HISTOLOGY OF RETINA. 487 



abundant according as the eye is black, brown, or gray. In 

 blue eyes the pigment is confined to the deeper layers and 

 modified in tint by light absorption in the anterior color- 

 .. less strata through which the light passes. 

 ( ~} ) The third coat of the eye, the retina, 15, is its essential 

 "^portion, being the part in winch the light produces those 

 changes that give rise to impulses in the optic nerve. It 

 is a still less complete envelope than the second tunic, ex- 

 tending forwards only as far as- the commencement of the 

 ciliary processes, at least in its typical form. It is extremely 

 .soft and delicate and, when fresh, transparent. Usually 

 when an eye is opened it looks colorless; but by taking 

 proper precautions the natural purple color of some of its 

 outer layers can be seen. Its most external layer, more- 

 over, is composed of black pigment cells. On its inner 

 surface two parts, different from the rest, can be seen in a 

 fresh eye. One-is the point of entry of the optic nerve, 

 16, the fibres of which, penetrating the sclerotic and 

 choroidj spread out in the retina. At this place the retina 

 is whiter than elsewhere and presents an elevation, the 

 optic mound. . The other peculiar region is the yellow spot 

 (macula luted], 18, which lies nearly at the posterior end of 

 the axis of the eyeball and therefore outside the optic mound; 

 in its centre the retina is thinner than elsewhere and so a 

 ~pit(fQveaccntrahs), 18, is formed. This appears black, the 

 thinned retina there allowing the choroid to be seen through 

 it more clearly than elsewhere. In Fig. 125 is represented 

 the left retina as seen from the front, the elliptical darker 

 patch about the centre being the yellow spot, and the 

 'white circle on one side, the optic mound. The vessels of 

 the retina arise from an artery (17, Fig. 124) Avhich runs 

 in with the optic nerve and from which branches diverge 

 as shown in Fig. 125. 



The Microscopic Structure of the Retina. A simplified 

 stratum, continuous with the proper retina, and formed of 

 a layer of nucleated columnar cells is continued over the 

 ciliary processes: elsewhere the membrane has a very com- 

 plex structure and a section taken, except at the yellow 

 spot or the optic mound, shows ten layers, partly sensory 



