142 MR JAMES DEWAR AND DR M‘KENDRICK ON THE 
application of the normal stimulus, but only through the intervention of a 
terminal apparatus. For each sense a special terminal organ is required, the 
function of which is to receive the normal stimulus. This stimulus affects a 
change in the terminal organ, and the result is conveyed by the nerve to the 
nerve centre. 
Now arises the question of what is the specific effect of the external stimulus 
on the terminal organ. With regard to the ear, for example, there is the hypo- 
thesis that the hair-like processes of the cells situated on the delicate rods of 
Corti found in the scala media of the cochlea vibrate synchronously with waves 
of sound, and communicate these vibrations to the minute filaments of the 
auditory nerve which are in connection with them. With reference to vision, 
numerous theories have been put forward. NewrTon, MELLONI, and SEEBECK 
stated that the action of light on the retina consisted of a communication of 
mere vibrations; Youne conjectured that it was a minute intermittent motion of 
some portion of the optic nerve; Du Bois-REymonp attributed it to an electrical 
effect; DRAPER advanced.the view that it depended on a heating effect of the 
choroid; and Mosier compared it to the action of light on a sensitive photo- 
graphic plate. Up to this date, however, our knowledge of the specific effect 
produced by light on the retina is without any experimental foundation. 
Now, it is evident that, in accordance with the principle of the transference 
of energy, the action of light on the retina must produce an equivalent result, 
and this result may be expressed and measured as heat, chemical change, or 
electro-motive force. The change in the retina will likewise excite correspond- 
ing changes in the nerve, which will be conducted to the brain. What are the 
properties of a living nerve which can be affected by changes in the condition of 
the retina ? : 
A nerve has three distinct properties : 1s, sensibility, or the property by 
which it is capable of receiving an impression and producing an influence ; 2d, 
conductivity, or the property of conducting the influence to or from a nerve 
centre; and, 3d, electro-motive force, or the difference between the electrical 
potentials of two surfaces of the nerve, such as the longitudinal surface and 
transverse section. These three properties exist only in living nerve. When a 
nerve is removed from the body, and deprived of an adequate supply of healthy 
blood, it dies, and the rapidity of its death may be measured by the gradual 
loss of these properties. Now, we know that these properties are affected by a 
stimulus applied to the nerve. When a portion of a nerve is subjected to the 
influence of a continuous current of electricity, that portion passes into a condi- 
tion termed the electro-tonic state, in which these three properties are altered, 
and altered differently in the neighbourhood of each pole. It is difficult, how- 
ever, to ascertain with accuracy, by physical methods, the alteration in the pro- 
perties of sensibility and conductivity; but it is comparatively a simple matter 

