CHAP, ii.] THE CONTRACTILE TISSUES. 143 



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 movements of the 

 body, the fundamental tone of the sound corresponds to about 19 

 or 20 vibrations a second, it has been concluded that the con- 

 traction 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 really correspond to the 

 shortenings and relaxations of the individual contractions, and 

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

 indication 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 movements 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, is not at present 

 perhaps absolutely determined ; but certain markings on the 

 myrographic tracings of these movements and other facts seem 

 to indicate that it is about 12 a second. 



81. The Influence of the Load. It might be imagined that 

 a muscle, which, when loaded with a given weight, arid 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 tension 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 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. 

 In a muscle viewed as a machine we have to deal not merely 



