THE BLIND SPOT. 581 



mented cells of epithelial character, which, on their outer face, 

 show a striking hexagonal outline. 



A nerve fibril then may be said to have the following course : 

 entering with the other fibrils at the porus opticus, it reaches the 

 immediate vicinity of the hyaloid membrane, and runs a certain 

 distance in contact with that membrane, it then turns outwards 

 towards the choroid and enters a nerve cell. From the nerve 

 cell pass on a couple of filaments which pierce the various granu- 

 lar and nuclear layers where they probably freely inosculate 

 with the fibrils from other cells and finally terminate in a rod 

 or a cone. The rods and cones may then be regarded as the ulti- 

 mate terminals of the nerves, and they lie in the active proto- 

 plasm of the peculiar, pigmented epithelium cells. 



It is this outer layer, consisting of rods and cones lodged in 

 epithelial protoplasm, which is the really effective part of the 

 retina. Of this we have the following evidence : 



1. The fact that the rods and cones must be regarded as the 

 real anatomical nerve terminals of the optic nerve. 



2. Where the optic nerve enters the eyeball and the nerve fibres 

 are fully exposed to the light, there are no rods and cones. This 

 part, the optic disk, cannot appreciate light, and hence is called 

 the " blind spot." This fact shows that the nerve fibres are quite 

 insensitive to light, and that we must look to the terminals for its 

 appreciation. The existence of the blind spot can be demonstrated 

 as follows. Shut the left eye, and hold the left thumb, at ordinary 



a 



A 



reading distance, in front of the right eye. While the eye is fixed 

 on the left thumb bring the right thumb within about four inches 

 of it and move it slowly an inch or so from side to side. A little 

 practice soon finds a place when the right thumb nail disappears. 

 It also can be demonstrated by steadily fixing the right eye on the 



