CHORDATA. 



353 



is the sensitive layer. Light falling directly on this from all 

 directions would produce no image, just as the photographic 

 plate exposed outside the camera would present a general blur. 

 We find then the necessity of the same optical devices as are 

 found in the camera: (i) a sensitive surface, the retina; (2) 

 a box for support and for keeping out the light except from 



FIG. 173. 



FIG. 173. Diagram showing some of the retinal elements in their relations to each 

 other. Layer i is directed toward the interior of the eye and consists of nerve fibres 

 (f) which finally enter the optic nerve at the blind spot; 2, the ganglion-cell layer, 

 made up of the nerve cells of which the fibres are a part; 3, the inner " molecular 

 layer" made up of the fine, much-branched nerve fibrils from 2 and 4; 4, the inner 

 nuclear layer, containing numerous ganglion cells (g) ; 5, the outer " molecular layer," 

 similar in structure to 3; 6, outer nuclear layer, containing the nuclei of the rod and 

 cone cells (r.c. and c.c.) ; 7, the layer of rods and cones (r, c). This is the nervous 

 epithelium, or the nerve-endings of vision. The rods and cones are partly imbedded 

 in a pigment epithelium (8). It must be remembered that hundreds of elements are 

 omitted where one is shown in the figure. 



Questions on the figure. Which portion of the retina does the light 

 first strike on entering the eye? To what point must it penetrate to arouse 

 nervous activity? Over what route must a nervous impulse pass to reach 

 the brain from the point of stimulation (rods and cones) ? Compare with 

 similar figures in other texts. 

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