CHAPTER XXIX 

 THE SENSE OF SIGHT 



The sense of sight is the sense by which the intensity and frequency of 

 the vibrations of the ether, the objective cause of light and color are 

 perceived. 



The physiologic mechanism involved in the sense of sight includes the 

 eyeball, the optic nerve, the optic tracts, the thalamo-occipital tract or the 

 optic radiation, 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 direc- 

 tion 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. 



If a horizontal section of the eyeball is made it will be seen to consist 

 of several concentrically arranged membranes enclosing various refracting 

 media essential to vision. (See Fig. 283.) 



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. 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 1 1 mm. The curvature is, therefore, sharper in the latter than in 



604 



