VISION. 555 



Accommodation through the ciliary muscle is governed 'by 

 a center situated in the hind part of the floor of the third ven- 

 tricle near the anterior bundles of the third nerve, which latter 

 is the medium of the change. When rays of light are focused 

 anterior to the retina, the eye is myopic ; when posterior to it, 

 hypermetropic. 



The presbyopic eye is one in which the mechanism of accom- 

 modation is at fault, chiefly the ciliary muscle. The point of 

 entrance of the optic nerve (blind-spot) is insensible to light; 

 and visual impulses can be shown to originate in the layers of 

 rods and cones, probably through stimulation from chemical 

 changes effected by light acting on the retiua. The sensation 

 outlasts the stimulus ; hence positive after-images occur. Nega- 

 tive after-images occur in consequence of excessive stimulation 

 and exhaustion of the retina, or disorder of the chemical pro- 

 cesses that excite visual impulses. "When stimuli succeed one 

 another with a certain degree of rapidity, sensation is continu- 

 ous. The eye can distinguish degrees of light within certain 

 limits, varying by about -j^ of the total. 



Objects become fused or are seen as one when the rays 

 from them falling on the retina approximate too closely on 

 that surface. The brain, as well as the eye itself, is concerned 

 in such discriminations, the former probably more than the 

 latter. 



The macula lutea, and especially the fovea centralis, are in 

 man the points of greatest retinal sensitiveness. When the 

 images of objects are thrown on these parts, they are seen with 

 complete distinctness; and it is to efl'ect this result that the 

 movements of the two eyes in concert take place. An object is 

 seen as one when the position of the eyes (visual axes) is such 

 that the images formed fall on corresponding parts of the retina. 

 Binocular vision is necessary to supply the sensory data for the 

 idea of solidity. It is important to remember that, before an ob- 

 ject is " seen " at all, the sensory impressions furnished by the 

 retina and conveyed inward by the optic nerve are elaborated in 

 the braiii and brought under the cognizance of the perceiving 

 ego. We recognize many visual illusions and imperfections 

 of various kinds, the course of which it is difficult to locate 

 in any one part of the visual tract, such as are referred to 

 " irradiation," " contrast," etc. There may also be visual phe- 

 nomena that are purely subjective, and others that result from 

 suggestion rather than any definite sensory basis of retinal 



