626 PHYSIOLOGY 



only the cones are present, and are very much larger than the 

 cones in any other part of the retina. The fibres from those cones, 

 passing to the inner nuclear layer, diverge as they leave the fovea 

 centralis, all the layers of the retina being displaced towards the 

 circumference in order to allow the light to fall on the cones without 

 having to pass through any of the other layers of the retina. 

 As we pass from the centre to the periphery of the retina the 

 cones become fewer and the rods more numerous. At the extreme 

 margin the rods also are scattered more diffusely, and at the ora serrata, 

 which lies a short distance behind the ciliary processes, the special 

 nervous elements come to an end, and the retina is continued forwards 

 over the ciliary processes and the posterior surface of the iris as a r layer, 

 two cells thick, closely packed with pigment granules (the uvea), 



The following facts show that the layer of the rods and cones 



FIG. 286. 



represents the end -organ of vision, and that for distinct vision to take 

 place the image of an external object must be formed in this layer : 



(a) The point of entry of the optic nerve, where the whole thickness 

 of the retina is composed of nerve fibres, is absolutely insensitive to light 

 and constitutes the blind-spot (Fig. 286). If the light of a small flame 

 be directed, by means of a mirror, into the eye so that it falls only on 

 to the optic disc, the individual receives no sensation of light. The 

 existence of the blind-spot is more easily shown by the following 

 experiment : On closing the left eye and gazing fixedly with the right 

 eye at the white cross in the figure, on approximating the book to the 

 eye a point will be found, when the book is at a distance of eight 

 inches from the eye, at which the white circle becomes invisible and 

 the whole figure appears to be covered by the black ground. By 

 measuring the apparent size of the blind-spot and its distance from 

 the point of fixation, we find that its situation on the retina corresponds 

 exactly to the point of entry of the optic nerve. The blind-spot is so 

 large that at a distance of about six feet the image of the head of a 

 man will fall on it and therefore be invisible. 



(6) At the point of most distinct vision, i.e. the fovea, centralis, a,H 



