74 THE SENSES 



again. The cause of this phenomenon is simply that the portion of retina 

 which is occupied by the entrance of the optic nerve is quite blind; and there- 

 fore that when it alone occupies the field of vision objects cease to be visible. 



FIG. 467. Diagram for Demonstrating the Blind Spot. 



2. In the fovea centralis and macula lutea, which contain rods and cones but 

 no optic-nerve fibers, light produces the greatest effect. In the latter, cones 

 occur in large numbers, and in the former cones without rods are found, 

 whereas in the rest of the retina, which is not so sensitive to light, there are 

 fewer cones than rods. We may conclude, therefore, that cones are even 

 more important to vision than rods. 3. If a small lighted candle be moved 

 to and fro at the side of and close to one eye in a dark room while the eyes 

 look steadily forward into the darkness, a remarkable branching figure 

 Purkinje's figures, is seen floating before the eye, consisting of dark lines on 

 a reddish ground. As the candle moves, the figure moves in the opposite 

 direction, and from its whole appearance there can be no doubt that it is a 

 reversed picture of the retinal vessels projected before the eye. The two 

 large branching arteries passing up and down from the optic disc are clearly 

 visible, together with their minutest branches. A little to one side of the 

 disc there is an area free from vessels. This corresponds to the yellow 

 spot, or macula lutea, figure 452. This remarkable appearance is due to 

 shadows of the retinal vessels cast by the candle. The branches of these 

 vessels are chiefly distributed in the nerve fiber and ganglionic layers; 

 and since the light of the candle falls on the retinal vessels from in front, 

 the shadow is cast behind them. It follows that those elements of the 

 retina which perceive the shadows must also lie behind the vessels. Here, 

 then, we have a clear proof that the light-perceiving elements of the retina 

 are not the fibers of the optic nerve forming the innermost layer of the 

 retina, but the external layers of the retina, the rods and cones. 



When light falls on the rods and cones it produces changes which de- 

 velop nerve impulses that are transmitted by the chain of neurones ex- 

 tending through the retina, the optic nerve and chiasma, the geniculate 

 bodies, etc., to the cerebral cortex of the occipital lobe, which is the sen- 

 sorium for visual sensations, figures 405 and 406. We have already seen 

 that the eye possesses a wonderful mechanical perfection for receiving and 

 focusing light on definite parts of the retina. A comparison of visual 

 sensations shows that there are corresponding qualities in the sensation, 

 as, for example, its intensity, duration, localization, complexity, etc. 



Duration of Visual Sensations. The duration of the sensation pro- 

 duced by a luminous impression on the retina is always greater than that 

 of the stimulus which produces it. However brief the luminous impression, 



