Manchester Memoirs, Vol. xlii. (1898), No. 1^. 21 



" bipolar cell." (I am purposely omitting various com- 

 plications existing in the retina, as not bearing directly 

 on my theme.) This, again, by the terminals of its axon 

 is linked on to a "ganglionic cell," whose axon is a fibre 

 first of the optic nerve and then of the optic tract. This 

 constitutes the first or retinal portion of the mechanism. 



Now, we can easily obtain evidence that when the 

 retina is stimulated, either naturally by light or artificially 

 by other means, currents of action are developed in the 

 optic fibres. We may take these currents as the objective 

 tokens of what we have called nervous impulses passing 

 along those fibres. Consequent upon the development of 

 these nervous impulses in the optic fibres, visual sensations 

 are developed if the remaining cerebral portion of the 

 mechanism be intact ; the nervous impulses in the optic 

 fibres give rise in the cerebral mechanism to visual sen- 

 sations. 



So far as the objective token of the electric current 

 is concerned, we can find no difference between these 

 optic impulses, these nervous impulses along the optic 

 fibres, and the nervous impulses of an ordinary nerve. 



And yet they are different. If we stimulate artificially 

 an ordinary nerve fibre at any part of its course, we can 

 produce effects which, within certain limits, are identical 

 with the effects of natural stimulation. If we artificially 

 stimulate a motor nerve, we can almost if not quite com- 

 pletely reproduce natural effects, movement and the like. 

 If we stimulate a sensory nerve, we can at least reproduce 

 the natural effect of pain, and many other natural effects. 

 If, on the other hand, we stimulate artificially the optic 

 nerve at any part of its course, we do not (according to 

 the most trustworthy observations, though all observers 

 are not agreed) give rise to visual sensations ; so far as 

 can be ascertained we do not produce any psychical effect 

 whatever. If the retina be stimulated, whether the stimulus 



