INTRODUCTION AND METHODS 



The amount of light which enters the eye is controlled by the iris, 

 a membrane having a circular aperture called the pupil. The iris has 

 the form of a very shallow truncated cone, the pupillary border of 

 which is supported by the lens. In darkness the pupil expands to its 

 greatest diameter, about 8 mm, while in very bright light it contracts 

 to about 1-5 mm. Since the focal length of the eye is about 20 mm, 

 the effective aperture can thus vary between /2-5 and /.1 3, giving a 

 range of nearly 30: 1 in the amount of hght admitted. 



THE RETINA AND ITS CONNECTIONS TO THE BRAIN 



The retina covers the inside of the posterior part of the eyeball. 

 Over most of its surface the retina is only loosely attached but it is 

 firmly bound along its periphery at the ora serrata and also at the 

 lamina cribrosa (through which pass the fibres of the optic nerve). 



Although the retina is only one or two hundredths of an inch thick, 

 it has a very complicated structure. A diagrammatic section is 

 shown in Fig. 1.2. In essence, the retina has two parts, a sensory 

 neuro-epithehum consisting of the percipient elements (the 'rods' and 

 *cones') and an inner, extremely complex, arrangement of nervous 

 tissue, similar both in structure and function to the central nervous 

 system of which it is, in fact, an outlying part. 



It is in the rod-cone layer that the image cast upon the retina by the 

 dioptric apparatus of the eye is in sharpest focus. The fight thus 

 fafiing on the rods and cones causes changes in them which disturb 

 the electrical equifibrium of the outer nuclear layer (Fig. 1.2). 



The electrical changes so initiated are subject to mutual interaction 

 within the nervous layers of the retina. The ultimate messages from 

 the retina are transmitted by the optic nerve fibres. The bundles of 

 nerve fibres proceeding from each eye divide at the optic chiasma, so 

 that those serving the left-hand halves of each retina continue to the 

 left side of the brain, and those serving the right-hand halves, to the 

 right side. Since (as in the camera) the retinal image is not only 

 inverted but also laterally transposed, this means that the right-hand 

 visual fields of both eyes are represented in the left brain, and vice 

 versa. Consequently, because the left brain relates to the right-hand 

 side of the body, the projection on the brain of the right visual field 

 (for example) is topographically close to those areas serving the right 

 musculature and sensory systems. 



On arrival at the brain the electric changes propagated through 

 the optic nerve fibres give rise, in some unknown and perhaps 



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