550 



THE EVOLUTION OF MAN. 



The history of the development of the eye is equally 

 remarkable and instructive. For although the eye, owing 

 to its exquisite optical arrangement and wonderful struc- 

 ture, is one of the most complex and most nicely adapted 

 organs, yet it develops, without a preconceived design, from 

 a very simple rudiment in the outer skin-covering. 



Fig. 241. — The human eye in transverse section: a, protective membrane 



{sclerotica) ; b, horn membrane (cornea) ; c, outer membrane (conjunctiva) ; 

 d, circular veins of iris ; e, vascular membrane (choroidea) ; /, ciliary 

 muscle ; g, corona ciliaris ; h, rainbow membrane (iris) ; i, optic nerve 

 (n. opticus) ; Jc, anterior limit of the retina; I, crystalline lens (lens crystal- 

 Una); m, inner cover of the horn membrane (water membrane, membrana 

 Descemeti); n, pigment membrane (pigmentosa); o, retina; p, "petits-c&nal;" 

 q, yellow spot of the retina. (After Helmholtz.) 



When fully developed, the human eye is a globular 

 capsule (the eyeball, bulbus, Fig. 241). This lies in the 



