16 



ANIMAL MECHANICS. 



time between the observation and recording of a pheno 

 menon may vary greatly in the same person according to the 

 state of health ; but, as the principal retardation in these cases 

 may, and probably does, occur in the brain itself (between the 

 reception of the sensation and the issuing of the volition), it 

 would not be philosophical to adduce such cases as proofs of 

 a difference of velocity in the nerves either of sensation or of 

 motion. 



Whatever be the rate of wave motion in the muscle or 

 nerve, the time of its molecular vibration may be determined by 

 the note of the susurrus caused by the muscles, and by the 

 note of the tinnitus produced by the nerves. 



In the Croonian Lecture, read before the Royal Society, on 

 November 16th, 1809, Dr. Wollaston called attention to the 

 sound, or susurrus, produced by the muscles when in a state 

 of contraction. He states correctly that this sound is best 

 produced by inserting gently the extremity of the finger into 

 the ear, bringing at the same time the muscles of the hand and 

 forearm into strong contraction. The muscular susurrus may 

 also be heard easily by using a stethoscope upon a con- 

 tracted muscle, either of the observer's or of any other person's 

 body. 



Having described the mode of obtaining the sound, Dr. 

 Wollaston correctly adds, that it " resembles most nearly that 

 of carriages at a great distance passing rapidly over a pave- 

 ment ;" to which admirable description of the sound I would 

 add, that it bears a most striking resemblance to the deep 

 hum produced by the blowing fan of a large foundry. 



Dr. Wollaston attempted to estimate the frequency of the 

 elementary muscular contraction that produces the susurrus 

 by allowing his ear to rest on the ball of his thumb, while his 

 elbow was supported by a horizontal board, in which he had 

 cut a number of equal notches, about one-eighth of an inch 

 asunder. Against these notches he rubbed a pencil with a 



