466 



THE RABBIT. PHYLUM CHORDATA 



eye a very complicated structure (Fig. 419) and is connected with 

 the optic nerve, by which the impulses which give rise to sight are 

 conveyed to the brain. The fibres of the optic nerve pass right 

 through the retina and spread out over its inner surface (that 

 which is turned towards the hollow of the eyeball). The percipient 

 cells are on the outer surface, against the pigment layer, so that 



NASAL 



internal rectus 

 muscle \1.> 



sclerotic 



choroid 



retina 



TEMPORAL 



point of attachment 

 of ciliary muscle 

 to sclerotic 



conjunctiva 



ris 



cornea 

 ens 



ens capsule 



aqueous 

 humour 



suspensory 

 "gament 



ciliary 

 body 



external rectus 

 muscle 



Fig. 365.— a diagrammatic horizontal section of a mammalian eye. The thickness 

 of the choroid and retina, and especially of the anterior continuation of the 

 latter over the posterior surface of the cihary body and iris, is exaggerated. 



light must pass through the layer of nerve fibres to reach them. 

 Where the nerve leaves the eye there are no sense cells, so that 

 there is a blind spot ; the fovea is an area of special acuity, on 

 which images are normally focused. In the front half of the eye 

 the retina loses its complicated structure and becomes very 

 thin, but it continues to line the posterior chamber up to the 

 edge of the pupil. 



For an object to be seen it is necessary that an image of it 

 should be formed on the retina. The formation of such an image 

 is due mainly to refraction of light by the lens, but refraction 

 also takes place at the surfaces of all the other media (cornea, 

 aqueous humour, vitreous humour) through which the light 

 passes in the eye. In the diagram below (Fig. 366) these effects 

 are combined and the refraction shown as taking place at a 



