THE ACTIVE FORCES OF LIVING ORGANISMS 35 



The blood carries the same raw materials to every 

 region of the body. By what means is the selection 

 in the various tissues made ? and what are the con- 

 ditions which determine the nature of the chemical 

 combinations whereby the renewal of them is assured ? 

 Complicated in the highest degree though the pro- 

 cesses of metabolism taken separately undoubtedly 

 are, the main factors in them are, on the contrary, 

 probably very limited. Amongst them it is necessary 

 to concentrate one's attention in the first place on the 

 tissues, the nervous system, and the ether. 



When new molecules are formed in any tissue The aeieo- 



. . tion of 



they resemble the old in their chemical composition, materials 

 Though the various substances which go to form the tissues. 

 tissues are of a complex nature, and are elaborated 

 or prepared in other regions than those for which 

 they are finally destined — though, indeed, in some 

 instances, as in that of lecithin, bodies are found in 

 many parts of the organism in that form which they 

 retain when built up into tissue — yet, speaking gene- 

 rally, the ultimate absorption of nutritive matter is a 

 process, not merely of physical attraction, but essen- 

 tially of chemical action also. The tissues constitute 

 the workshop in which new tissue is manufactured, 

 and they must therefore be regarded in a great 

 measure as the moulding agency. But if the tissues 

 are to exercise an influence of this kind they must be 

 active — they must, that is to say, transmit force either 

 directly or through some medium to the materials in 

 the lymph which bathes them. Now, if we suppose 



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