CHEMICAL BASIS OF THE BODY. 55 



The attempt to investigate the composition of a living tissue by 

 chemical analysis, must cause its death, and thus alter the arrange- 

 ments of its constituents, so that its true molecular constitution 

 when alive, cannot be determined. 



We know that the composition of all living textures is extremely 

 complicated, every one being made up of a great number of com- 

 ponents, most of which contain many chemical elements associated 

 together in very complex proportions. 



But as has already been pointed out, the complexity of their 

 chemical constitution is not so wonderful as another fact which 

 sounds paradoxical, that, in order to preserve their elaborate com- 

 position, they must constantly undergo a change or renewal, which 

 is necessary for, and forms the one essential characteristic of, their 

 life. In fact, their complexity and instability is such, that they 

 require constant reconstruction to make up for the changes insep- 

 arable from their functional activity. 



Their chemical constituents are easily and permanently disso- 

 ciated, and the various components are themselves readily decom- 

 posed, generally uniting with oxygen to form more stable com- 

 pounds. 



The investigation of the chemical changes, known as assimila- 

 tion, forms a great part of physiological study, and therefore will 

 occupy many chapters of this book. Here we can only call atten- 

 tion to the chief characteristic substances to be found in the animal 

 body, as the result of the primary dissociation or death of the 

 textures, and briefly enumerate the products of their further de- 

 composition as obtained by the analysis of the different substances. 



The tissues of the higher animals present a greater variety of 

 substances, materially differing in chemical composition ; they 

 have, however, all been made from protoplasm, and contain a pro- 

 portion of some substance forming a leading chemical constituent 

 of protoplasm. Every living tissue contains either protoplasm or 

 a derivative of it, and the special characters of each tissue depend 

 upon the greater development of some one of these substances. 



It is of little use to classify the numerous chemical constituents 

 found in the animal body, in such a systematic manner as to sat- 

 isfy the rules of modern chemistry, because their classification from 



