CHAPTER XXIX 

 THE SENSE OF SIGHT 



The physiologic mechanism involved in the sense of sight includes the 

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

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

 jPeripheral 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. 



, , 



Tcor.a.-Tte n 1. the thick op,, membrane 

 of th, ball. It is compo,do layers o 



tL optic axl By virtue of its firmness and density the sclera gives form to 

 meridian 11 mm. The curvature is, therefore, sharper in the latt 



