620 THE SENSES. 



gray matter of the brain. An examination of the manner in which the 

 impressions of light are perceived brings into view the following facts. 



The optic nerve and its fibres are insensible to light. Notwith- 

 standing that this nerve is capable of transmitting luminous impressions 

 from the retina to the brain, yet in order to do this, it must first receive 

 its own stimulus from the retina. The optic nerve fibres themselves, 

 though sensitive to mechanical or galvanic irritation, cannot be called 

 into activity by the direct influence of luminous rays. This is shown 

 by the experiment of Bonders, in which, by aid of the ophthalmoscope, 

 a light of a certain degree of intensity is concentrated upon the optic 

 nerve, without being allowed to reach the tissue of the retina. When 

 the bottom of the eye is illuminated by the ophthalmoscope, the ob- 

 server sees the general surface of the retina of a red or brownish color, 

 while the papilla, which corresponds to the entrance of the optic nerve, 

 presents itself as a white circular spot. This spot is occupied entirely 

 by optic nerve fibres, while the elements of the retina commence only 

 beyond its borders. If the minute image of a candle flame at some dis- 

 tance be thrown by reflection upon the retina, its light is perceived by 

 the person under observation, as well as its image by the observer. If 

 the eye however be turned in such a direction as to bring the image of 

 the flame upon the white circle of the optic nerve, this circle, and the 

 nerve fibres of which it is composed, are visibly illuminated to a certain 

 depth, owing to the translucency of their substance ; but the light of 

 the candle flame is no longer perceived by the person under examina- 

 tion. The moment, on the other hand, the image of the flame is allowed 

 to pass beyond the limits of the white spot, and to touch the retina, its 

 light becomes perceptible. 



The Blind Spot. The region, accordingly, occupied by the entrance of 

 the optic nerve, and in which the elements of the retina proper are ab- 

 sent, is a blind spot, where luminous rays make no perceptible impres- 

 sion. The real diameter of this spot, according to the average measure- 

 ments obtained by Listing, Hannover, and Helmholtz,is 1.G5 millimetre, 

 and it covers in the field of vision a space equivalent to about 6 degrees. 

 Notwithstanding the existence of this insensible part at the bottom 

 of the eye, no dark point is usually observed in the field of vision, for 

 the following reasons. The blind spot is not situated in the visual axis 

 of the eye, but is placed, corresponding with the entrance of the optic 

 nerve, nearer the median line (Fig. 189). Consequently the image of 

 an object which is directly examined in the normal line of vision can- 

 not fall upon this spot, but is always outside of it, at the end of the 

 visual axis. Even an object which is perceived in the field of vision out- 

 side the direct line of sight, can never reach the blind spot of both eyes 

 at the same time. If it happen to be so placed that its image falls upon 

 the blind spot of one eye, it will necessarily reach the retina of the other 

 eye at a different point, and is accordingly perceived. If, on the other 

 hand, one eye alone be employed, there is always a small portion of the 

 field of vision which is imperceptible. This deficiency is not generally 



