

MUSCLE 581 



the stress of the change falls chiefly upon the relaxation. 

 This indicates that the relaxation is by no means a mere 

 elastic recoil, but a physiological process as important as 

 the contraction itself; and this conclusion is strengthened 

 by the fact we have now to mention, that not only is heat 

 produced during the actual shortening, but also during the 

 relaxation. It is in accordance with this that as fatigue 

 comes on the production of heat diminishes earlier than the 

 height of the contraction. We must suppose that the 

 diminution is related to the increasing slowness of the re- 

 laxation, and indicates that the chemical changes are less 

 rapid when relaxation is more difficult. 



The fraction of the total energy transformed which 

 appears as muscular work varies with the conditions of the 

 contraction. The greater the resistance,* the larger is the 

 proportion of the energy which appears as-work, the smaller 

 the proportion which appears as heat ; but even in the most 

 favourable case, an excised frog's muscle never does work 

 equal to more than \ of the heat given off. Generally the 

 ratio is much less, and may sink as low as Y V In the intact 

 mammalian body the muscles work somewhat more economi- 

 cally than the excised frog's muscles at their best ; for both 

 experiment and calculation show (p. 501) that in a normal 

 man under the most favourable conditions as much as 

 one-third of the energy is converted into work. But in 

 any case the heat-producing mechanism and the work-pro- 

 ducing mechanism of muscle are certainly in some respects 

 distinct, and a variation in the activity of the one is not 

 necessarily associated with a corresponding variation in the 

 activity of the other. 



(4) Chemical Phenomena of the Muscular Contraction. We as 

 yet know but little regarding the chemical composition of living 

 muscle, and are unlikely ever to know much, since most chemical 

 operations cause the immediate death of the tissue. The composition 

 of dead mammalian muscle may be stated, in round numbers, as 

 follows, but there are considerable variations, even within the same 

 species. 



* This statement, based on experiments with excised frog's muscles is 

 not, of course, inconsistent with the fact mentioned on p. 502, that in the 

 intact body the fraction of the energy transformed into heat is greater in 

 hard than in moderate work. 



