REFRACTIVE MEDIA AND SURFACES 673 



palpebral and lachrymal arteries, there are at least two other distinct sets 

 of vessels supplying the tunics of the eyeball: i. The vessels of the sclera, 

 choroid, and iris, and 2, the vessels of the retina. The first are the short and 

 long posterior ciliary arteries which pierce the sclerotic in the posterior half 

 of the eyeball, and the anterior ciliary which enter near the insertions of the 

 recti. These vessels anastomose and form a very rich choroidal plexus; 

 they also supply the iris and ciliary processes, forming a very highly vas- 

 cular circle round the outer margin of the iris and adjoining portion of 

 the sclerotic. The distinctness of these vessels from those of the conjunctiva 

 is well seen in the difference between the bright red of blood-shot eyes 

 (conjunctival congestion), and the pink zone surrounding the cornea which 

 indicates deep-seated ciliary congestion. 



The central artery of the optic nerve enters the retina from the center of 

 the optic disc and sends out branches over the retinal cup lying in the nerve 

 fiber layer, figure 451. These blood vessels, however, are absent from the 

 fovea centralis and reduced in size in the macula lutea, figures 451 and 452. 



THE OPTICAL APPARATUS. 



The optical apparatus may be supposed, for the sake of description, to 

 consist of several parts: i, a system of transparent refracting surfaces and 

 media by means of which images of external objects are brought to a focus 

 upon the back of the eye; 2, a sensitive screen, the retina, which is a special- 

 ized sensory apparatus in connection with the terminations of the optic 

 nerve, and capable of being stimulated by luminous objects, and of sending 

 such impressions as to produce in the brain visual sensations. To these 

 main parts may be added, 3, an apparatus for focusing light from objects al 

 different distances from the eye; and 4, since both eyes are usually employed 

 in vision, an arrangement by means of which the eyes may be turned in the 

 same direction so that binocular vision is possible. The arrangement of 

 the optic nerve fibers, and of the continuation of these backward in the optic 

 chiasma, and thence to special districts of the brain have already been 

 discussed. 



The eye may be compared to a photographic camera, in which the trans- 

 parent refracting media correspond to the photographic lens. In a camera 

 images of external objects are thrown upon a screen, the sensitive plate, at 

 the back of the camera box. In the eye, the camera proper is represented 

 by the eyeball with its choroidal pigment, the sensitive screen by the retina, 

 and the lens by the refracting media. In the case of the camera, the screen 

 is djusted to receive clear images of objects at different distances by a 

 mechanical apparatus for focusing. The corresponding adjustment in the 

 eye is accomplished by the accommodating mechanism. 



Refractive Media and Surfaces. At first sight it would seem as if 

 the refracting apparatus of the eye were very complicated, since it consists 

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