THE EYE. 413 



fissure closes and permanently disappears. The ex- 

 ternal coats of the eye that is, the sclera and the 

 choroid develop from the surrounding connective 

 tissue. 



Fig. 285. Plastic representation of the optic cup with lens and vitre- 

 ous body (Hertwig): ab, Outer wall of the cup; ib, its inner wall; h, 

 cavity between the two walls, which later disappears entirely ; Sn, fun- 

 dament of the optic nerve (stalk of the optic vesicle with a furrow on 

 its lower surface); aus, optic (choroid) fissure; gl, vitreous body; /, lens. 



The parts of the adult eyeball may be tabulated as 

 follows : 



I. Tunica externa. 



1. Sclera. 



2. Cornea. 

 II. Tunica media. 



1 . Choroid coat. 



2. Ciliary body. 



3. Iris. 

 III. Tunica Interna. 



1. Retina. 



2. Pigment membrane. 



The refracting media, or transparent media of the 

 eye traversed by a ray of light, are: 

 i. The cornea. 



