264 THE ANATOMY OP THE HORSE. 



dilator muscle of the pupil, which are derived from the sympathetic 

 system. 



Directions. — In the immersed eye from which the cornea and part of 

 the sclerotic have been removed, the portion of choroid exposed is to be 

 torn away with two pairs of forceps from the subjacent retina. The 

 inner surface of the membrane will be seen, through the transparent 

 vitreous humour, in the submerged half of the eye that was frozen. 



The Eetina is the most delicate of the coats of the eyeball. It is 

 formed by the radiation of the optic nerve on the inner surface of the 

 choroid, and like that coat it is bell-shaped. Its external or choroidal 

 surface is covered by a layer of hexagonal pigment cells, which were at 

 one time referred to the texture of the choroid. Its inner surface is 

 moulded on the vitreous humour. This surface shows a little to the 

 inner side of the summit of the bell, or of the antero-posterior axis of 

 the eyeball, a disc-like elevation — the papilla optica, which is the point 

 at which the optic nerve begins to expand. In the centre of this spot 

 the arteria centralis retince appears, and divides into branches which 

 radiate on the inner surface of the retina. The nervous structures of 

 the retina terminate at a wavy line — the ora serrata — behind the 

 ciliary processes ; but the retina is continued beneath these processes 

 in the form of an epithelial layer — ^the pars ciliaris retince, which forma 

 the edge of the bell. 



In the human eye a yellow spot — the macula lutea — is placed a little 

 external to the papilla optica, and almost exactly in the antero-posterioi 

 axis of the eyeball. This is not present in the eye of the horse or in any 

 mammal lower than the quadrumana. 



The perfectly fresh retina is translucent and of a pale pink colour, 

 but it speedily becomes opaque. In consistence it is delicate and 

 jelly-like. 



Structure. — Ten distinct layers are described as composing the thick 

 ness of the retina. These enumerated from within to without are as 

 follows : — 



The Memhrana Limitans Interna. — This, although appearing as s 

 distinct line in a transverse section, is not a distinct stratum, but merelj 

 the inner limiting line of a sustentacular framework — the radial fibres o: 

 Miiller — which pervades and supports the nervous elements in the 

 other layers of the retina. 



2. The La^er of Nerve fibres. — This layer results from the rMiatioii 

 of the optic nerve, whose fibres at their point of entrance into the eye 

 ball lay aside their medullary sheath. 



3. The Layer of Nerve Cells. — This is a single layer of multipolai 

 nerve cells. 



4. The Inner Molecular Layer is a thick stratum of fibres and inter 

 mediate granular matter. 



