108 ZOOLOGY SECT. 



its outer or choroidal surface with a layer of black pigment (P. E.). 

 It extends as far as the outer ends of the ciliary processes, where 

 it appears to end in a wavy line, the ora serrata (o. s.) : actually, 

 however, it is continued as a very delicate membrane (p. c. R.) over 

 the ciliary processes and the posterior face of the iris. The optic 

 nerve (O.N.) pierces the sclerotic and choroid and becomes con- 

 tinuous with the retina, its fibres spreading over the inner surface 

 of the latter. Microscopic examination shows that these fibres, 

 which form the innermost layer of the retina (Fig. 794, o. n.), turn 

 outwards and become connected with a layer of nerve-cells (n. c.). 

 External to these come other layers of nerve-cells and granules, 

 supported by a framework of delicate fibres, and finally, forming 

 the outer surface of the retina proper, a layer of bodies which 

 correspond to modified sensory cells and are called, from their 

 shape, the rods and cones (r.). These are placed perpendicularly 

 to the surface of the retina, and their outer ends are imbedded in 

 a single layer of hexagonal pigment-cells, loaded with granules of 

 the black pigment already referred to. 



Immediately behind and in close contact with the iris is the 

 transparent biconvex lens (Fig. 793, L.), formed of concentric layers 

 of fibres, each derived from a single cell. The lens is enclosed in 

 a delicate capsule, attached by a suspensory ligament (sp. I.) to the 

 ciliary processes. The suspensory ligament exerts a pull upon the 

 elastic lens so as to render it less convex than when left to itself ; 

 when the ciliary muscles contract they draw the suspensory ligament 

 towards the iris, relaxing the ligament and allowing the lens to 

 assume, more or less completely, its normal curvature. It is in 

 this way that the accommodation of the eye to near and distant 

 objects is effected. 



The space between the cornea in front and the iris and lens 

 behind is called the aqueous chamber of the eye, and is filled by a 

 watery fluid the aqueous humour. The main cavity of the eye, 

 bounded in front by the lens and the ciliary processes and for the 

 rest of its extent by the retina, is called the vitreous chamber, and 

 is filled by a gelatinous substance, the vitreous humour (V. H.). 



The cornea, aqueous humour, lens, and vitreous humour together 

 constitute the dioptric apparatus of the eye, and serve to focus the 

 rays of light from external objects on the retina. The iris is the 

 diaphragm by which the amount of light entering the eye is regu- 

 lated. The percipient portion or actual organ of sight is the retina, 

 or, more strictly, the layer of rods and cones. The great peculiarity 

 of the vertebrate eye, as compared with that of a Cephalopod 

 (Vol. I, p. 750), to which it bears a close superficial resemblance, 

 is that the sensory cells form the outer instead of the inner layer 

 of the retina, so that the rays of light have to penetrate the remaining 

 layers before affecting them. 



The mode of development of the eye is as characteristic as its 



