VISION. 



Light and vision are to some degree correlatives of each 

 other. Light is supposed to have as its physical basis the vibra- 

 tions of an imponderable ether. Such is, however, to a non- 

 seeing animal as good as non-existent, so that we may look at 



Fig. 375.— Eye partially dissected (after Sappey). 1. optic nerve; 8, 3, 4, sclerotic dis- 

 sected back so as to uncover the choroid coat; 5, cornea, divided and folded back 

 with sclerotic coat; 6, canal of Schlemm; 7, external surface of choroid, traversed 

 by one of the long ciliary arteries and by ciliary nerves; 8; central vessel, into 

 which the vasa vorticosa empty; 9, 10, choroid zone; 11, ciliary.nerves; 12, long 

 ciliary artery; 13, anterior ciliary arteries; 14, iris; 16, vascular circle of iris; 16, 

 pupil. 



this subject either with the eyes of the physiologist or the phys- 

 icist, according as we regard the cause of the effects or the 

 latter and their relations to one another. It is, however, im- 

 possible to understand the physiology of vision without a 

 sound knowledge of the anatomy of the eye, and an apprehen- 



