CHORDATA 



357 



inner angle of the eye outward. Sometimes all three lids are 

 present together. In the lower groups the lids are wanting. 



The essential part of the eye is the sensory expansion of the 

 optic nerve — the retina — which occupies the innermost posi- 



PlG 



Fig. 176. 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 (J) 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 («); 5, the outer "mo- 

 lecular 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. — Prom what direction would the light come in this 

 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. 



tion, bounding the posterior portion of the cavity of the eyeball. 

 This is a very complicated layer, but a general idea of it can be 

 obtained from the diagram (Fig. 176). The layer of rods and 

 cones, in close connection with a layer of pigment, is the sensi- 



