44 PHYSIOLOGY OF MUSCLES AND NERVES. 



dinally ; for these also emit a sound although no change 

 of form is externally perceptible. 



This raises a question as to how many of these irri- 

 tations are really requisite in order to bring a muscle 

 into an enduring condition of contraction. By means of 

 Wagner's hammer (fig. 12), just described, or by means 

 of an electric wheel (fig. 11), the number of the irrita- 

 tions may be regulated. It will be found that from 16 to 

 18 distinct irritations in each second are quite sufficient to 

 cause a constant contraction of the muscle. In a living 

 body also, where the muscles are voluntarily contracted, 

 the condition of tetanus appears to be produced by the 

 same number of irritations. It has been found that the 

 height of the muscle-note heard during voluntary con- 

 traction of the muscles is about equal to c* or d^, which 

 represents from 32 to 36 vibrations in the second. But 

 Helmholtz was able to show, with great probability, 

 that this is not the true number of muscle-vibrations, 

 but that the vibrations within the muscle are really 

 only half as many. As, however, notes of this pitch 

 are indistinguishable to our ears, we hear the next 

 higher tone instead, which represents twice the num- 

 ber of vibrations.^ 



6. As yet we have noticed only the shortening of 

 muscles. This alone determines the amount of labour 

 accomplished, which consists in raising weights. Bat 

 on looking at a contracted muscle, it is evident that 

 it has become, not only shorter, but thicker. This 



• According to Preyer, some men are capable of distinguishing 

 notes of as many as fifteen to twenty-five vibrations per second ; 

 and, according to tlie same authority, the muscle-note sounds very 

 like that produced by from eighteen to twenty vibrations per 

 second, which corresponds very well with the views of Helmholtz 



