THE THERMAL RESPONSE. 



403 





the excitations are strong enough and frequent enough to produce a full 

 thermal effect, they may be altered in both these respects without interfering 

 with the result. When the frequency of the stimuli is not only insufficient 

 to produce a maximal tetanus, but fails to evoke a continuous contraction 

 at all, as is the case in a series of maximal excitations, following one 

 another at intervals of such length as to allow the muscle time to relax 

 between each contraction and its successor, more heat is produced than in 

 a tetanus, whether complete or incomplete, of the same duration. 



It may be noted that Fick l summarises these results by the statements 

 (1) That in tetanus the quantity of heat which is assignable to each 

 excitation varies inversely with the frequency, and (2) that in a series of 

 responses to instantaneous excitations, the thermogenetic effect of each 

 excitation far exceeds that which is attributable to each member of a series 

 of stimulations of such frequency as to evoke complete tetanus. 2 



For the reasons stated on p. 394, oxidation must be considered as 

 the first in order of time of the processes which constitute the response 

 of a muscle to excitation. In the excitation of a muscular element, the 

 direct action of the stimulus is to induce in it chemical changes, whereby 

 either heat is produced or work is done. But we have seen that not 

 only the proportion between the inogenetic and thermogenetic changes 

 varies according to the mechanical conditions, but that the total amount 

 of chemical work clone is also dependent on them. We are thus led, with 

 Fick, to adopt the general principle that the " chemical process is in part 

 determined by the external conditions under which the muscle is placed 

 when it is excited." How far this influence extends is one of the most 

 important problems in muscle physiology ; for in all discussions of the 

 relation between the useful result obtained, and the quantity of material 

 expended in its production, it is this problem that we have to encounter. 



In voluntary movement we may well suppose that when the central 

 nervous system sends its mandate to a muscular element to contract, it 

 does so without definite information (if I may so express myself) as to 

 the amount of material which it will be necessary to use or waste in 

 fulfilling that mandate, and that if the demand first made is found to be 

 inadequate, as, e.g., in consequence of unanticipated resistance, the supply 

 will be proportionately increased. There is, in short, no difficulty in 

 supposing that during the performance of a voluntary act, expenditure 

 is continuously adapted to requirement. In skilled movements, such 

 as those of drawing, writing, or playing on an instrument, it is difficult 

 to see how such adaptation could, be dispensed with. We might without 

 difficulty found upon this consideration the hypothesis that in general 

 the adaptation of supply to demand is a function of the nervous system. 

 Inasmuch, however, as in muscles rendered nerveless by curare, this 

 adaptation is as remarkable (see p. 361), though not so perfect, as in 

 muscles working under the guidance of nerve cells, no such general 

 statement can be made. 



We have already seen (p. 357) that if a muscle sustaining a weight 

 which it is just able to lift, is tetanised for a limited period, during 

 which its load is gradually diminished in such a way that throughout 

 the period of excitation the lifting power of the muscle is only just in 

 excess of the resistance it has to overcome, it will do more work than 

 under any ordinary conditions, for throughout its effort it is doing its 

 utmost. It eventually attains its maximum degree of shortening, at 



1 " Mech. Arbeit," S. 216. 2 Loc. cit., S. 213. 



