782 A MANUAL OF PHYSIOLOGY 



other layers of the retina greatly thinned ; over the optic disc neither 

 rods nor cones are present. The disc is pierced by the retinal blood- 

 vessels (Fig. 289). 



External to the rods and cones is a sheet of pigmented epithelial 

 cells of hexagonal shape, belonging to the choroid, but remaining 

 attached to the retina when the latter is separated, and therefore 

 often reckoned as its most external layer. 



A little behind the cornea and anterior to the retina is the /ens, 

 enclosed in a capsule, and attached to the choroid by the suspensory 

 ligament, or zonule of Zinn. The iris hangs down in front of the lens 

 like a diaphragm, with a central hole, the pupil. Between the iris 

 and the posterior surface of the cornea is the anterior chamber of 

 the eye, filled with the aqueous humour. Between the iris and the 



anterior surface of the lens lies 

 the posterior chamber, which is 

 rather a potential than an actual 

 cavity. The space between the 

 lens and the retina is accurately 

 occupied by an almost structure- 

 less semi-fluid mass, the vitreous 

 humour, enclosed by the delicate 

 hyaloid membrane, which in 

 front is reflected over the folds 

 of the ciliary processes, and 

 blends with the suspensory liga- 

 ment of the lens. 



Refraction in the Eye- 

 Formation of the Retinal 



FIG. 289. RETINAL BLOODVESSELS . 



(HENLE). Image. The amount of re- 



The arteria centralis is seen issuing from fraction which a ray of light 



&firi2lfc l S^SffiCk undergoes at a curved sur- 



representing the yellow spot with the fovea f ace depends UDOn two 

 centralis in its centre. r t r 



factors, the radius of cur- 

 vature of the surface, and the difference between the 

 refractive indices of the media from which the ray comes and 

 into which it passes. The smaller the radius of curvature, 

 and the greater the difference of refractive index, the more 

 is the ray bent from its original direction. A ray of light 

 passing into the eye meets first the approximately spherical 

 anterior surface of the cornea, covered with a thin layer of 

 tears. Since the refractive index of the tears and of the 

 cornea is greater than that of air, refraction must occur 

 here. At the parallel posterior surface of the cornea, how- 

 ever, the ray is but slightly bent, for the refractive indices 



