12 MANUAL OF HISTOLOGY. 



We are, to be sure, tolerably well acquainted with the general inter- 

 change of matter which takes place in the system, but not so that in the 

 individual organs. We are, indeed, justified in concluding that this in- 

 terchange of material in the latter possesses varying degrees of intensity ; 

 that it increases during the action of the part, and decreases during rest ; 

 but we are possessed of almost no facts which would enable us to de- 

 monstrate with desirable accuracy the amount of this traffic, as it were, 

 which goes on, even in a single tissue. 



If in this way the destiny of many constituents of our body is veiled 

 in obscurity, how much more so, then, their real chemical relations. 

 Although of many substances we are able to say, " They are products 

 of decomposition, residues, relics of broken down tissue, their sojourn in 

 the body has no other significance," still, in dealing with others, great 

 difficulties arise when it is to be decided to what side of metamorphosis 

 they belong to the formative or to the retrogressive. Of the sources of 

 many products of decomposition we know nothing certain ; and even 

 the changes effected by chemical action are either but very unsatisfac- 

 torily understood, or not at all. Superfluous alimentary matter, so often 

 present in the system, may possibly be hardly distinguishable in its 

 derivatives from the matters resulting from metamorphosis of some con- 

 stituents of the body itself. Finally, we are uncertain still in regard to 

 many mineral substances, whether they are essential integral components 

 of our body, or are only casually present in the latter. 



Now it is, properly speaking, the theme of physiology to follow up 

 this behaviour of material in detail, and to interpret its full significance 

 for animal life ; but histochemistry will be frequently obliged to enter 

 upon physiologico-chemical research, for only in this way can a know- 

 ledge of the precise nature of the substances of which tissues and organs 

 are composed be acquired.. 



Commencing with the axiom, that the physiological characters of a 

 matter are in the first place dependent on its chemical constitution, we 

 choose as an introduction to the elements of composition of the human 

 body a section principally chemical. 



A. Albuminous or Protein Compounds. 



Absent from no organism, and taking part in the construction of all 

 tissues, these matters, which constitute the most important materials of 

 nutrition, appear of the highest significance in animal life ; indeed, they 

 may be regarded with all propriety as the chemical substrata of the 

 latter. Their histogenic qualities come even more prominently before us 

 in the embryonic body than in the mature ; for in the latter, many parts 

 consist of other than albuminous substances : for instance, of collagen, 

 chondrigen, elastic matter, and fats ; whereas, in the earliest periods of 

 existence, protein compounds are everywhere present. Those matters 

 just named, however, must also be looked upon as derivatives of the 

 latter, produced by metamorphosis of albuminous principles. 



The great instability and tendency to decomposition of all the members 

 of this group cause the appearance of a considerable number of substances 

 in the system, which in some cases participate still, though in a minor 

 degree, in the formation of tissues, and are in others (having undergone some 



