MUSCLE 577 



hither and thi'her along the muscle, the ear might from time to time 

 pick out of the turmoil of feeble aerial waves those corresponding to 

 its resonance tone, just as a tuning-fork or a piano-string attuned 

 to a particular note would catch it up amid a thousand other sounds 

 and strengthen it. 



But while this renders it highly probable that the resonance 

 of the ear contributes to the production of the muscle-sound, 

 and shows that we cannot from the pitch of the muscle- 

 sound alone deduce the rate at which the muscle-substance 

 is vibrating, it does not invalidate Helmholtz's objective 

 observations with the oscillating reeds. And several observers 

 (Schafer, Horsley, v. Kries) have noticed periodic oscillations, 

 at the rate of 8 to 10 per second, in the curves taken from 

 voluntarily contracted muscles, and from muscles excited 



FIG. 181 CONTRACTIONS CAUSED BY STIMULATION OF THE SPINAL CORD, 



through stimulation of the motor areas of the surface of the 

 brain. Since this rate remains the same whether the motor 

 cortex, the corona radiata, or the spinal cord is excited, and, 

 unlike the rate of response to excitation of peripheral nerves, 

 is independent of the frequency of stimulation, it has been 

 supposed to represent the rhythm with which impulses are 

 discharged from the motor cells of the cord (Fig. 181). 

 Other observers have seen a rhythm of 20 per second while 

 Haycraft denies that regular oscillations occur at all, and 

 thinks that irregularities in the contraction, connected with 

 a want of co-ordination of all the fibres, cause the muscle- 

 sound by drawing forth the resonance tone of the ear itself, 

 Loven, however, found the rhythm of strychnia tetanus in 



37 



