152 THE ENERGY OF MUSCLE. [BOOK i. 



by various anatomical relations ; but the fact that the completeness 

 and rapidity of the return are dependent on the condition of the 

 muscle, that is on the complex changes within the muscle making 

 up what we call its nutrition, the tired muscle relaxing much more 

 slowly than the untired muscle, shews that the relaxation is due 

 in the main to intrinsic processes going on in the muscle itself, 

 processes which we might characterize as the reverse of those 

 of contraction. In fact, to put the matter forcibly, adopting 

 the illustration used in 57, and regarding relaxation as a 

 change of molecules from a 'formation' of one hundred in 

 two lines of fifty each to a formation of ten columns each 

 ten deep, it would be possible to support the thesis that the 

 really active forces in muscle are those striving to maintain the 

 latter formation in columns and that the falling into double lines, 

 that is to say the contraction, is the result of these forces ceasing to 

 act ; in other words that the contracted state of the muscular fibre 

 is what may be called the natural state, that the relaxed condition 

 is only brought about at the expense of changes counteracting the 

 natural tendencies of the fibre. Without going so far as this 

 however we may still recognize that both contraction and relaxation 

 are the result of changes which, since they seem to be of a chemical 

 nature in the one case, are probably so in the other also. And 

 though in the absence of exact knowledge it is dangerous to specu- 

 late, we may imagine that these chemical events leading to 

 relaxation or elongation are of an opposite or antagonistic character 

 to those whose issue is contraction. 



It has not been possible hitherto to draw up a complete equa- 

 tion between the latent energy of the material and the two forms 

 of actual energy set free, heat and movement. The proportion of 

 energy given out as heat to that taking on the form of work 

 varies under different circumstances ; and it would appear that on 

 the whole a muscle would not be much more efficient than a 

 steam-engine in respect to the conversion of chemical action into 

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

 heat given out is not, as in the steam-engine, mere loss, but by keep- 

 ing up the animal temperature serves many subsidiary purposes. It 

 might be supposed that in a contraction by which work is actually 

 done, as compared with the same contraction when no work is 

 done, there is a diminution of the increase of temperature corre- 

 sponding to the amount of work done, that is to say, that the 

 mechanical work is done at the expense of energy which other- 

 wise 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 or substances. The muscle does consume oxygen, and 



