250 



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) ; f, ciliary 

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

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

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

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

 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 



