THE VISUAL APPARATUS 209 



The latter connection is for responses of purely reflex type, 

 chiefly those concerned with the movements of the eyeballs and 

 accommodation of the eyes; the thalamic connection is a station 

 in the cortical visual path. 



From these relations it follows that there is nothing in the 

 visual organs which corresponds to a peripheral nerve. The 

 retina as a part of the brain is directly excited by the light waves 

 which penetrate its substance. The so-called optic nerve is a 

 tract within the brain, whose fibers for the most part come from 

 visual neurons of the third order in the retina, though there are 

 others also which come from the brain and pass outward to end 

 by free arborizations within the retina (Fig. 98, c.n.). The 

 function of these centrifugal fibers to the retina is unknown. 

 Identically the same nerve-fibers which make up the so-called 

 optic nerves peripherally of the optic chiasma are called the 

 optic tracts centrally of that point. It would be more logical 

 to name these fibers optic tracts for their entire length, these 

 tracts being very similar to those of the lemniscus system. Like 

 the lemniscus fibers, they decussate completely in the optic 

 chiasma in lower vertebrates before terminating in the thalamus 

 and midbrain. It is only in animals with an overlapping of the 

 fields of vision of the two eyes and stereoscopic vision that the 

 decussation of the optic tracts in the chiasma is incomplete. 



The significance of the crossed and uncrossed fibers of the 

 optic tracts is seen in Fig. 101. In this diagram the shaded por- 

 tions of the retinae receive their light from the left side of the 

 median plane of the body; the unshaded portions, from the right 

 side. The nasal part of each retina recives visual images from 

 objects lying on the same side of the body exclusively, i. e., 

 from the temporal portion of the visual field, while the temporal 

 part of the retina may receive images from objects on the oppo- 

 site side of the body. Accordingly, in order that the visual 

 images derived from all objects lying on one side of the body may 

 be represented by nervous excitations within the opposite half 

 of the brain, it is necessary that the nerve-fibers from the nasal 

 part of each retina cross in the chiasma, while those from the 

 temporal part pass through the chiasma without decussation. 



The reflex optic centers in the roof of the midbrain occupy 

 most of the colliculus superior, which corresponds to the optic 



14 



