RETINAL CHANGES INVOLVED IN VISION 561 



Important differences are found in the structure of the retina in its 

 different parts. At the point of entrance of the optic nerve the optic disc 

 the only elements present are the nerve fibres, which diverge from this 

 point over the whole inner surface of the retina. A short distance externally 

 to the optic disc is found the macula lutea with a small depression in the 

 middle, the central spot oifovea centralis (Fig. 287). When we fix our gaze 

 on any object the visual axes are so directed that the image of the object falls 

 on the fovea centralis. At this spot the retina is thinned by the gradual 

 disappearance of all its layers except the outermost. The outermost layer 

 is moreover distinguished by the fact that the rods have disappeared and that 



FIG. 287. 



Section through half the fovea centralis. 

 and GOLDING BIRD.) 



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 pro- 

 cesses and the posterior surface of the iris as a layer, two cells thick, closely 

 packed with pigment granules (the uvea). 



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



