766 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES 



When a muscle, excited by maximal stimuli, is made to lift con- 

 tinuously increasing weights, both the work done and the heat given 

 out increase up to a certain limit. The muscle, as it were, burns the 

 candle at both ends. The heat-production reaches its maximum some- 

 what sooner than the work. 



It is certain that when work is done by a muscle an equivalent 

 amount is subtracted from its sum-total of energy, and under proper 

 conditions this can be actually demonstrated by the deficiency in the 

 heat-production. This is done by means of a contrivance called a 

 work-adder. It consists of a wheel, the rotation of which raises a 

 weight attached to a cord wound round its axle. The muscle acts on 

 the periphery of the wheel, and by rotating it raises the weight a little 

 at each contraction. At the end of the contraction the wheel is pre- 

 vented from moving back by a catch. The work done in a series of 

 contractions is calculated from the total height to which the weight 

 has been raised. Suppose a frog's gastrocnemius is made to contract 

 a certain number of times while attached to the work-adder, and that 

 simultaneously the heat- production is measured by means of a thermo- 

 pile. Let H represent the heat actually produced, and h the heat 

 equivalent of the work done. Now let the muscle be disconnected from 

 the adder and made to raise the same weight, directly attached to it, 

 by a series of contractions elicited in precisely the same way as the 

 previous ones, except that the weight is allowed to fall with the muscle 

 when it relaxes after each contraction. Here heat corresponding to the 

 external work disappears from the muscle during the contraction just 

 as in the first experiment, but this heat is returned to the muscle during 

 the relaxation, since on the whole no external work is done. The heat 

 produced in the second experiment is found, as a matter of fact, 

 allowing for unavoidable errors, to be equal to H'+h. 



According to Hill, the true ' efficiency ' of the muscle is not the 

 ratio W/H, where W is the external work and H the total heat liberated, 

 but T/H where T is the maximum increase of tension set up during the 

 twitch when the muscle is contracting isometrically. This fraction T/H 

 is constant whatever be the initial tension, the number of fibres excited, 

 or the strength of excitation of each fibre. For the theory of the 

 muscular contraction the tension auring an isometric muscle twitch, 

 which represents the potential energy suddenly developed in conse- 

 quence of the excitation, is accordingly much more important than the 

 height of the contraction, which is related to the work actually done. 

 The essential thing in muscular contraction may be the abrupt develop- 

 ment of this tension through a chemical reaction which liberates certain 

 substances at some membrane or surface in the muscle. The potential 

 energy once in being may or may not be transformed into work, and if 

 so transformed the change may be accomplished economically or waste- 

 fully, according to the conditions of the contraction. The ratio T/H 

 decreases in fatigue, and with the time during which the muscle has 

 been deprived fi its blood-supply. Hill has calculated the absolute 

 value of the hfcat-production in tetanus of a sartorius or semimem- 

 branosus muscle of the frog. This quantity, reckoned per centimetre 

 of length of the muscle, per gramme weight of the tension developed 

 and per second of maintenance of the tension, is relatively constant at 

 about 0-000015 'gramme-calorie. Including the recovery processes of 

 oxidation following the contraction the total heat-production would 

 amount to about 0-000025 calorie. The potential energy possessed by 

 a muscle of a length of a centimetre when maintaining a tension of a 

 gramme is abowt 0-000004 calorie. So that to maintain this state of 

 potential energy six or seven times as much energy must be liberated 



