446 



STRUCTURE OF VERTEBRATA. 



indeed, it arises. Outside of the retina is a pigmented 

 epithelium, and outside of this a vascular membrane ; 

 together these are often called the choroid. The vascular 

 part may be compared to the pia matter 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 

 from the skin. It is fringed in front by a pigmented and 

 muscular ring — the iris, which is for the most part a con- 



FiG. 142. — Diagram of the E)^. (Compiled). 



C, Cornea ; a.h., aqueous humour ; c.b., ciliarj' body ; /., lens ; 

 /., Iris; Sc, sclerotic; Ch., choroid; R., retina; 7'.//., \'itreous 

 humour ; y.sp., yellow spot ; w., optic nerve. 



tinuation of the choroid. The space enclosed by the iris 

 in front of the lens is called the pupil. Protecting and 

 closing the front of the eye is the firm cornea continuous 

 with the sclerotic, and covered externally by the conjunctiva 

 — a delicate epithelium continuous with the epidermis. 

 Between the cornea and the iris is a lymph space containing 

 aqueous humour, while the inner chamber behind the lens 

 contains a clear jelly — the vitreous humour. The lens is 



