464 



THE SPECIAL SENSES 



nerves) pass to the back of the orbits, 

 eyeball, and, spreading out their fibers, 



where they enter the 

 have their endings in 

 the end organs of the 

 retina. The projec- 

 tion of an image upon 

 the retina stimulates 

 these end organs and 

 thus arouses the im- 

 pulses of vision. 

 Plow the brain cells 

 interpret the impul- 

 ses produced by an 

 inverted image into 

 a sensation of sight 

 is unknown. 



Structure of the 

 retina. — The optic 

 nerve fibers enter 

 the retina on its 

 anterior surface. 

 Near this surface 

 the fibers connect 

 with the axis cylin- 

 ders of large gang- 

 hon cells. These cells 

 give off other cylin- 

 ders which connect 

 with other cells (see 

 Fig. 221), and these 

 connecting cells and 

 cylinders finallj^ terminate in modified nerve cells at the 

 very back of the retina next to the choroid layer. This last 



i Direction of Light through Retina 



^~4. 



i-^l-^ 



Optic^Nerve 



Fig. 221 — A, Diagram of structure of the retina 

 seen under the compound microscope; S, the 

 essential nervous elements; C, the direction of 

 light stimuli; 1, internal membrane; 2, nerve 

 fibers; 3, nerve cells; 4, terminal brushes of 

 neura; 6, inner nerve cell layer; 6, 2d set 

 terminal brushes; 7, nerve cells; 8, external mem- 

 brane; 9, rods and cones; 10, pigment layer. 



