108 



CONSCIOUS NERVOUS OPERATIONS 



Direction of Light through Retina 



only about one fiftieth of an inch in thickness at the point 

 opposite the pupil, where it is thickest, it consists of ten 

 different layers (Fig. 64). Beginning with the side toward 

 the center of the eyeball, the first layer, called the internal 

 limiting membrane, is in contact with the vitreous humor 

 which fills the largest cavity of the eye ( 146), while the 

 tenth, or pigment-cell layer, is next the choroid. 



In the second layer, that of the optic nerve fibers, the 

 minute filaments of the optic nerve are distributed. From 

 this second layer they turn backward to enter the deeper 

 layers of the retina. 



The third layer is that of ganglion cells. They are 



large nucleated cells, 

 whose axis cylinder 

 processes are con- 

 tinuous with the 

 optic nerve fibers 

 of the second layer. 

 In the ninth layer 

 are found nerve cells 

 of peculiar shapes, 

 called rods and cones. 

 There are more rods 

 than cones, three or 

 four .rods usually 

 lying between two 

 cones. It is under- 

 stood that in these cells arises the nervous impulse which 

 results in vision. 



From the pigment-cell layer extends a thick fringe to 

 support the outer ends of the rods. Many of the rays of 

 light which fall upon the retina are absorbed by the pig- 

 ment, only a small part of the rays being reflected back 



Direction of Optic Impulse 

 To Brain 



Opti<rNerve 



Fig. 65. Diagram of a section of the retina. 



Showing that the rays pass from the anterior 

 to the posterior parts of the retina to reach the 

 rods and cones. From the latter the optic im- 

 pulse passes to the anterior parts of the retina, 

 and thence by the optic nerve fibers through the 

 posterior parts to the brain. 



