SECTION III 

 THE STRUCTURE OF THE EYEBALL 



THE EYEBALL is a sphere, about 20 mm. in diameter. It lies near the front 

 of the orbital cavity protected by the eyelids. The greater part of its 

 external surface is formed by a firm white membrane called the sclera. In 

 front this is replaced by a transparent structure called the cornea. This 

 has a greater curvature than the rest of the eye, the radius of its surface being 

 about 8 mm. Attached to the eyeball behind and slightly to the inner 



CORNEA 



FIG. 249. Transverse section through equator of left eye seen from above. 



side is the optic nerve, the function of which is to convey to the brain the light 

 impressions received by the eye. Attached to it also, about 6 mm. from the 

 corneal margin, are the tendons of four of the ocular muscles, as described 

 in Section II. The sclerotic is lined within by a highly vascular and deeply 

 pigmented coat called the choroid. In front this coat has a circular aperture, 

 in relationship with which the choroid becomes modified into several impor- 

 tant structures, namely the iris, ciliary muscles and ciliary glands. Spread 



500 



