THE EYE AND THE SENSE OF SIGHT 217 



and with a narrow part, in which a nucleus lies, passing 

 inwards. The inner processes of both the rods and cones 

 l)i rak up into delicate fibrils, which meet the fibrils of other 

 nucleated cells deeper in the retina, and these meet in their 

 turn numerous fine nerve fibrils, springing from large nerve 

 cells, in which the fibres of the optic nerve end. So that 

 there is a connection, although not a simple one, between the 

 fibres of the optic nerve and the rods and cones. These, the 

 essential or nervous elements of the retina, are supported by a 

 delicate connective tissue framework, and so the structure of 

 the retina is made up. 



The layer of rods and cones is at the outer surface of the 

 retina, that is, the surface next to the choroid coat, while the 

 layer of nerve fibres and of nerve cells is next to the vitreous 

 humour ; so that the light, since it comes through the vitreous 

 humour, has to pass through the retina itself before it can 

 excite the rods and cones ; hence the whole retina is trans- 

 parent. In the centre of the back of the eye is a small oval 

 area, stained yellow and called the yellow spot. In the 

 central part of this yellow spot all the parts of the retina 

 except the layer of rods and cones are extremely thin, and here, 

 unlike the rest of the retina, the cones are more numerous 

 than the rods, indeed, at quite the centre are alone present, the 

 rods being absent. Owing to this extreme thinness of the 

 rest of the retina, light can here most easily pass to the cones. 

 It is in the yellow spot that vision is most distinct ; hence when 

 we wish to see a thing distinctly we look straight at it, so that 

 its image falls on the yellow spot. We see the other tiling 

 around, but not so distinctly, since their images are falling on 

 other parts of the retina. 



Since the fibres of the optic nerve are on the inner side of 

 the retina next to the vitreous humour, they must pierce not 

 only the sclerotic and choroid coats, but the retina itself, 

 before they spread out on its inner surface. There can, there- 

 fore, be no rods and cones at the spot where the optic nerve 

 HUT-., \\liich is not exactly at the centre of the back of the 

 eye, but somewhat on the inner side of it nearer the nose. 

 Light falling on this spot produces no effect. It is, there- 

 fore, called the blind spot. This is a proof that the sensa- 

 tion of light cannot arise without either rods or cones. The 



