Chap, ir.] THE CONTRACTILE TISSUES. 123 



Now it has been thought that the vibrations of the muscle giving 

 rise to the " muscle sound " are identical with the single con- 

 tractions making up the tetanus of the muscle. And since, in 

 the human body, when a muscle is thrown into contraction in a 

 voluntary effort, or indeed in any of the ordinary natural move- 

 ments of the body, the fundamental tone of the sound corresponds 

 to about 19 or 20 vibrations a second, it has been concluded that 

 the contraction taking place in such cases is a tetanus of which 

 the individual contractions follow each other about 19 or 20 times 

 a second. But investigations seem to shew that the vibrations 

 giving rise to the muscle sound do not leally correspond to the 

 shortenings and relaxations of the individual contiactions, and 

 that the pitch of the note cannot therefore be taken as an indica- 

 tion of the number of single contractions making up the tetanus ; 

 indeed, as we shall see in speaking of the sounds of the heart, a 

 single muscular contraction may produce a sound which though 

 differing from the sound given out during tetanus has to a certain 

 extent musical characters. Nevertheless the special characters 

 of the muscle sound given out by muscles in the natural move- 

 ments of the body may be taken as shewing at least that the 

 contractions of the muscle in these movements are tetanic in 

 nature, and the similarity of the note in all the voluntary efforts of 

 the body and indeed in all movements carried out by the central 

 nervous system is at least consonant with the view that the 

 repetition of single contractions is of about the same frequency 

 in all these movements. What that frequency is, and whether 

 it is exactly identical in all these movements, has not at present 

 been clearly determined; though certain markings on the myro- 

 giaphic tracings of these movements and other facts seem to 

 indicate that it is about 12 a second. 



§ 76. The Influence of the Load. It might be imagined that 

 a muscle, which, when loaded with a given weight, and stimulated 

 by a current of a given intensity, had contracted to a certain 

 extent, would only contract to half that extent when loaded with 

 twice the weight and stimulated with the same stimulus. Such 

 however is not necessarily the case ; the height to which the 

 weight is raised may be in the second instance as great, or even 

 greater, than in the first. That is to say, the resistance offered \ 

 to the contraction actually augments the contraction, the ten- 

 sion of the muscular fibre increases the facility with which the 

 explosive changes resulting in a contraction take place. And we 

 have other evidence that anything which tends to stretch the 

 muscular fibres, that any tension of the muscular fibres, whether j 

 during rest or during contraction, increases the metabolism of the ■ 

 muscle. There is, of course, a limit to this favourable action of 

 the resistance. As the load continues to be increased, the height 

 of the contraction is diminished, and at last a point is reached at 

 which the muscle is unable (even when the stimulus chosen is 

 the strongest possible) to lift the load at all. 



