XIII 



PHYLUM CHORDATA 



105 



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 anterior 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 posterior chamber, 

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



The cornea, aqueous, lens, and vitreous together constitute the 

 dioptric apparatus of the eve, and serve to focus the rays of light 



FIG. 740. Diagram of the retina, the supporting structures to the left, the nervous and epithelial 

 elements to the right ; ad. fibrous supporting structures ; gr. gr'. granular layers ; n.c. n.c'. 

 n.c". n.c'". nerve cells; nu. nuclear layer of rods and cones; o.n. fibres of optic nerve; 

 r. rods and cones. (From Wiedersheim's Vertebrata.) 



from external objects on the retina. The iris is the diaphragm by 

 which the amount of light entering the eye is regulated. 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. 720), 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. 



