112 ZOOLOGY SECT. 



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 vitreoiis 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, 



invl 



tjot.sl 



FIG.. "786; Early (A) and later (B) stages in the development of the eye of a Craniate. 

 dien. diencephalon ; inv. I. invagination of ectoderm to form lens ; I. lens ; opt. c. outer, and 

 opt. c'. inner layer of optic cup ; opt. st. optic stalk ; opt. v. optic vesicle ; ph. pharnyx ; 

 pty. pituitary body. (Altered from Marshall.) 



or, more strictly ; the layer of rods and cones. The great peculi- 

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

 (Vol. I, p. 772), 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 

 structure. At an early stage of development a hollow outgrowth 

 the optic vesicle (Fig. 786, A, opt. v.} is given off from each side of 

 the diencephalon (dien.}. It extends towards the side of the head, 

 where it meets with an inpushing of the ectoderm (inv. I.) which 

 deepens and forms a pouch, and finally, separating from the 

 ectoderm, a closed sac (B, I.) with a very small cavity and thick 

 walls. This sac is the rudiment of the lens : as it enlarges it pushes 

 against the optic vesicle, and causes it to become invaginated (B), 



