TILE SENSES. 203 



to this there are certain sustentacular fibres of connective tissue (radiating 

 fibres of Miiller) which spring from the meinbranal imitans internet almost 

 vertically, and traverse the retina to the limitans externa, whence very 

 delicate connective tissue processes pass up between the rods and cones. 

 The framework which they form is represented in Fig. 370, A. The nerv- 

 0//.V elements of the retina are represented in Fig. 370, B, and consist of 

 delicate fibres passing up from the nerve-fibre layer to the rods and cones, 

 and connected with the ganglionic corpuscles and granules of the internal 

 and external layer. 



Blood-vessels of the Eyeball. The eye is very richly supplied 

 with blood-vessels. In addition to the conjunctival vessels which are 

 derived from the palpebral and lachrymal arteries, there are at least two 

 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 cili- 

 ary processes, forming a very highly vascular circle round the outer mar- 

 gin of the iris and adjoining portion of the sclerotic. 



The distinctness of these vessels from those of the conjunctiva is well 

 seen in the diiference between the bright red of blood-shot eyes (con- 

 junctival congestion), and the pink zone surrounding the cornea which 

 indicates deep- seated ciliary congestion. 



(2.) The retinal vessels (Fig. 372) 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. 



OPTICAL APPARATUS. 



The eye may be compared to the camera used by photographers formed 

 by a convex lens. In this instrument images of external objects are 

 thrown upon a ground-glass screen at the back of a box, the interior of 

 which is painted black. In the eye the convex lens is represented by the 

 crystalline lens, the dark box by the eyeball with its choroidal pigment, 

 and the screen by the retina. In the case of the camera the screen is 

 enabled to receive clear images of objects at different distances, by being 

 shifted forward and back: while the convex lens too can be screwed in 

 and out. The corresponding contrivance in the eye will be described 

 under the head of Accommodation. 



Conditions Necessary. The essential constituents of the optical 

 apparatus of the eye may be thus enumerated: (1) A nervous structure 

 (the retina) to be stimulated by light and to transmit by means of the 

 optic nerve, of which it is the terminal expansion, the impression of the 

 stimulation to the brain, in which it excites the sensation of vision; (2) 



