254 



THE EYE IN EVOLUTION 



an unusually wide range of adaptive demands, for vertebrates have 

 succeeded in making themselves at home in every environment where 

 life is possible. These differences have been very considerable and the 

 adaptations demanded have been great. The vertebrate eye was 

 initially evolved for vision in shallow water ; it has been asked to 

 adapt itself for vision in the abyss, in the rivers, in the mud of the 

 swamps, on land and in the air, and on occasion to readapt itself for 



Fig. 268. — Diagram of the Longitudinal Section of the Human Eyeball. 



a, angle of anterior chamber. ac, 

 anterior chamber. aCV, anterior ciliary 

 vessel. C, cornea. CB, ciliary body. 

 Ch, choroid. CO, ocular conjunctiva. 

 CS, canal of Schlemm. DS, dural 

 sheath. F, fovea. I, iris. L, lens. 

 ON, optic nerve. OS, ora serrata. 



PC, posterior chamber. PCV, posterior 

 ciliary vessel. PP, pars plana. R, 

 i-etina. RM. rectus muscle. S, sclera. 

 SCT, subconjunctival tissue. V, 



vitreous. VS, vaginal sheath. VV, vortex 

 vein. Z, zonule. 



vision in the seas ; it has been asked to fit itself for vision at night, in 

 twilight or in dark cavernicolous surroundings and in the brightest of 

 dajdight ; it has been asked to cater for panoramic vision where the 

 detection of movement is paramount, or to accommodate itself to the 

 finest stereoscopic prowess, to meet the needs of a sluggish or an active 

 habit (ji Jife, to be content with a vague apperception or to evolve the 

 capacit\ for minute resolution in form vision and master the intricacies 

 of colou, iiion. All this — and more — it has done ; and in so doing 

 it has trit-i, and often discarded, now this expedient, now that. 



