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, 

 innervated 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; 



f.b. 



op.v. 



FIG. 265. Development ot the eye. After Balfour and 

 Hertwig. 



i. Section through first embryonic vesicle, showing outgrowth of 



optic vesicles (c>p.v.) to meet the skin ; j.b. y thalamencephalon ; 



G., the gut. 

 2-4. Sections illustrating the formation of ^ the lens (/.) from the 



skin, and the modification of the optic vesicle into an optic 



cup ; /?., retina ; v.k., vitreous humour. 

 5. External aspect of embryonic eye; /., 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 



