20 PHYSIOLOGY CHAP. 



in which one tone predominates. On introducing one finger into 

 the auditory meatus and then forcibly contracting the muscles of 

 the arm, a dull murmur is heard similar to that from a distant 

 vehicle moving rapidly along the surface of a road. He regarded 

 the tremor often noticed in the muscular movements of old people 

 as the effect of an abnormal slowing of the muscular vibrations 

 due to debility and age. From his studies of voluntary muscular 

 contraction Wollaston concluded that the sound in the contracting 

 muscle corresponds to a frequency that oscillates between 14 and 

 15 per second at the minimum, 35 and 36 at the maximum. 



Helmholtz (1864) investigated the subject of the muscle sound 

 with better methods. He observed that if in the dead of night 

 the auditory meatuses are stopped and the masse ters forcibly 

 contracted, a murmur is heard in which there is a ground tone 

 that lasts as long as the voluntary contraction, and does not 

 change materially with increase of muscular tension. 



FIG. 13. Vibrations of biceps muscle of rabbit's femur on stimulating the spinal cord or sciatic 

 nerve with forty-two induction shocks per second. (Kronecker and Stanley Hall.) The middle 

 line s gives the vibrations of a tuning-fork in T jt 5 sec. ; the upper line n is the tracing of the 

 vibration of the muscle during stimulation of the sciatic ; the lower line m, the vibrations of 

 the muscle during stimulation of the cord. Both tracings were obtained by applying a 

 sensitive lever to the surface of the exposed muscle. 



The same tone is heard on firmly contracting the eye-muscles 

 or applying the stethoscope to the arm-muscles during voluntary 

 contraction. Helmholtz pointed out that the vibrations which 

 give rise to the sounds did not follow in regular sequence like 

 those of a musical tone. To determine the frequency objectively, 

 he applied watch springs or strips of paper to the muscles which 

 were vibrating in unison, and found the vibrations to be 18-20 

 per second. He confirmed the fact previously observed by Du Bois- 

 Keymond, that vibrations of the same frequency as those of 

 voluntary contraction are produced when a tetanising current of 

 high frequency is applied not only to the nerve or muscle but 

 also to the spinal cord of an animal. Subsequently, Helmholtz 

 pointed out (1864) that the tone perceived by the ear corresponds 

 not to the effective number of muscular vibrations, but to the 

 resonance tone proper to the ear of the observer, which corresponds 

 with the first over-tone or the octave of the fundamental tone of 

 the muscle, and is difficult to determine, because it lies at the limit 

 of the perceptible tones. He stated in effect that the tone heard 

 on voluntary contraction of the masseter muscles corresponds to 



