CHEMICAL BASIS OF THE BODY. 63 



bj detected, but these must be regarded rather as accidental than 

 indispensable ingredients. 



The attempt to investigate the composition of a living tissue 

 by chemical analysis, must cause its death, and thus alter the 

 arrangements of its constituents, so that its true molecular con- 

 stitution when alive, cannot be determined. 



We know that the composition of all .living textures is ex- 

 tremely complicated, every one being made up of a great number 

 of components, 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 

 composition, 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 inseparable from their functional activity. 



Their chemical constituents are easily and permanently disso- 

 ciated, and the various components are themselves readily de- 

 composed, generally uniting with oxygen to form more stable 

 compounds. 



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 attention 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 decomposition 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 

 proportion of some substance forming a leading chemical con- 

 stituent of protoplasm. Every living tissue contains either 

 protoplasm or a derivative of it, and the special characters of 



