234 THE SPECIAL SENSES 



frequency, and only certain ones are capable of affecting the 

 eye. The impulses they generate pass to the brain by way of 

 the optic nerves, giving the sensation of light. 



If the optic nerves are examined in a superficial manner, 

 they will be seen to leave each eye and pass backward through 

 the optic foramina until they reach the body of the sphenoids. 

 Here they cross one another in the form of an X (optic chiasm), 

 the fibers intermingling, and the right nerve apparently passing 

 over to the left side and the left nerve to the right side. The 

 posterior limbs of the X pass backward, and are called the optic 

 tracts. The optic tracts in their course curve around the crura 

 cerebri to terminate in the nerve cells of the pulvinar, anterior 

 quadrigemina, and external geniculate bodies. From these, 

 fibers, called the optic radiations, pass backward to terminate 

 in the cells of the cortex of the posterior part of the occipital 

 lobes. A closer examination of the optic nerves will show that 

 each consists of two bundles of fibers laterally placed. The 

 inner set of fibers comes from the inner half of the retina; the 

 outer bundle comes from the outer half of the retina. If these 

 bundles are traced to the optic chiasm, it is noted that the 

 inner bundles decussate and pass to the opposite side of the 

 brain. Thus the left pulvinar, left anterior quadrigeminate, 

 and external geniculate bodies receive fibers from the inner 

 half of the right eye, and from the outer half of the left eye. 

 The commissure of Gudden, which connects the internal 

 geniculate bodies, probably plays no part in vision. When an 

 image is properly received on the retina, it excites the rods and 

 cones to activity. When the impulses reach the basal ganglia, 

 the sensation of light is not aroused. Light is not perceived 

 until the impulses reach the cortex of the cerebrum. The 

 pulvinars, the external geniculate bodies, and the anterior 

 quadrigemina form the primary vision centre. 



The character of the sensations aroused depends upon three 

 modifications of light: 



1. Color, which depends upon the rate of vibration of the 

 ether waves. 



2. Intensity, which depends upon the energy of the vibra- 

 tions. 



3. Saturation, which depends upon the amount of white 

 light mixed with light of one wave length. 



