 of notice, however, that even simple forms of protoplasm, like that constituting 

 the body of a white corpuscle, forms of protoplasm which we may fairly consider 

 as 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 so far as to say, that in all 

 forms of living protoplasm, the proteid basis is found upon analysis to have some 

 carbohydrates 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, protoplasm gives rise by metab- 

 olism to members of the same three classes ; and, as far as we know at present, 

 carbohydrates and fats, when found 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 con- 

 tains 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, gelatin, etc. ; 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 

 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 

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