THE SENSE OF SIGHT 



705 



Lais anatomic arrangement, a portion of the light coming through the pupil 

 till be intercepted by the vessels and a shadow projected on the layer of 

 bds and cones. Ordinarily, these shadows are not perceived, for the reason 

 ;hat the shaded parts are more sensitive, so that the small amount of light 

 tiassing through the vessels produces as strong an impression on this part 

 Is does the full amount of light on the unshaded parts of the retina, and 

 Lrhaps because the mind has learned to disregard them. But if light be 

 fnade to enter the eye obliquely, the position of the shadows will be changed, 

 jvhen at once they become apparent. This can be shown in the following 

 >vay: If in a darkened room a lighted candle be held several inches to the 

 idde'and to the front of the eye, and then moved up and down, there will 

 oe perceived, apparently in the field of vision, an arborescent figure corres- 

 ponding to the retinal blood-vessels. This is due to the falling of the 

 shadows on unusual portions of the layer of rods and cones. 





Excitability of the Retina.-^ retina is not equally excitable in .M parts 

 of its extent The maximum degree of sensibility is found m the macula 

 tatea and especially in its central portion, the fovea. In this region the 

 "almost entirely disappear, the layers of rods and cones 



rx'tr *%^&tttt*-*f * *" ked 



g^lS3is3s^ 



45 



