TRANSACTIONS OF SECTION D. 729 



for hearing, is aflfected by each tone of perceptibly different pitch. Although the 

 physical diiFerence between high and low tones is simply a difference in frequency 

 of the sound waves, that is not supposed by Helmholtz to be the cause of the 

 different sensations of pitch. According to his theory, the function of frequency of 

 vibration is simply to excite by sympathy different nerve terminals in the ear. The 

 molecular movement in all the nerve fibres is supposed to be identical, and the 

 different sensations of pitch are ascribed to a highly specialised condition of cells- 

 in the hearing centre, whereby each cell, so to speak, produces the sensation of a 

 tone of definite pitch, which in no way depends on the frequency of incoming 

 nerve impulses, but simply on the specific activity of the cell concerned. 



In my lecture on the sense of hearing I pointed out in detail the great 

 anatomical ditficulties attending the theory in question. I endeavoured to show 

 the ph)'sical defect of a theory which does not suppose that our sensations of 

 harmony and discord must immediately depend upon the numerical ratios of nerve 

 vibrations transmitted from the ear to the central organ, and I offered a new theory 

 of hearing based upon the analogy of the telephone. According to that theory, 

 there is probably no analysis of sound in the ear ; the hair-cells at the peripheral 

 ends of the auditory nerve are probably affected by every audible sound of what- 

 ever pitch. When stimulated by sound they probably produce nerve vibrations, 

 simple or compound, whose frequency, amplitude, and wave-form correspond to 

 those of the sound received. The nerve vibrations arriving in the cells of the 

 auditory centre probably induce simple sensations of tones of different pitch, or 

 compound sensations of harmonies or discords strictly dependent on the relative 

 frequencies of the nerve vibrations coming in through the nerve. 



I cannot now recapitulate the evidence derived from anatomical, experimental, 

 and pathological observations that give support to ray theory of hearing, but I ma.y 

 briefly say that it is opposed to the theory of specific activities, in so far as it has 

 been applied to explain the different qualities of sound sensation. It is, however, 

 in strict accord with the fundamental proposition stated by Fechner ' in his great 

 work on psj'chophysics in these words : 'The first, the fundamental hypothesis is, 

 that the activities in our nervous system on which the sensations of light and 

 sound functionally depend are, not less than the light and sound themselves, to be 

 regarded as dependent on vibratory movements.' It is evident that, if we could 

 only comprehend the nature of the molecular movement in the nerve that links the 

 vibration of the physical agent to that in the sensory cell, we could advance 

 towards a true theory of the physiological basis of different qualities of sensation in 

 the different sense-organs. As j'et no definite answer can be given to the question, 

 what sort of molecular movement constitutes a nerve impulse, but in recent years 

 our knowledge of the subject has been extended in a direction that opens up a 

 new vista of possibilities. 



A nerve impulse travels at a rate not much more than 100 feet per second — an 

 extremely slow speed compared with that of electricity in a wire. It has been 

 thought to be of the nature of a chemical change sweeping along the nerve, but 

 that hypothesis is opposed by the fact that the most delicate thermo-pile shows no 

 production of heat, even when an impulse is caused to sweep repeatedly along the 

 same nerve. Again, it is far easier to fatigue a muscle than a nerve. A living 

 frog's nerve removed from the animal, and therefore deprived of all nutrition, can 

 retain its excitability for nearly an hour, although subjected all the while to thirty 

 or forty stimulations per second. An excised muscle, when similarly stimulated, 

 is exhausted far sooner, because the mechanical energy entirely springs from 

 chemical change in the muscular substance, and therefore the muscle is more easily 

 fatigued than the nerve. The molecular commotion in the excited nerve produces 

 a momentary electrical current ; but that result is not peculiar to nerve. The 

 same occurs in muscle when stimulated. Possibly the molecular movement is of 

 tho nature of a mechanical vibration ; at all events, we now know that a nerve 

 can transmit hundreds, even thousands, of impulses, or let us simply say vibrations, 

 per second. That fact is so important and significant in relation to the physiology 



' Memente tier Pnyclwjiliysik, 1860. 2nd edition, 1889, part ii. p. 282. 



