502 THE SENSE OF SIGHT. 



fibres to form the retina. This is the only point of the surface 

 of the retina from which the power of vision is absent. 



FIG. 181. 



C7i 



The posterior half of the retina of the left eye viewed from before (after Henle); 

 *, the cut edge of the sclerotic coat ; ck, the choroid ; r, the retina ; in the interior at 

 the middle, the macula lutea with the depression of the fovea centralis is represented 

 by a slight oval shade ; towards the left side the light spot indicates the colliculus or 

 eminence at the entrance of the optic nerve, from the. centre of which the arteria 

 centralis is seen spreading its branches into the retina, leaving the part occupied by 

 the macula comparatively free. 



On making a vertical section of the retina, it is seen, under 

 the microscope, to be composed of several layers, which differ 

 from each other in structure and arrangement, while besides 

 these there are fibres, the so-called fibres of Muller, which ex- 

 tend through the different layers, and perforate them, so to 

 speak. Fig. 182 represents a vertical section of a small piece 

 of the retina. On examination it will be seen that there are 

 three principal layers, bounded on the inner aspect by a mem- 

 brana limitams, and on the outer by the choroid coat. 1. The 

 outermost is the membrane of Jacob, or the columnar layer. 

 2. In the middle is the granular layer. 3. The innermost is 

 the nervous layer. Each of these layers, again, is composed of 

 different strata, after the fashion shown in the figure. 



The columnar layer (Jacob's membrane) is composed of cyl- 

 indrical or staff-shaped transparent and highly refractive 

 bodies, arranged perpendicularly to the surface of the retina, 

 with their outer extremities imbedded, to a greater or less 

 depth, in a layer of black pigment of the choroid coat. Re- 



