ANATOMY OF THE BALL 3! I 



perior and inferior oblique antagonize each other. The 

 former rotates the globe so that the pupil is directed out- 

 ward and downward ; the latter so that it looks outward and 

 upward. The associated action of all these muscles can pro- 

 duce almost any variety of movements, and no effort of the 

 will is necessary to properly associate them when it is desired 

 to direct the line of vision toward a certain object. For in- 

 stance, when it is desired to look at an object on the right it 

 takes no distinct voluntary effort to contract the external 

 rectus of the right eye and the internal rectus of the left. It 

 will be seen later that vision for the two eyes is normal only 

 when impressions are made upon exactly corresponding 

 parts of the two retinae, so that they may act as a single or- 

 gan ; and for this to be done not always the same movements 

 are called for in both balls. 



Anatomy of the Ball. The eye-ball is a globular body 

 consisting of several coats enclosing refracting media. Of 

 these coats the external is the sclerotic, dense and fibrous, 

 covering the posterior five-sixths of the organ and continu- 

 ous with the cornea, which covers the anterior one-sixth. It 

 is not well supplied with blood-vessels. The cornea is trans- 

 parent, and upon its external surface are several layers of 

 delicate nucleated epithelium; underneath this layer of cells 

 is a thin membrane, the anterior elastic lamella, which is a 

 continuation of the conjunctiva. The substance proper of 

 the cornea is composed of pale interlacing fibers among 

 which are connective tissue corpuscles and quite a quantity 

 of fluid. These fibers are continuous from the sclerotic, but 

 they lose their opacity at the corneo-sclerotic margin. On 

 the posterior surface of the cornea is the transparent elastic 

 membrane of Descemet, a part of which, at the circumfer- 

 ence of the iris, passes into the ciliary muscle. The cornea 

 is very sensitive, but contains no blood-vessels. 



Next inside the sclerotic is the choroid coat of the eye. It 

 does not lie under the cornea, but is confined to the sclerotic 

 area of the ball Behind the optic nerve penetrates it, and in 



