974 



THE SENSES 



Rods. 



Cones. 



that this compound type of eye has already been abandoned ; the single 

 system of curved refracting surfaces so characteristic of the vertebrate 

 eye has made its appearance; and the formation of a clean-cut image 

 of the object on the retina, with the excitation of a sharply-bounded area 

 of that membrane, follows as a geometrical consequence from the theory 

 of lenses. 



We have to consider (i) the mechanism by which an image is 

 formed on the retina, and (2) the events that follow the formation 

 of such an image and their relations to the stimulus that calls them 

 forth. 



Structure of the Eye. The eye may be described with sufficient 

 accuracy as a spherical shell, transparent in front, but opaque over 



the posterior five-sixths of its 

 surface, and filled up with a 

 series of transparent liquids 

 and solids. The shell consists 

 of three layers concentrically 

 arranged, like the coats of an 

 onion: (i) An external tough, 

 fibrous coat, the sclerotic, the 

 anterior portion of which 

 appears as the white of the eye. 

 In front this external layer is 

 completed by the transparent 

 cornea. (2) A vascular layer, 

 the choroid, which, in the re- 

 stricted sense of the term, ends 

 in front in a series of folds or 

 plaits, the ciliary processes. The 

 choroid contains a greater or 

 smaller quantity of the black 

 pigment melanin. The ciliary 

 processes abut on the outer 

 boundary of the iris, which 

 may be looked upon as an 

 anterior continuation of the 

 choroidal or middle coat of the 

 eyeball. Between the corneo- 

 sclerotic junction and the an- 

 Fig. 398- Diagram of Structure of Retina terior portion of the choroid is 

 (after Cajal). H, layer of nerve-fibres; G, interposed a ring of unstriped 



\^\7f^r r\r oron mirvn rol1o TT internal rr*r\lo 



layer of ganglion cells; F, internal mole- 

 cular layer; E, internal nuclear layer; 

 C, external molecular layer; B, external 

 nuclear layer; external limiting membrane ; 

 A , layer of rods and cones. 



muscular fibres, the ciliary 

 muscle. (3) The inner or sen- 

 sitive coat, termed the retina 

 (Fig. 398). This covers the 

 choroid as a delicate mem- 

 brane, extending to the ciliary processes, where it ends in a 

 toothed margin, the ora serrata. The optic nerve forms a kind of 

 stalk to which the eyeball is attached. Its point of entrance at 

 the optic disc is a little nearer the median line than the antero-posterior 

 axis, which nearly passes through the centre of a small depression, 

 the fovea centralis, situated in the middle of the macula lutea, or 

 yellow spot. From the optic disc (sometimes called the optic 

 papilla) the optic nerve spreads over the retina as a layer of non- 



