Chap. XX] INTERNAL AND EXTERNAL SENSES 425 



The eyebalL — The eyeball is spherical in shape, but its trans- 

 verse diameter is less than the antero-posterior, so that it projects 

 anteriorly, and looks as if a section of a smaller sphere had been 

 engrafted on the front of it. 



The eyeball is composed of three coats, or tunics, and contains 

 three refracting media or humors. They are as follows : — 



Tunics. — 1. Sclera and cornea. 



2. Choroid, ciliary body, and iris. 



3. Retina. 

 Refracting media. — 1. Aqueous. 



2. Crystalline lens and capsule. 



3. Vitreous. 



The sclera. — The sclera, or " white of the eye," covers the 

 posterior five-sixths of the eyeball. It is composed of a firm, 

 unyielding, fibrous membrane, thicker behind than in front, and 

 serves to protect the delicate structures contained within it, and 

 maintain the shape of the eyeball. It is opaque, white, and 

 smooth externally, and behind is pierced by the optic nerve. In- 

 ternally it is stained brown where it comes in contact with the 

 choroid coat. It is supplied with very few blood-vessels, and the 

 existence of nerves in it is doubtful. 



The cornea. — The cornea covers the anterior sixth of the eye- 

 ball. It is directly continuous with the sclera, which, however, 

 overlaps it slightly above and below, as a watch crystal is over- 

 lapped by the case into which it is fitted. The cornea, like the 

 sclera, is composed of fibrous tissue, which is both firm and un- 

 yielding, but, unlike the sclera, it has no color, and is perfectly 

 transparent ; it has been aptly termed the " window of the eye." 

 The cornea is well supplied with nerves and lymph-spaces, but is 

 destitute of blood-vessels, so that it is dependent on the lymph 

 contained in the lymph-spaces for nutriment. 



Choroid. — The choroid, or vascular coat of the eye, is a thin, 

 dark brown membrane lining the inner surface of the sclera. It is 

 composed of delicate connective tissue, the cells of which are large 

 and filled with pigment, and it contains a close network of blood- 

 vessels. The pigment cells and blood-vessels render this mem- 

 brane dark and opaque, so that it darkens the chamber of the eye 

 by preventing the reflection of light. It extends to within a short 

 distance of the cornea. 



