58 



CLASSIFICATION 



cornea, which permits light rays to enter. Within the sclerotic 

 is a much softer choroid coat containing numerous blood-vessels; 

 this, however, does not line the cornea and its immediate 

 neighbourhood, but runs transversely across the front part of 

 the eye to form the coloured part we call the iris, in the centre 



of which is a hole, 

 the pupil. Opti- 

 cally speaking the 

 iris is a diaphragm 

 for regulating the 

 amount of light 

 entering the eye, 

 _'* and the size of its 

 central aperture, the 

 pupil, varies in size 

 according to cir- 

 cumstances, becom- 

 ing very small in 

 a bright light and 

 very large in a dim 

 light. Lining the 

 choroid is a third 

 eye-coat, pulpy in consistency, and known as the retina. It 

 contains the end -organs for sight (rods and cones), and acts 

 as a sensitive screen upon which light falls. The second cranial 

 or optic nerves run into the retina, branching within it to supply 

 the rods and cones. 



The eye is filled with refracting structures, of which the most 

 important is the biconvex lens, placed just behind the iris. The 

 small space in front of the lens is filled by a clear fluid, the 

 aqueous humour, and the much larger space at its back by 

 the jelly-like vitreous h^lmour. Just as the lens of a photo- 

 grapher's camera forms a picture of surrounding objects upon 

 the sensitive plate at the back of the apparatus, so do the re- 

 fracting substances within the eye act with reference to the 

 retina. •" 



A study of the development of the eye shows that its sensitive 

 part, the retina, is formed from an outgrowth of the brain, and 

 as this organ is in reality a piece of skin folded in and much 

 modified, the retina must be looked upon as of the same nature. 



^i&' 33- — Representation of a vertical cut through the Eyeball in its Socket 



A, Cornea: b, aqueous humour; c, pupil; d, iris; e, lens; H, sclerotic; I, choroid; 

 K, retina ; l, vitreous humour ; m, optic nerve ; N, an eye-muscle ; p, an eyelid- 

 muscle, with an eye-muscle (o) below it ; Q r, eyelids. 



