CHAPTER XX. 



THE RETINA. 



The view of the interior of the eyeball which one gets with an 

 ophthalmoscope reveals a whitish lining, the retina, over thejsurface 

 of which are fine branches of arteries which supply it with blood. 

 There are two spots on it which differ in their appearance from the 

 rest of the surface. One of these, situated directly opposite the 

 centre of the pupil, is the MACULA LUTEA. This is slightly raised, 

 yellowish in colour, and has a tiny depression in its centre, the 

 FOVEA CENTRALIS, the part of the retina capable of the most accurate 

 vision. The blood vessels skirt around the yellow spot and do not 

 pass across it. Towards the nasal side of this and some little 

 distance from it there is a shining white spot, the OPTIC DISC, which 

 marks the passage through the retina of the fibres of the optic 

 nerve. 



In order that the student may have the structure of the retina fresh in his 

 mind as a basis for his experiments, he is reminded of the details of its anatomy 

 and histology which follow. 



The fibres of the optic nerve enter the eyeball from outside arranged in a 

 cylindrical fashion, enclosing in the middle an artery, branches of which we have 

 seen ramifying over the surface of the retina. When they have pierced the outer 

 layers of the retina, the nerve fibres of the cylinder divide and spread out over its 

 whole surface, thus forming the innermost retinal layer of all. The whole retina 

 extends forward in the eyeball only as far as the beginning of the ciliary processes. 

 From here on it is represented simply by the layer of black pigment cells which 

 clothes the posterior surface of the ciliary body. The cells which are sensitive to 

 light waves, the fundamental part of the whole organ of vision, are contained in one 

 of the layers of the retina. The retina itself is to be regarded not simply as com- 

 parable to a group of sensory nerve endings, connected each by a single fibre 

 directly to the central nervous system, but as a part of the brain itself, from an 

 outgrowth of which it is formed during the development of the foetus. Accord- 

 ingly it contains, as well as the light-sensitive cells themselves, two layers of relay 

 cells through which the impulses arising in the sensitive elements must pass on 

 their way to the brain (Fig. 46). The elements which respond to light waves, the 

 rods and cones, are placed farthest away from all the incoming light. They 

 are the free ends of the outer cell layer, sticking out towards the choroid coat from 

 which they are separated only by a layer of black pigment cells. The nuclei 



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