VISUAL OR OPTIC CENTER 



645 



Visual or Optic Center. The termination of the optic nerve in each 

 eye, the retina, to the structure of which we shall return when treating of the 

 eye, is so arranged that when we look at an object with both eyes, symmetrical 

 parts of the retinae are used. For example, if we examine an object to the 

 left of the center of vision, an image of that object is focused upon the right 

 half of both retinae, viz., upon the temporal side of the right retina, and upon 

 the nasal side of the left retina. The optic nerve fibers of these symmetrical 

 parts of the retinae are gathered together behind where the optic nerves de- 

 cussate, viz., in the optic chiasma. The fibers which come from the right 

 side of both eyes are contained in the optic tract of the same side, viz., the 

 right, those from the right eye being outside of the others. In the same way 

 the left optic tract contains internally fibers from the left side of the right eye 

 and externally those from the left side of the left eye. The optic tract thus 

 formed then passes backward and terminates in three distinct nuclei, viz., 



FIG. 405. Diagrams to Show Flechsig's Sensory and Association Areas on the Surface of the 

 Cerebral Hemisphere. (From Cunningham, after Flechsig.) 



the pulvinar of the thalamus, the anterior corpus quadrigeminum, and the 

 lateral corpus geniculatum. These nuclei atrophy if the eyes are removed 

 from an adult animal; and if the eyes are removed from a newly-born 

 animal, they do not fully develop. Through the superior corpora quadri- 

 gemina the optic tract establishes synapses that bring it into relation with the 

 nucleus of the third nerve, and which form the basis of the eye reflexes to 

 light stimulation. 



It appears that some of the fibers of the optic tract pass directly into the 

 cerebral cortex without joining with the thalamus, corpus quadrigeminum, 

 or corpus geniculatum. 



It was shown above that the fibers of the cerebral cortex, known as the 

 optic radiation, pass from the occipital region to the three nuclei about 

 which we are speaking, viz., into the pulvinar of the thalamus, the 

 anterior corpus quadrigeminum, and lateral corpus geniculatum, and 

 it is known that when the occipital cortex is removed, these three 



