464 THE EYE. 



cone-visual cells, but end on the cell-bodies of the ganglion ceils 

 of the ganglion-cell layer. The horizontal cells send their dendrites 

 into the outer molecular layer, while their neuraxes extend hori- 

 zontally and give off numerous collaterals to the same layer, ending 

 there in telodendria. These cells are of two varieties: the smaller, in- 

 directly connecting the cone-visual cells with one another by means 

 of their dendrites and neuraxes ; and the larger, more deeply situated 

 cells, connecting in a similar manner the basilar ends of the rod- 

 visual cells. A few cells of the second variety give off one or 

 two dendrites each, which penetrate through the inner nuclear layer 

 into the inner molecular layer. 



3. The inner molecular layer. This is composed of five strata. 

 The majority of the spongioblasts (amacrine or parareticular cells) 

 in the inner nuclear layer send their processes upward into the inner 

 molecular layer, in which some end in fine arborizations in the first, 

 others in the second, and still others in the third interstice, separat- 

 ing the strata of the inner molecular layer from one another. Be- 

 sides these so-called stratum spongioblasts, there are also others in the 

 inner nuclear layer, the diffuse spongioblasts, whose ramifications end 

 simultaneously in several or in all of the strata of the inner molecu- 

 lar layer. Besides the ramifications of the spongioblasts just men- 

 tioned, autochthonous cells are also present. These lie in one of 

 the interstices of the molecular layers, their ramifications spreading 

 out in a horizontal direction. Besides all these structures the den- 

 drites of the cells in the ganglion-cell layer also ramify throughout 

 the inner molecular layer. 



4. The ganglion - cell layer. The cell-bodies are irregularly 

 oval ; their dendrites extend into the inner molecular layer, and 

 their neuraxes into the nerve-fiber layer. According to the 

 manner of their dendritic termination, the ganglion cells may be 

 divided into three groups: (i) those the dendrites of which ex- 

 tend into. but one stratum of the molecular layer ; (2) those the 

 dendrites of which extend into several strata of the molecular layer ; 

 and (3) those the dendrites of which are distributed throughout the 

 entire thickness of the molecular layer. Thus, these three groups 

 are made up of the so-called mono-stratified, poly -stratified, and 

 diffuse cells ; by means of their dendrites they come in contact with 

 one or several of the neuraxes of the bipolar cells of the inner 

 nuclear layer. 



5. The nerve-fiber layer of the retina. This layer consists of 

 centripetal neuraxes from the ganglion cells of the ganglion-cell 

 layer, and of centrifugal nerve -fibers ending in various layers of 

 the retina, including the outer molecular layer. 



8. THE OPTIC NERVE. 



Within the orbit the optic nerve possesses an external sheath, 

 which is an extension of the dura mater and is continuous with the 

 scleral tissue, and an inner sheath, which is a prolongation of the pia 



