730 REPORT— 1892. 



of the sense organs that I show you an experiment to render it more intelligible. 

 A frog's muscle has been hooked to a light lever to record its movement on a 

 smoked cylinder. The nerve of the muscle has been laid on two electrodes con- 

 nected with the secondary coil of an induction machine. In the primary circuit a 

 vibrating reed has been introduced to serve as a key for making and breaking the 

 circuit, and so stimulating the nerve with periodic "induction shocks. If we make 

 the reed long enough to vibrate ten times per second, ten impulses are sent through 

 the nerve to the muscle and ten distinct contractions produced, as shown by the 

 wavy line upon the cylinder. If we shorten the reed so that it will vibrate, say, 

 fifty times per second, the muscle is thrown into a continuous contraction aiid 

 traces a smooth line on the cylinder ; but if we listen to the muscle we can hear a 

 tone having a pitch of fifty vibrations per second, from which we know that fifty 

 nerve impulses are entering the muscle and inducing fifty shocks of chemical dis- 

 charge in the muscular substance. If we take a reed that vibrates, say, COO times 

 per second, we hear, on listening to the muscle, a tone having the pitch of 500 

 vibrations. Observe that we are not dealing with the transnaission of electrical 

 shocks along the nerve, but with the transmission of nerve impulses. By stimu- 

 lating the nerve with wires of a telephone it has been shown by D'Arsonval that a 

 nerve can transmit upwards of 5,000 vibrations per second, and that the wave- 

 forms may be so perfect that the complex electrical waves produced in the 

 telephone by the vowel sounds can be reproduced in the sound of a muscle after 

 having been translated into nerve vibrations and transmitted along a nerve. Such 

 •experiments go far in helping us towards a comprehension of the capabilities of 

 nerves in transmitting nerve vibrations of great frequency and complicated wave 

 form ; but although they enable us reasonably to suppose that all the fibres of the 

 auditory nerve can transmit nerve vibrations, simple or complex, and with a 

 frequency similar to that of all audible tones, we encounter superlative difficulty 

 in applying such a theory to the sense of sight. In objective sound we have to 

 deal with a comparatively simple wave motion, whose frequency of vibration is not 

 difficult to grasp even at the highest limit of audible sound— about 40,000 vibra- 

 tions per second. But in objective light the frequency of vibrat ion is so enormous — 

 amountmg to hundreds of billions per second — that ever^'one feels the difficulty of 

 forming any conception of the manner in which different frequencies of ether waves 

 induce diftereuces in colour sensation. 



But before passing to colour sense I wish to allude for a moment to the sense 

 ■of smell. The terminals of the olfactory nerve in the nose are epithelial cells. It 

 has been recently shown by Yon Brunn' that in man and other mammals the cells 

 have at their free ends verj- delicate short hairs, resembling those long known in 

 lower vertebrates. These hairs must be the terminal structures affected by sub- 

 stances that induce smell, and are therefore analogous to the hairs on the terminal 

 cells in our organ of hearing. No one ever suggested that the hairs of the 

 auditory cells can analyse sounds by responding to particular vibrations, and I 

 think it quite as improbable that the hairs on any particular olfactory cell respond 

 to the molecular vibrations of any particular substance. If we follow those who 

 have had recourse to the doctrine of specific activities to explain the production 

 of different smells, we must suppose that at least one special epithelial cell and 

 nerve fibre are aff"ected by each different smelling substance. Considering how 

 great is the variety of smells, and that their number increases with the production 

 of new substances, it would be a somewhat serious stretch of imagination to sup- 

 pose that for each new smell of a substance yet to emerge from the retort of the 

 ■cheniist there is in waiting a special nerve terminal in the nose. It seems to me 

 far simpler to suppose that all the hairs of the olfactory cells are affected by every 

 smelling substance, and that the different qualities of smell residt from differences 

 in the frequency and form of the vibrations initiated by the action of the chemical 

 molecules on the olfactory cells and transmitted to the brain. • That hypothesis i 

 was, I believe, first suggested by Professor Ramsay," of Bristol, in 1882, and it | 



' Von Brunn, Arcliir fiir mih-osltophclie Anatomic, 1892, Band 39, p. 633. 

 ? Karnsay, A"(!^j(r{', vol. xsvi. 1882, p. 189. . . 



