460 



THE EYE. 



3. REGION OF THE OPTIC PAPILLA. 



The optic papilla is the point of entrance of the optic nerve 

 into the retina. At the center of the papilla, in the region where 

 the nerve-fibers spread out radially in order to supply the various 

 areas of the retina, is a small, funnel-shaped depression, the physi- 

 ologic excavation. The fibers of the optic nerve lose their medullary 

 sheaths during their passage through the sclera and choroid, and then 

 continue to the inner surface of the retina, over which they spread in 

 a layer which gradually becomes thinner toward the ora serrata. On 

 account of the deflection of the nerve- fibers, and because, during 



Physiologic excavation. 



Layer of nerve-fibers.... 



Inner nuclear layer. "" 



Outer molecular lay, T. - 



Outer nuclear layer..- 



Rods and cones. ' 



Pigment layer. .-' 



Sclera. 



Lamina cribrosa.--' 



Fig. 359. Section through point of entrance of human optic nerve ; X 4- 



their passage through the sclera, they lose their medullary sheaths 

 at one and the same point, the optic nerve becomes suddenly 

 thinner. The result is a deeply indented circular depression in this 

 region. On this depression border the three ocular tunics. At this 

 point the retina is interrupted, the outer layers extending to the bot- 

 tom of the depression, while the inner cease at its margin. In many 

 cases the outer layers of the retina are separated from the optic nerve 

 by a thin lamina of supporting tissue (intermediate tissue). 



4. REGION OF THE MACULA LUTEA. 



At the center of the macula lutea is a trough-like depression, 

 the fovea centralis, the deepest part of which, \hzfnndits, lies very 

 close to the visual axis. Here the layers of the retina are practic- 

 ally reduced to the cone-visual cells. The margin of this depression 

 is somewhat thickened, owing to an increase in the thickness of the 

 nerve-fiber and ganglion-cell layers. Toward the fundus of the fovea 

 each of the four inner retinal layers becomes reduced in thickness, 

 the inner layer first and the three others in their order : the inner 

 molecular layer, however, seems to extend as far as the fundus. As 

 we have seen, only the cone-visual cells are found in the fovea cen- 

 tralis, there being an entire absence of the rod-visual cells. Since 

 the nuclei of the cone -visual cells are in the immediate neighborhood 



