304 STUDIES IN PHYSIOLOGY 



images of objects near at hand, however, are not in good 

 focus, and so they appear blurred. Hence, to get a clear 

 picture of something close in front of me, I must push out- 

 ward and so make more convex the outer surface of my 

 crystalline lens ; this is accomplished by the contraction of 

 the ciliary muscles of the choroid coat, to which we have 

 already referred (p. 302 and Figs. 137 and 138). 



Sensations of Sight. We will now try to see how it is 

 that the eye helps us to get sensations of sight. If an ob- 

 ject, say an arrow, is held in front of the eye, rays of light 

 pass in a great many directions from every part of the 



FIG. 139. The Formation of an Image on the Retina. 



arrow tip. A considerable number of these rays strike the 

 convex surface of the cornea and the crystalline lens, and 

 are thereby focused, or made to converge upon a point on 

 the retina. In the same way the light rays from every 

 other point of the arrow are brought to focus on the inner 

 surface of the retina. By this means a smaller, inverted 

 image of the arrow is projected on the inner lining of the 

 eye. The influence of these light rays then passes through 

 the transparent layers of the retina, and so the rods and 

 cones become stimulated. 



From the region of each of these sensitive cells there ex- 

 tend back into the brain fibers of the optic nerve. Most of 

 them cross on the ventral surface of the brain, and pass to 

 the midbrain; thence fibers run backward and end in the 

 cells of the occipital lobes of the forebrain. We do not know 



