S£JVS£ ORGANS. 



449 



snakes and lizards, and its nerves are abortive in the 

 mole. 



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; 

 together these are 



often called the cho- 

 roid. The vascular 

 part may be com- 

 pared to the pia 

 mater covering the 

 brain, and like it is 

 derived from meso- 

 blast. Outside of the 

 choroidis aprotective 

 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 

 directlyfrom the skin 



Fig. 191. — Diagram of the eye. 



C, Cornea; a.k., aqueous humour; c.b., ciliary 

 body; L, lens; /., Iris; Sc, sclerotic; Ch., 

 choroid; R. t retina; z/.A., vitreous humour; 

 y.sp., yellow spot ; «., optic nerve. 



It is fringed in front 

 by a pigmented and muscular ring — the iris, which is for the 

 most part a continuation 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 

 moored by " ciliary processes " of the choroid, and its shape 

 29 



