THE SENSES. 215 



the general decay of old age. In consequence the patient would be obliged 

 in reading to hold his book further and further away in order to focus 

 the letters, till at last the letters are held too far for distinct vision. The 

 defect is remedied by weak convex glasses, which are very commonly 

 worn by old people. It is due chiefly to the gradual increase in density 

 of the lens, which is unable to swell out and become convex when near 

 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 exci- 

 tation of the retina, the only layer of which capable of reacting to the 

 stimulus being 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 distance 

 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 increasing 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 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. (2.) In the fovea centralis and macula lutea, which contain rods 

 and cones but no optic nerve-fibres, light produces the greatest eifect. 

 In the latter, cones occur in larger 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.) 

 tf 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 dark- 

 ness, a remarkable branching figure (Purkinje's figures) is seen floating 



