THE SENSES. 709 



other distinct sets of vessels supplying the tunics of the eyeball. (1) The 

 vessels of the sclerotic, choroid, and iris, and (2) the vessels of the retina. 

 (1. ) These 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 



Fig. 419. Section through the macula lutea and fovea central is of human retina, a, fovea; 6, 

 descent of the macula toward fovea. The numbers indicate the layers of the retina. (Kuhnt.) 



ciliary processes, forming a very highly vascular 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 

 (conjurictival congestion), and the pink zone surrounding the cornea 

 which indicates deep seated ciliary congestion. 



(2.) The retinal vessels (fig. 418) are derived from the arteria cen- 

 tralis retince, which enters the eyeball along the centre of the optic 

 nerve. They ramify all over the retina, chiefly in its inner layers. 

 They can be seen by direct ophthalmoscopic examination. 



The Optical Apparatus. 



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

 to consist of several parts. Firstly, of a system of transparent refract- 

 ing surfaces and media by means of which images of external objects are 

 brought to a focus upon the back of the eye; and secondly, of a sensitive 

 screen, the retina, which is a specialized termination of the optic nerve, 

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

 the optic nerve, such an impression as to produce in the brain visual 

 sensations. To these main parts may be added, thirdly, an apparatus 

 for focussing objects at different distances from the eye, called accommo- 

 dation. Even this does not complete the description of the whole organ 

 of vision, since both eyes are usually employed in vision, and fourthly, 

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

 the same direction by a system of muscles, so that binocular vision is 

 possible. 



