140 REPETITION OF CONTRACTIONS IN TETANUS. [BOOK i. 



e.g. the rabbit, some muscles (such as the adductor magnus 

 femoris) are pale, while others (such as the semitendinosus) are 

 red. The red muscles are not only more richly supplied with 

 blood vessels, but the muscle substance of the fibres contains more 

 haemoglobin than the pale, and there are other structural differ- 

 ences. Now the single contraction of one of these red muscles 

 is more prolonged than a single contraction of one of the pale 

 muscles produced by the same stimulus. Hence the red muscles 

 are thrown into complete tetanus with a repetition of much less 

 frequency than that required for the pale muscles. Thus, ten 

 stimuli in a second are quite sufficient to throw the red muscles 

 of the rabbit into complete tetanus, while the pale muscles require 

 at least twenty stimuli in a second. 



So long as signs of the individual contractions are visible on the 

 curve of tetanus it is easy to recognise that each stimulation 

 produces one of the constituent single contractions, and that the 

 number so to speak of the vibrations of the muscle making up the 

 tetanus corresponds to the number of stimulations ; but the 

 question whether, when we increase the number of stimulations 

 beyond that necessary to produce a complete tetanus, we still 

 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 contractions which as we have said are 

 the kind of contractions by which the voluntary, and indeed other 

 natural, movements of the body are carried out ? What is the 

 evidence that these are 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 contracted 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 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 



