ORGANS OF THE ENERGIZING SYSTEM 87 



which these substances are distributed. The fatty acids and 

 glycerol are combined as fats in the intestinal wall. The soluble 

 sugars are, in part, distributed to the tissues but go, in the main, 

 to the liver where they are converted into glycogens. 



316. On Structural x\ssimilation. It is sometimes said 

 that some bodily parts (the nerve-cells and fibres, the muscle-cells, 

 etc.) are '* alive " while other substances, the glycogen of the liver, 

 the carbohydrate of the muscle-fibre, the neutral fat of adipose 

 tissue, etc., are " not alive." But in the obvious difficulty of 

 defining " living substance " the distinction suggested is not clear. 



There is, how^ever, some difference in a fundamental sense 

 between the substance of the tissue-mechanism (say the sarcostyles 

 of a muscle-fibre, regarded as contractile tissue) and the energy- 

 yielding substances associated with the tissue-mechanism. (Say 

 the carbohydrate in the muscle fibres that disintegrate and yield 

 energy to set up the muscular tensions.) Also we think about the 

 bodily fats, which are used up in starvation, as different in some 

 deep sense from the connective tissue-cells in which the fats are 

 " stored," or the matrices of bone and cartilage are not so obviously 

 " vital " as the bone and cartilage cells that lay down these 

 matrices. In some way " stored," or " reserve," or mechanical- 

 supporting substances are different from, and subsidiary to, the 

 tissue-mechanisms. 



The latter are subject to waste and must be repaired. Also 

 there is actual increase in their mass, in bodily growth. Therefore 

 the materials that have been chemically assimilated must, to some 

 extent, be structurally assimilated, or incorporated, or incarnated 

 into the obviously functioning parts of the body. 



So there is one kind of flesh of beasts and another of fowis 

 and so on. The flesh, etc., that is eaten is broken down into its 

 chemical parts from which some other kind of flesh can be con- 

 structed. The analytic changes are imperfectly known, but the 

 synthetic ones, involving not only chemical transformations but 

 also specific tectonic ones are certainly among the unsolved 

 problems of physiology. 



32. ON THE ORGANS OF EXCRETION 

 On the analogy of the heat-engine we expect various " products 

 of combustion " to be eliminated from the animal body. Since 



