222 STUDIES IN ELECTRO-PHYSIOLOGY: 



connecting by means of various nerve processes and gan- 

 glionic cells with the brain. 



" At the fovea each cone is connected to a separate 

 chain of neurons, whereas in other regions the rods and 

 cones are connected in groups to these chains. ... At 

 the exit of the optic nerve the only structures present are 

 nerve-fibres. . . . The nerve-cells in the retina remind us 

 that the optic, like the olfactory nerve, is not a mere nerve 

 but an outgrowth of the brain." (Halliburton.) 



The clearest, if not the most comprehensive, exposition 

 of the structure and functioning of the eye, so far as my 

 reading goes, is contained in Thornton's Human Physiology. 

 Briefly summarising this, I learn that the outermost layer 

 of the retina next to the choroid consists of a single stratum 

 of hexagonal epithelium containing black but, according 

 to Schafer, dark brown pigment. They are present in 

 all parts of the retina, except at the entrance of the optic 

 nerve. The outer surface of the cells is smooth and flat, 

 but the inner part is prolonged into fine processes which 

 extend between the rods. About 7,000 cones are said to 

 exist in the fovea. Near the macula lutea the retina 

 contains one cone to four rods ; midway to its termination 

 at the ora serrata one cone to twenty -four rods ; at the 

 peripheral part rods only. 



Visual impulses begin in the rods and cones on the outer 

 side of the retina, after the rays of light have passed 

 through most of the retinal layers, and the processes 

 started in these sensory epithelial cells of the retina pass 

 back to the layer of fibres on the inner surface of the retina 

 and thence by the optic nerve to the brain. 



We know that the retinal vessels are distributed in the 

 inner layers (nerve-fibres and ganglionic cells) of the 

 retina, and the shadows cast behind them must be per- 

 ceived by something posterior to those vessels. This is a clear 

 proof, it is said, that the external layers of the retina nearest 



