THE RETINA. 501 



use of the choroid is to absorb, by means of its pigment, those 

 rays of light which pass through the transparent retina, and 

 thus to prevent their being thrown again upon the retina, so 

 as to interfere with the distinctness of the images there formed. 

 Hence animals in which the choroid is destitute of pigment, 

 and human albinos, are dazzled by daylight, and see best in 

 the twilight. The choroid coat ends in front in what are called 

 the ciliary processes (Fig. 180*). 



FIG. 180. 



Ciliary processes as seen from behind. 1, posterior surface of the iris with the 

 sphincter muscle of the pupil ; 2, anterior part of the choroid coat ; 3, one of the 

 ciliary processes, of which about seventy are represented. 



The retina (Fig. 181) is a delicate membrane, concave, with 

 the concavity directed forwards and ending in front, near the 

 outer part of the ciliary processes in a finely notched edge 

 the ora serrata. Semi-transparent when fresh, it soon becomes 

 clouded and opaque, with a pinkish tint from the blood in 

 its minute vessels. It results from the sudden spreading out 

 or expansion of the optic nerve, of whose terminal fibres, ap- 

 parently deprived of their external white substance, together 

 with nerve-cells, it is essentially composed. 



Exactly in the centre of the retina, and at a point thus cor- 

 responding to the axis of the eye in which the sense of vision 

 is most perfect, is a round yellowish elevated spot, about ^4 of 

 an inch in diameter, having a minute aperture at its summit, 

 and called, after its discoverer, the yellow spot of Scemmering. 

 It is not covered by the fibrous part of the retina, but a layer 

 of closely set cells passes over it, and in its centre is a minute 

 depression called fovea ceniralis. About y 1 ^ of an inch to the 

 inner side of the yellow spot, and consequently of the axis of 

 the eye, is the point at which the optic nerve spreads out its 



