VISUAL OR OPTIC CENTER 585 



distinct than the main portion. This is indicated by the lighter shading. 

 Lesions of this area in the cortex lead to loss of sensibility in definite regions of 

 the opposite side of the body. 



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 each retina are used. For example, if we look at an object to the left, 

 an image of that object is focussed 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 retina are 

 gathered together behind where the optic nerves decussate, 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., the pulvinar of the optic 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 quadrigemina the optic tract establishes synap- 

 ses 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 optic 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 optic thalamus, the anterior 

 corpus quadrigeminum, and lateral corpus geniculatum, and it is known that 

 when the occipital cortex is removed, these three atrophy. It has been further 

 shown that in a newly born animal the removal of such a region is followed 

 by imperfect development of the parts in question. 



If one optic nerve be divided, blindness of the corresponding eye results ; 

 but if one optic tract be divided there is a half blindness in both eyes, which is. 

 called hemianopsia, or hemiopia, right or left, according as the right or left field 

 of vision is cut off. It is evident that the occipital lobe, figures 412, 413, and 

 particularly the cuneus, is concerned as a visual center, since not only is it 

 connected with the optic nerves, as we have seen, but also because the re- 

 moval of the right occipital lobe in an animal (monkey) is followed by left 

 hemiopia, removal of the left by right hemiopia, and removal of both occipital 

 lobes by total blindness. 



Olfactory Center in the Cortex. The olfactory nerve differs from 



