VISIOK. 235 



of substance, one within another, like an onion; and the 

 layers increase in density towards the centre, which is so 

 firm that it is sometimes called the nucleus of the lens. 

 The layers are composed of fibres extending from front to 

 back, each with a nucleus, and remarkable in having serrated 

 edges by which they fit into one another. 



The capsule of the lens is in contact in front with the 

 inner edge of the iris, and there is a space left between it 

 and the cornea. This is filled with fluid, the aqueous 

 humour ; and as much of the space as lies in front of the 

 iris is called the anterior chamber; while the remaining part, 

 forming a slight interval between the back of the iris and 

 the lateral part of the lens, is distinguished as the posterior . 

 chamber. 



Fig. 119. DEVELOPMENT OP THE EYE, a diagram, <z, Cuticular 

 epithelium; b, lens developed by invagination of cuticle; C 9 

 entrance from the cavity of the brain into the primary optic 

 vesicle ; d, secondary optic vesicle ; e, e, pia mater ; /, choroid 

 coat ; g, retinal artery entering at the bottom of the cleft of the 

 eye; h, cerebral substance continued into the optic nerve and 

 retina ; i, epithelium of cerebral cavity continued into the pig- 

 mentary epithelium of the choroid ; k, the same continued into 

 Jacob's membrane. 



173. Reviewing the whole structure of the eyeball, it may be 

 interesting to the student to know that, in the early embryo, 

 it is developed partly from the integument and partly from 

 the brain. The lens is originally an invagination of the 

 skin, which becomes converted into a closed sac; and its 

 fibres may be fairly considered as elements of the same series 

 as the elongated cells -of the deepest stratum of the cuticle. 



