530 THE NERVOUS SYSTEM. 



eye be covered, and the right eye fixed upon the left-hand card, the 

 other will disappear from view at a distance of about eight feet from 

 the wall. 



It is evident, furthermore, that the optic nerve fibres are not directly 

 sensitive to light, even outside the blind spot. These fibres radiate 

 from the entrance of the optic nerve, forming a continuous sheet on 

 the inner surface of the retina ; terminating at successive points in 

 the retinal membrane to its extreme border. A luminous ray striking 

 the retina near the fundus of the eye must, therefore, traverse a con- 

 siderable number of nerve fibres, connected at their peripheral extremi- 

 ties with different parts of the retina ; and, though coming from a sin- 

 gle point, it would thus cause the sensation of an extended line. As 



FIG. 137. 



DIAGRAM, for observing the situation of the blind spot. (Helmholtz.) 



distinct points are separately perceived, although the rays emanating 

 from each have passed through the whole layer of nerve fibres in the 

 retina, it follows that these fibres are not directly affected by the action 

 of light. 



The sensitive elements of the retina are in its posterior or external 

 layers. This is apparent partly from the phenomena observed when 

 the retinal blood-vessels are made visible within the eye. These vessels 

 and their branches radiate from the entrance of the optic nerve. Their 

 ramifications, down to a certain size, are situated in the innermost layer 

 of the retina, and it is only the finest subdivisions which pass into 

 the layer of ganglionic cells. The two outer layers, namely, that of 

 the nuclei, and that of the rods and cones, are destitute of blood-vessels. 

 Owing to this arrangement, the outer layers of the retina, situated 

 behind the main branches of the blood-vessels, lie in the shadow of 

 these branches, since the light comes directly from the front through 

 the pupil. The shadows thus thrown are not usually perceived, because 

 the portions of retina covered by them are habitually in shadow at the 

 same points, and their sensibility to light is greater in proportion. But 

 they may be rendered perceptible by throwing them, under oblique 

 illumination, upon unaccustomed points of the retina. 



Let a lighted candle be held, in a dark room, about three inches from 



