352 



ZOOLOGY. 



brane) works from the inner angle of the eye outward. Some- 

 times 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- 

 tion, bounding the posterior portion of the cavity of the eye- 

 ball. This is a very complicated layer, but a general idea of 

 it can be obtained from the diagram (Fig. 173). The layer 

 of rods and cones, in close connection with a layer of pigment, 



FIG. 172. 



FIG. 172. Diagrammatic horizontal section through the right eye of Man. The line 

 a p is the axis of vision. The optic nerve leaves the eye on the median side of this 

 line, a.c., central artery; o./t., aqueous humor; b., blind spot, the entrance of the 

 optic nerve; c, conjunctiva; ch., choroid layer of the eye-ball; c.l., crystalline lens; 

 c.m.c., circular fibres of the ciliary muscles; c.m.r., radial fibres of the ciliary muscles; 

 co., cornea, the transparent portion of the sclerotic; c.p., ciliary process; c.s., canal of 

 Schlemm, a lymphatic vessel; fo., fovea centralis, the point of clearest vision; o.n., 

 optic nerve; o.s., ora serrata, the anterior wavy margin of the visual portion of the 

 retina; r, the retinal layer; sc., sclerotic layer; sh., sheath of the optic nerve; v.h., 

 vitreous humor. 



Questions on the figure. Which is the essential sensory portion of the 

 eye? Which parts are concerned in bringing the rays of light to a focus? 

 How many refractive surfaces are present? How many refractive media? 

 Which portions of the eye are primarily protective and supportive? What 

 is the function of the various parts of the choroid layer? In what way j s 

 an image formed on the retina, of objects in front of the lens? 



