CHAPTER XXV. 

 THE SENSE OF SIGHT. 



The physiologic mechanism involved in the sense of sight in- 

 cludes the eyeball, the optic nerve, the optic tracts, their cortical 

 connections, and nerve- cells in the cuneus and adjacent gray matter. 

 Peripheral stimulation of this mechanism develops nerve impulses 

 which transmitted to the cortex evoke (>i) the sensation of light and 

 its different qualities colors; (2) the perception of light and color 

 under the form of pictures of external objects; and (3) in connection 

 with the ocular muscles, the production of muscle sensations by 

 which the size, distance, and direction of objects may be judged. 



The specific physiologic stimulus to the terminal end-organ, 

 the retina, is the impact of ether vibrations. In general, it may 

 be said that, at least for the same color, the intensity of the objective 

 vibration determines the intensity of the sensation. 



THE PHYSIOLOGIC ANATOMY OF THE EYEBALL. 



The eyeball is situated at the fore part of the orbit cavity, and 

 in such a position as to permit of an extensive range of vision. It 

 is loosely held in position by a fibrous membrane, the capsule of 

 Tenon, which is attached, on the one hand, to the eyeball itself, 

 and, on the other, to the walls of the orbit cavity. Thus suspended, 

 the eyeball is susceptible of being moved in any direction by the 

 contraction of the muscles attached to it. 



The ball is spheroid in shape, measuring about 24 millimeters 

 in its antero-posterior diameter and a little less in its transverse 

 and vertical diameters. When viewed in profile, it is seen to con- 

 sist of the segments of two spheres, of which the posterior is the 

 larger, occupying five-sixths, and the anterior is the smaller, occupy- 

 ing one-sixth of the ball. It is composed of several concentrically 

 arranged membranes enclosing various refracting media essential to 

 vision. 



The membranes, enumerating them from without inward, are 

 as follows: the sclera and cornea, the chorioid and iris, and the 

 retina. The refracting media are the aqueous humor, the crystal- 

 line lens, and the vitreous humor. 



The Sclera and Cornea. The sclera is the thick opaque mem- 

 brane covering the posterior five-sixths of the ball. It is composed 



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