644 THE EYE 



It is limited anteriorly by the internal surface of the iris, and an- 

 tero-externally by the ciliary processes. Its postero-internal boundary 

 is formed by the marginal portion of the lens, together with the adjacent 

 portion of the hyaloid membrane, which incloses the vitreous humor. 



THE INTERNAL COAT THE NERVOUS TUNIC 



The internal coat of the eyeball is divisible into three portions: 

 1, the pars optica retinas or retina proper; 2, the pars ciliaris retinae, 

 and 3, the pars iridica retinas. 



The last two portions, though morphologically continuous with the 

 pars optica retinas, differ therefrom in their physiological function ; they 

 respectively form the innermost layer of the ciliary body and iris. As 

 such they have already been described. 



GENERAL CONSIDERATION OF THE RETINA 



The retina (pars optica retince) may be said to be formed by the 

 radial expansion of the fibers of the optic nerve which enter the eye at 

 the inner side of its posterior pole, piercing the sclera and choroid and 

 spreading out over the inner surface of the eyeball. 



These nerve fibers arise from groups of nerve cells which are disposed 

 in layers to form the optic and retinal ganglia (ganglion nervi optici 

 and ganglion retince). The association of nerve cells and fibers with 

 their supporting tissues forms the inner, cerebral, or neural portion of 

 the retina. The dendritic arborizations of many of these nerve cells lie 

 within the outer half, or neuro-epithelial portion of the retina. 



The retina may be said to extend forward from the entrance of the 

 optic nerve (optic disk) as far as the posterior margin of the ciliary 

 body, where it apparently ends abruptly with an indented border, the 

 ora serrata. From this border the retina is continued farther forward, 

 but only as the dark pigmented layers of the ciliary processes and iris. 

 In the usual preparations these layers contrast intensely with the opaque 

 white color of the true retina. Like all the other tissues which are 

 placed in the optical axis of the eye, the retina, during life, with the ex- 

 ception of its pigment layer, is perfectly transparent, but becomes 

 opaque immediately after death or local injury. 



The retina presents on its inner surface a slightly elevated yellow 

 spot, the macula lutea, which lies exactly at the posterior pole of the 



