THE ORGANS OF SENSE. 131 



The Electrical Changes. When an excised eye is placed in circuit 

 with a string galvanometer, it is found that a current passes through 

 the eye from the posterior to the anterior pole. When light is allowed 

 to fall upon the retina, there is first a small negative variation of 

 this current, followed by a marked positive variation. 



THE FUNCTION OF THE RODS AND CONES. 



The layer of rods and cones is the part of the retina in which the 

 impulses are excited which give rise to visual sensations. This is 

 proved by three facts. (1) In the fovea, which is the area for most 



i I * 



FIG. 41. Sections of the frog's retina. (From Starling's 

 Principles of Physiology. ) 



A, after exposure to light. JB, kept in the dark. (Engelmann.) 



distinct vision, cones only are present, the other layers of the retina 

 being absent. (2) No sensation is excited when light falls on the 

 optic disc, where the rods and cones are absent and nerve fibres only 

 are present. The optic disc is therefore called the blind spot, and it 

 is to be noted that there is no sensation of darkness arising from it, 

 but merely the absence of any sensation at all. The existence of the 

 blind spot can be demonstrated in the following way. If the left eye 

 be closed and the right eye gaze steadily at the cross in fig. 42, and 

 if the book be moved to and fro, it will be found that at a distance of 

 about eight inches from the eye the white circle will disappear. The 

 black background will appear to be continuous, showing that the gap 

 is- unconsciously filled up. (3) The level of the layer which is stimu- 

 lated by light can be determined by means of Purkinje's images. If, 

 in a darkened room, a strong beam of light is focussed on the sclera 

 just external to the cornea, images of the retinal vessels will be seen. 



