402 THE PROPERTIES OF STRIPED MUSCLE. 



whether evoked by repeated or continuous excitation, should be a 

 heat producing process. If a muscle to which a weight is attached 

 contracts persistently, it does no external work, whatever the load 

 may be. And if we are to regard the state of contraction as nothing 

 more than one in which the natural length of each fibre (i.e. the 

 length it possesses when unexcited) is diminished, there is no more 

 reason why we should expect that in the state of extension the 

 excited muscle should produce heat, than that an elastic band, or an un- 

 excited muscle, under the same conditions, should do so. We know that 

 it actually does produce heat when contracted, although no external work 

 is done, and therefore conclude that the difference between the excited 

 and unexcited muscle does not consist exclusively in the difference of 

 the relation of length to tension. To explain the active thermogenesis 

 of a muscle which holds up a load persistently at the same height, it 

 must possess some property which does not belong to a stretched elastic 

 cord. 



This consideration reminds us of the suggestion that every muscular 

 contraction consists essentially of two processes, of which the second is 

 mechanically a reversal of the first ; and that when a muscle is subjected to 

 frequently repeated excitation (tetanus), so as to alternate between states of 

 increasing and diminishing tension, these are accompanied, or rather ushered 

 in, by corresponding chemical actions. In the first stage it has been suggested 

 that, as the immediate result of stimulation, some body (e.g. lactic acid) comes 

 into existence l and occasions an increase of tension, which in the second is 

 annulled by the oxidation of its efficient cause. Both processes are in the 

 chemical sense disintegrative, i.e., consist in the transformation of bodies of 

 greater heat- value into others of less \ in other words, both are thermogenetic. 



Even as regards tetanus produced by repeated stimulations, it is difficult 

 to see how this explanation facilitates the understanding of the result ; for if 

 we suppose the frequency of the excitations to be no more than 100 per second, 

 it is difficult to imagine that the muscular element can pass through the two 

 stages of the chemical process above referred to, hi the interval between two 

 successive excitations. The difficulty is still greater in applying this 

 explanation to prolonged contraction of central origin, in respect of which we 

 have no evidence that the excitation by which it is evoked is discontinuous. 

 It therefore seems that we must seek for some other solution of the fact that 

 all continuous muscular effort, whether effectual or not, is accompanied by the 

 production of heat, in quantity which in the main depends on the intensity 

 of the effort. Just as, in comparing the thermal effect of single excitations 

 which are not adequate to produce a maximal contraction, it was found that 

 this effect depends on the efficacy of the excitation as indicated by the lift, so 

 in tetanus, as long as the frequency of the excitations is insufficient to evoke a 

 maximal tetanic contraction, the heat produced by a tetanus of given duration 

 increases with the height to which the load is lifted. Thus the thermogenetic 

 effect, no less than the mechanical effect, depends on the frequency of the 

 excitations. But from the moment that the contraction becomes maximal, 

 frequency ceases to have any influence ; so that, in a series of similar 

 experiments, in each of which a muscle is completely tetanised but with 

 stimuli of different frequency, the quantity of heat produced in the same time 

 is always the same. It is also found, by comparing equal periods of a 

 tetanus brought about by a series of maximal stimuli of constant frequency, 

 that more than twice as much heat is produced in the first period, i.e., that in 

 which the muscle shortens, than in any succeeding period. Provided that 



1 Gad, "Zur Thcorie der Erregungsvorgange im Muskel," Arch. f. Physiol., Leipzig, 

 1893, S. 166. 



