340 PHYSIOLOGY CHAP. 



ground ; the black disc then disappears and the background 

 appears entirely white. 



Tne blind spot is so large that at a distance of 1-7-2 m. the 

 head of an adult man may be invisible. Helmholtz succeeded in 

 obtaining an exact measurement of the blind spot in his right eye 

 by looking fixedly at a point on a white surface, and then moving 

 the blunt end of a pencil to and fro within the blind area. By 

 drawing on the paper the points at which he began to see the 

 end of the pencil and joining these points by a continuous line, he 

 obtained Fig. 165, in which a represents the point fixated, d the 

 shape of the blind spot, the line AB the third part of the distance 

 between point a and the observer's eye. As the figure shows, 

 the blind spot corresponds exactly with the shape of the papilla of 

 the optic nerve and the main trunks of the central arteries issu- 

 ing from it. Bonders also demonstrated the insensibility of the 

 papilla to light. When the image of a flame is thrown on to the 



optic papilla by an oph- 

 thalinoscope, the subject 



though a slight inclina- 

 tion of the mirror makes 

 the image perceptible. 



It is thus evident 

 that the nerve-fibres of 

 the optic papilla are 

 totally inexcitable to 



Fio. 165. Form of Mariotte's blind spot. (After Helmholtz.) light, and that the 



are the specific receptor 



apparatus of luminous stimuli for direct and distinct vision. 

 We know that the cones become gradually less, and the rods 

 between them more numerous, towards the outer parts of the 

 retina. Indirect and blurred vision in the periphery of the visual 

 . field may be explained on the assumption that cones and rods alike 

 are excitable to light, but that cones alone are capable of arousing 

 visual sensations, the sole function of the rods being to excite 

 unconscious reflexes. This hypothesis proposed by Gad (1894) 

 is, however, disproved by the fact that only rods are present 

 in the retina of rabbits and other rodents, cones being en- 

 tirely absent even from the fovea. Again, we know from the 

 work of Cajal and Ketzius that rods as well as cones are in 

 connection with the external bipolar cells (second neurone) and 

 with the ganglion cells (third neurone), even if the conducting 

 paths are more reduced and concentrated than in the cones 

 (Fig. 159). 



The "duplicity theory," first formulated by M. Schultze (1886) 

 and confirmed by the subsequent work of Parinaud and v. Kries, 

 assumes that the rods as well as the cones are organs of visual 



