122 REPETITION OF CONTRACTIONS IN TETANUS. [BOOK i. 



plete tetanus will depend chiefly on the length of the individual 

 contractions, and this varies in different animals, in different 

 muscles of the same animal, and in the same muscle under differ- 

 ent conditions. In a cold blooded animal a single contraction is 

 as a rule more prolonged than in a warm blooded animal, and 

 tetanus is consequently produced in the former by a less frequent 

 repetition of the stimulus. A tired muscle has a longer contrac- 

 tion than a fresh muscle, and hence in many tetanus curves the 

 individual contractions, easily recognised at first, disappear later 

 on, owing to the individual contractions being lengthened out by 

 the exhaustion caused by the tetanus itself. In many animals, 

 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 

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

 'is more prolonged than the 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 contrac- 

 tions 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. 



