ANATOMY OF THE EYE 277 



a])|tai;itus and corrcspniKls to tlu' plate of a camera. Histologist.s divide the 

 substance of the retina into eight layers, viz. .starting from the si(U' on whii li 

 the light falls, i.e. next to the vitreous hiinioiii- : 



1. Stratum opticum. Layer of non-myelinated nerve fibres. 



2. (Janglionie nerve cell layer. 



3. Inner molecular layer. Interlacing dendrites of 2 and -i. 



4. Inner nuclear layer. Bipolar nerve cells. 



5. Outer molecular layer. Dendrites of 4 and fi. 



6. Outer nuclear layer. Neurones of rods and cones. 



7. Bacillary layer. Layer of rods and cones. 



8. Stratum pigmenti. 



For our purpose we may consider four sets of elements in the retina, viz. 

 neurones, rods, cones, and pigment-containing cells. 



(fl) The bacillary layer contains structures known as rods and cones. 

 These structures are believed to be the actual sensitive structures of the eye. 

 Lender certain conditions, e.g. when the eye has been in the dark for some time 

 before death, fine processes of the pigment cells can be seen to pass up between 

 the cells of this layer. (6) An intermediate layer of bipolar cells which 

 function as connector neurones and link up the rods and cones with the next 

 layer of neurones, (c) The ganglion cells. 



The axons of the ganglion cells pass horizontally across the inner surface of 

 the retina and converge on a point at the back of the eyeball slightly internal 

 to and just below the antero-posterior axis, to pierce the chorioid and sclera 

 to form the optic nerve. These layers, as mentioned above, do not, however, 

 extend so far anteriorly as the ciliary region, but are represented in this 

 region by a double layer of pigmented cells constituting the }xirs ciliaris 

 retin(e. The sensitive retina itself shows variations in structure in different 

 regions. 



At a point where the antero-posterior axis meets the retina there is an 

 area which is yellow in colour, the macula lutea, in the centre of which is a 

 depression, the fovea centralis. At the fovea, which is the area for direct 

 vision, only cones are found, and here the cones are larger than in other areas 

 of the retina. In addition to the absence of rods at the fovea the remaining 

 layers of the retina are not represented, the centrally directed processes of the 

 cones diverging towards the periphery of the macula to end in relation to the 

 ganglion cells, which are at this region found to be of several layers deep. 



The retina receives its own arterial blood supply from the arteria centralis 

 retinae, a small artery which enters the eyeball at the site of exit of the optic 

 nerve. Branches of this artery radiate on the inner surface of the retina, 

 supplying all areas excepting the fovea centralis. 



The three coats as described constitute the walls of the eyeball and enclose 

 the three refractive media, i.e. the aqueous humour, the lens and the vitreous 

 body. 



The lens is a laminated biconvex, transparent, elastic structure, the posterior 

 surface of which is more convex than the anterior surface. The lens is placed 

 just behind the iris and centred with the pupil, and is enclosed in an elastic 

 membrane called the capsule. The periphery of the capsule is attached by a 

 thickened portion of the hyaloid membrane {vide injra), known as the 

 suspensory ligament, to the ciliary processes. 



The anterior compartment of the eyeball contains a clear fluid, the aqueous 

 humour, and the posterior portion a more jelly-like substance, the vitreous 

 body. The vitreous body (or humour) is enclosed in a membrane, the 



