Manchester Memoirs, Vol xlii. (1898), No. 12. 23 



the cone fibre. At the linkage of the cone fibre with the 

 bipolar cell there takes place, we may infer, the first 

 differentiation. We may assume that whatever be the 

 exact nature of the changes which take place in the 

 bipolar cell, these are different from those taking place in 

 the cone cell. At the linkage of the bipolar cell with the 

 ganglionic cell, a second differentiation takes place. 

 Through the two linkages, in accordance with the general 

 principles on which, a little while back, I dwelt so long, 

 the simpler events started in the cone cell ^\w^ rise to the 

 subtler events which travel onwards from the ganglion 

 cell along its axon, the optic fibre. 



The differentiation thus begun in the retinal portion 

 of the mechanism is continued in the central cerebral 

 portion. The optic fibre, the axon of the ganglion cell, 

 runs a course the length of which is immense compared 

 with that of the axon of the bipolar cell. But that is 

 simply due to mechanical morphological causes ; the optic 

 nerv^e and tract are long in order to place the retina in a 

 suitable position for being affected by light. We have no 

 reason whatever for thinking that visual impulses, the 

 differentiated nervous impulses or nervous changes which 

 sweep along the optic fibre are in any way essentially 

 changed from the time they leave the perikaryon of the 

 retinal ganglionic cell until they reach their furthest 

 terminals. Each optic fibre ends by linking itself with some 

 cell, some nerve unit in one or other of the three nervous 

 masses of which I spoke at the beginning, in the corpus 

 quadrigeminum, in the corpus geniculatum, or in the 

 pulvinar of the optic thalamus. Here again, a further 

 differentiation, we may assume, takes place. Confining our- 

 selves, for simplicity's sake, to one of the three bodies, say 

 the corpus geniculatum, we find that the terminal of the 

 optic fibre links itself to a cell or, rather, to more than one 

 cell. The axon of that cell (or of each of those cells) either 



