THE PRINCIPLES OF VISION 191 



accommodation is paralyzed. Paralysis of the ac- 

 commodation is often caused by contusion of the eye- 

 ball, influenza, diabetes, and diseases of the central 

 nervous system. Diphtheria is a common cause in man. 

 Belladonna and its alkaloids will produce it tem- 

 porarily. Spasm of the accommodation often occurs, 

 and when an eye is hyperopic it becomes falsely myopic. 

 It is partly for this reason that a cycloplegic should be 

 used when the eyes are being tested for refractive errors. 

 The retina is the receptive coat of the eye (as the dry 

 plate receives the image in the camera), and the direct 

 image received by the retina falls upon the ^'macula 

 lutea," which is in the direct visual axis. The retina is 

 composed of nerve-elements intimately associated with 

 the optic nerve-fibers. These nerve-elements, the so- 

 called rods and cones — particularly the latter, of which 

 the macula is principally composed — are exceedingly 

 sensitive to the vibration of light rays. The image is 

 produced by the vibration of these Hght rays refracted 

 or focused upon the macula, which is transmitted to the 

 optic nerve-fibers, thence to the center of vision in the 

 occipital lobes of the brain. If the image falls upon the 

 same center of each eye, two images are naturally pro- 

 duced, which become one in the visual center, just as 

 two pictures are fused into one while looking through 

 the spheroprisms of a stereoscope. Double vision (dip- 

 lopia) is experienced when one or more of the extrinsic 

 muscles are paralyzed. 



