8 A TYPICAL VERTEBRATE EYE: THE HUMAN 



is more sharply curved than the latter (Fig. 3). A substantial portion of 

 the thickness of the cornea represents the skin of the head, which during 

 evolution became affixed to the eyeball, leaving loose places, to permit 

 eye movements, up underneath the eyelids where it merges with their 

 linings to join the ordinary outer skin at the lid margins. Only some of 

 the inner layers of tissue in the cornea represent a clear window in the 

 original, ancestral, fibrous capsule. As a matter of fact, the sclera itself 



Fig. -I — Fibrous and vascular tunics of the human eyeball, x 135. 

 Modified from Maximow and Bloom, after Schaffer. 



a, sclera and chorioid. 



a- artery; c- choriocapillaris layer of chorioid; Iv- lamina vitrea; s- sclera; v- vein; vl- vascular, 

 pigmented layers of chorioid. 



b, cornea. 



b- Bowman's membrane; d- Descemet's membrane; e- epithelium; m- mesothelium; p- sub- 

 stantia propria. 



is almost as transparent as the cornea in many of the lower vertebrates. 

 The 'white' of the human eye is differentiated from the clear cornea not 

 because the latter has become transparent secondarily, but rather because 

 the sclera has become clouded. What has happened in evolution also 

 takes place in individual development, and the clear parts of the em- 

 bryonic eye are clear from the start and remain so — they do not become 



