SENSE OF SIGHT. 617 



membrane. These layers, counting from the internal to the external 

 surface of the retina, are as follows: 1. The layer of nerve fibres, de- 

 rived from the expansion of the optic nerve; 2. The ganglionic layer 

 of nerve cells; 3. The layer of nuclei; 4. The layer of rods and 

 cones. 



1. Layer of Nerve Fibres. The optic nerve joins the posterior part 

 of the eyeball at a point about 2 millimetres inside its longitudinal axis, 

 and slightly below the horizontal plane of this axis. The neurilemma 

 of the nerve at once becomes continuous with the sclerotic coat of the 

 e}*eball, while the nerve fibres alone penetrate into its cavity. Up to 

 this point the fibres of the optic nerve present the usual dark-bordered 

 appearance of medullated nervfc fibres, and have, according to Kolliker, 

 a diameter of from 1 to 4.5 mmm. But at their entrance into the cavity 

 of the eyeball the nerve fibres not only lose the prolongations of con- 

 nective tissue which previously surrounded their different bundles, but 

 also become much smaller in size, being reduced, on the average, to 

 less than 2 mmm., and many of them to less than 1 mmm. in diameter. 

 Owing to these changes, the nerve appears suddenly diminished in size 

 at its passage through the sclerotic and choroid membranes. Internally 

 it forms a slight prominence on the inner surface of the wall of the eye- 

 ball, the so-called papilla ; and from a depression at its middle part, the 

 central artery and vein of the retina send out their branches to supply the 

 retinal capillary plexus. From the papilla as a centre the optic nerve 

 fibres, which have thus reached the inner surface of the retina, diverge 

 in every direction under the form of a closely set layer. This layer 

 diminishes gradually in thickness from within outward, and from behind 

 forward, owing to the fact that the nerve fibres of which it is composed 

 terminate successively in the deeper parts of the membrane, thus estab- 

 lishing a connection between every point of the retina and the nervous 

 centres in the brain. The longest fibres continue their course until they 

 reach the ora serrata at the anterior limit of the retina, beyond which 

 none are visible. 



2. Ganglionic Layer of Nerve Cells. This layer is situated imme- 

 diately outside the former, and contains, as its special distinguishing 

 element, multipolar nerve cells, similar to those of the gray matter of 

 the brain. According to Kolliker, they vary in size from 9 to 36 mmm. 

 in diameter, and are provided with a number of pale, ramified prolonga- 

 tions. Some of these prolongations are directed outward, penetrating 

 into the more external portions of the retina; others pass in a horizon- 

 tal direction, and, according to some observers (Kolliker, Miiller, Corti), 

 become connected with optic nerve fibres. For the most part, however, 

 it is only the identity in appearance between some of the prolongations 

 of these nerve cells and the more slender optic nerve fibres, which leads 

 to the presumption of their direct terminal continuity. It is, in any 

 case, possible that some of the fibres of expansion of the optic nerve 

 are connected with prolongations of the nerve cells, while others con- 

 tinue their course to the deeper layers of the retinal tissue. 



40 



