CHAP, ii.] THE CONTRACTILE TISSUES. 99 



work, were it not that in warm-blooded animals the heat given 

 out is not, as in the steam-engine, mere loss, but by keeping up the 

 animal temperature serves many subsidiary purposes. It might be 

 supposed that when in a contraction work is actually done, the 

 increase of temperature is less than when the same contraction 

 takes place without doing actual work, that is to say, that the 

 mechanical work is done at the expense of energy which otherwise 

 would go out as heat. Probable as this may seem it has not 

 yet been experimentally verified. 



Of the exact nature of the chemical changes which underlie a 

 muscular contraction we know very little, the most important fact 

 being, that the contraction is not the outcome of a direct oxidation, 

 but the splitting up or explosive decomposition of some complex 

 substance. The muscle does consume oxygen, and the products of 

 muscular metabolism are in the end products of oxidation, but the 

 oxygen appears to be introduced not at the moment of explosion 

 but at some earlier date. There is no evidence of nitrogenous 

 products being given off as waste ; such nitrogenous crystalline 

 bodies as are present in muscle, kreatin, &c., may be regarded 

 rather as the wear-and-tear of the machine than as products of 

 the material consumed in the work. Yet it is hardly consonant 

 with what we know elsewhere, to suppose that the contraction 

 of a muscular fibre has for its essence the decomposition of a 

 non-nitrogenous substance ; and we may suppose that the explosion 

 does involve some nitrogenous products, which however are re- 

 tained within the tissue, and used up again. We may even go so 

 far as to entertain with Hermann the view that a single complex 

 substance, an hypothetical inogen, splits up partly into nitrogenous, 

 partly into non-nitrogenous factors, the former, possibly of the 

 nature of myosin, being rapidly built up again into new inogen, 

 while the latter, such as the carbonic acid, are discharged at once 

 from the muscle. But our knowledge of these matters is not yet 

 ripe enough for the construction of an adequate and wholly 

 satisfactory theory. It may be worth while to point out that 

 during even the most complete repose muscle is undergoing chemi- 

 cal changes, which, as far as we know, are the same in kind, and 

 only differ in degree from those characteristic of a contraction. 

 Thus carbonic acid is constantly being produced, as are probably 

 other substances, all being got rid of as they form, just as they are 

 got rid of in larger quantities during the repose which follows 

 contraction. Supposing the existence of a substance which splits 

 up into these various products, and which we may speak of as the 

 true contractile material, it is evident that this material being thus 

 constantly used up, must be as constantly repaired. Thus a stream 

 of chemical substances may be conceived of as flowing through 

 muscle, the raw material brought by the blood being gradually 

 converted into true contractile stuff, the breaking-down again of 

 which is- gentle and gradual so long as the muscle is at rest, but 



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