110 THE CONTRACTILE TISSUES. 



increase the number of constituent single contractions is one not so easy to 

 answer. And connected with this question is another difficult one. What is 

 the rate of repetition of single contractions making up those tetanic contrac- 

 tions which as we have said are the kind of contractions by which the vol- 

 untary, and indeed other natural, movements of the body are carried out? 

 What is the evidence that these ar really tetanic in character ? 



When a muscle is thrown into tetanus, a more or less musical sound is 

 produced. This may be heard by applying a stethoscope directly over a 

 contracting muscle, and a similar sound but of a more mixed origin and 

 less trustworthy may be heard when the masseter muscles are forcibly con- 

 tracted or when a finger is placed in the ear, and the muscles of the same 

 arm are contracted. 



When the stethoscope is placed over a muscle, the nerve of which is 

 stimulated by induction-shocks repeated with varying frequency, the note 

 heard will vary with the frequency of the shocks, being of higher pitch with 

 the more frequent shocks. Now it has been thought that the vibrations of 

 the muscle giving rise to the " muscle sound " are identical with the single 

 contractions 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 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 show 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 move- 

 ments of the body may be taken as showing 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 move- 

 ments 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 fre- 

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

 exactly identical in all these movements, is not at present perhaps abso- 

 lutely determined ; but certain markings on the myographic tracings of 

 these movements and other facts seem to indicate that it is about 12 a 

 second. 



79. 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 con- 

 traction 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 favorable action of the 



