496 STRUCTURE OF VERTEBRATA. 
The adult eye is more or less globular, and its walls con: 
sist of several distinct layers. The innermost layer bound- 
ing the posterior part of the globe is the sensitive retina, 
innetvated by fine branches from the optic nerve. It may 
be compared to the nervous matter of the brain, from which, 
indeed, it arises. Outside of the retina is a pigmented 
epithelium, and outside of this a vascular membrane; 
Fic. 265.—Development of the eye.—After Balfour and 
Hertwig. 
x. Section through first embryonic vesicle, showing outgrowth of 
optic vesicles (of.v.) to meet the skin; 7.., thalamencephalon ; 
G., the gut. : 
2-4. Sections illustrating the formation of the lens (2) from the 
skin, and the modification of the optic vesicle into an optic 
cup; &., retina; v.4., vitreous humour. 
5. External aspect of embryonic eye; Z., lens. 
together these are often called the choroid. The vascular 
part may be compared to the pia mater covering the brain, 
and like it is derived from mesoblast. Outside of the 
choroid is a protective layer or sclerotic, comparable to, 
and continuous with, the dura mater covering the brain, 
and also mesoblastic in origin. Occupying the front of 
the globe is the crystalline lens, a clear ball derived directly 
