THE RABBIT. 



117 



Fig 63. STRUCTURE OF RETINA. 



represents the lower right-hand part of Fig. 62 

 magnified (very diagrammatic). 



form the layer of the retina nearest the choroid, except for 

 a layer of deeply pigmented (i.e. black) tissue, which 

 embeds the ends of the rods and cones. Within the 

 rod-and-cone layer 

 (i.e. nearer the 

 vitreous humour) 

 comes a layer of 

 bipolar nerve-cells, 

 with only short 

 arborizing pro- 

 , and then a 



larger 



whose 



cesses, 

 layer of 

 nerve-cells, 

 axis-cylinders be- 

 come the fibres of 

 the optic nerve. 

 These fibres con- 

 verge from all 

 parts of the retina 

 (forming the inner- 

 most layer of it) 

 to a point called the blind spot, where all the other 

 layers of the retina are wanting : here they turn sharply 

 out and run along the optic nerve. The structure of 

 the retina is more complicated than the above description 

 would imply, because besides the essential parts described 

 there are supporting structures (neuroglia) around and 

 among them. All the inner layers are sufficiently 

 transparent to allow the light to pass through them, and 

 reach the sensitive rods and cones. 



10. The Blind Spot. We may pause to call the 

 student's attention to a little point in the physiology 

 of nerves, very happily illustrated here. The function 

 of a nerve fibre is conduction pure and simple ; the light 

 radiates through the fibrous layer of the retina without 

 producing the slightest impression, and at the blind spot, 

 where the rods and cones are absent, and the nerve-fibres 

 are gathered together, no impression is made at all. If 

 there is any doubt as to the existence of a blind spot 



