310 THE SENSES 



that the pupil is directed outward and downward; the latter so 

 that it looks outward and upward. The associated action of all 

 these muscles can produce 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 instance, 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 organ; 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 con- 

 sisting 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 continuous with the 

 cornea, which covers the anterior one-sixth. It is not well sup- 

 plied with blood-vessels. The cornea is transparent, and upon 

 its external surface are several layers of delicate nucleated epithe- 

 lium; underneath this layer of cells is a thin membrane, the an- 

 terior elastic lamella, which is a continuation of the conjunc- 

 tiva. 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 trans- 

 parent elastic membrane of Descemet, a part of which, at the 

 circumference 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 

 front it is connected with the iris. The choroid is very vascular. 

 Its color is dark brown on account of the abundance of pigment 



