CHAPTER XXVII. 

 THE SENSE OF SIGHT. 



The physiologic mechanism involved in the sense of sight includes 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 turned 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 consist of the segments of 

 two spheres, of which the posterior is the larger, occupying five-sixths, and 

 the anterior is the smaller, occupying 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 crystalline lens, and the vitreous humor. 



The Sclera and Cornea. The sclera is the thick opaque membrane 

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

 connective tissue which are arranged transversely and longitudinally. It 

 is pierced posteriorly by the optic nerve about 3 or 4 millimeters internal to 

 the optic axis. By virtue of its firmness and density the sclera gives form to 

 the eyeball, protects delicate structures enclosed by it, and serves for the 

 attachment of the muscles by which the ball is moved (Figs. 302, 307). The 

 cornea is the transparent membrane forming the anterior one-sixth of the ball. 

 It is nearly circular in shape, measuring in its horizontal meridian 12 mm., 

 in its vertical meridian n mm. The curvature is therefore sharper in the 



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