CONTRACTION AND WORK OF THE MUSCLES. 47 



number as the minimum necessary to produce the state of 

 apparent immobility of the electrically tetanized muscle. 



If voluntary contraction, studied with the aid of the myo- 

 graph, furnishes no trace of vibrations, we must not be sur- 

 prised, since the essential character of that act consists in the 

 coalescence of shocks. But the existence of the sound which 

 accompanies the contraction of the muscle sufficiently proves 

 the complexity of this phenomenon. Let us add another proof 

 in favour of this theory. "When a muscle receives excitations 

 of equal intensity, the contraction which results from them is 

 all the stronger in proportion to their frequency. Now, in 

 contracting the muscles of the jaws with more or less force, 

 we have been able to convince ourselves that ^e acuteness of 

 the muscular sound increased with the energy of the effort. 

 We may thus obtain variations of a fifth in the tone of the 

 muscular sound. 



We shall also see hereafter how the electric state of the 

 muscles in contraction proves still more the complexity of this 

 phenomenon. 



The conclusion at which we have arrived is, that during 

 voluntary contraction, the motor nerves are the seat of suc- 

 cessive acts, each of which produces an excitation of the 

 muscle. The latter, in its turn, causes a series of acts, each 

 of which gives birth to a muscular wave producing a shock. 

 It is in the elasticity of the muscle that we must seek for the 

 cause of the coalescence of these multiplied shocks ; they are 

 extinguished just as the jerks of the piston of a fire-engine 

 disappear in the elasticity of its reservoir of air. 



Of work done by the muscles. After having seen how 

 mechanical force is produced, let us try to measure it — that 

 is to say, to compare it with the kilogrammetre, the unit of 

 measure of work. If we suspend a weight to the tendon 

 of a muscle which we cause to contract, we easily obtain the 

 measure of work by multiplying this weight by the height to 

 which the muscle raises it. 



In animated motors, the measure of work is less easy to 

 obtain. Sometimes, indeed, the strength of an animal is 

 utilized in the lifting of a weight, but the greater part of the 

 acts in which the strength of animals is employed can only 



