474 
THE NERVOUS SYSTEM OF MAN 
retina, and this picture must be carried to the brain by 
the eye (optic) nerve. When these two conditions take 
place, we see. 
As we have learned, the stimulus for the eye is always 
light. In physics we learn that the rays of light travel in 
straight lines. This fact explains why we cannot see round 
a corner. When the rays of light are made to pass through 
a glass lens, the rays 
B- which pass through the 
A ' thin edges of the lens are 
bent and do not travel 
Figure 417. —How We See the Pencil. to foe same p l ace they 
would have reached had they not passed through the lens. 
In the same way light rays from an object pass through the 
lens in our eyes and are bent. This results in the image of 
the object, the pencil in this instance, being inverted on the 
retina. The light rays of the pencil stimulate the nerve 
cells in the retina, and this stimulus, after being carried to 
the brain, is interpreted to us as a pencil, though we do not 
know how stimuli travel on nerves. 
The inverted image of the picture on the retina is due to 
the shape of the lens. When the stimulus reaches the living 
cells of the retina and through them is passed on to the optic 
nerve and the brain, a series of changes takes place in these 
living cells. There is no evidence as to how an inverted 
or upright picture passes through these living cells in the 
retina and brain. We do know that we have to learn the 
meaning of all stimuli. For example, a baby reaches for 
things far beyond the length of its arms and it is only after 
many trials that it eventually acquires precision in reaching 
for an object. It is probable that each one of us passed 
through a similar stage of learning to interpret the stimuli 
that arose from light. In coming to understand light stimuli, 
the sense of touch was of great assistance. Try to explain 
how this would be so. 
