THE SENSES. 723 



objects are looked at, and also to a weakening of the ciliary muscle, and 

 a general loss of elasticity in the parts concerned in the mechanism. 



VISUAL SENSATIONS. 



Excitation of the Retina. Light is the normal agent in the ex- 

 citation of the retina. The only layer of the retina capable of reacting 

 to the stimulus is the rods and cones. The proofs of this statement 

 may be summed up thus: 



(1.) The point of entrance of the optic nerve into the retina, where 

 the rods and cones are absent, is insensitive to light and is called the 

 blind spot. The phenomenon itself is very readily demonstrated. If 

 we direct one eye, the other being closed, upon a point at such a dis- 

 tance to the side of any object, that the image of the latter must fall 

 upon the retina at the point of entrance of the optic nerve, this image 

 is lost either instantaneously, or very soon, If, for example, we close the 

 left eye, and direct the axis of the right eye steadily toward the circular 



spot here represented, while the page is held at a distance of about six 

 inches from the eye, both dot and cross are visible. On gradually in- 

 creasing the distance between the eye and the object, by removing 'the 

 book farther and farther from the face, and still keeping the right eye 

 steadily on the dot, it will be found that suddenly the cross disappears 

 from view, while on removing the book still farther, it suddenly comes 

 in sight again. The cause of this phenomenon is simply that the por- 

 cion of retina which is occupied by the entrance of the optic nerve, is 

 quite blind; and therefore that when it alone occupies the field of vision, 

 objects cease to be visible. (2.) In the fovea centralis and macula 

 lutea which contain rods and cones bat no optic nerve-fibres, light pro- 

 duces the greatest effect. In the latter, cones occur in large numbers, 

 and in the former cones with out 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. 



