208 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



According to this schema, the general relations of each occipital 

 lobe to the retinas of the two eyes is such that the right occip- 

 ital cortex represents the cortical center for the two right halves 

 of the retinas, while the left occipital lobe is the center for the 

 two left halves — a relation that agrees completely with the results 

 of experimental physiology and clinical studies. 



In addition to the fibers described, which may be regarded as the visual 

 fibers proper, there are other fibers in the optic tracts and optic nerves whose 

 physiological value is not entirely clear. The fibers of this kind that have been 

 described are: (1) Inferior or Gudden's commissure. Fibers that pass from 

 one optic tract to the other along the posterior border of the chiasma. These 

 fibers form a commissural band connecting the two internal (or median) 

 geniculate bodies, and possibly also the inferior colliculi. It seems probable 

 that they belong to the central auditory path rather than to the visual system. 

 (2) Fibers passing from the chiasma into the floor of the third ventricle. 

 The further course of these fibers is not clearly known, but it is possible that 

 they make connections with the nuclei of the third nerve. They will be 

 referred to in the section on Vision in connection with the light reflex of the 

 iris. (3) A superior commissure. Several observers have claimed that there 

 is a commissural band along the anterior margin of the chiasma which connects 

 one retina with the other. 



The Amount of Decussation in the Chiasma. — According 

 to the schema given above, half of the fibers in each optic nerve 

 decussate in the chiasma. There is, however, no positive proof 

 that the division of the fibers is so symmetrically made. In the 

 lower vertebrates — fishes, amphibia, reptiles, and most birds — the 

 crossing is said to be complete, while in the mammaha a certain 

 proportion of the fibers remain in the optic tract of the same side. 

 In a general way, it would appear that the higher the animal is 

 in the scale of development, the larger is the number of fibers that 

 do not cross in the chiasma. At least it is true that a larger num- 

 ber remain uncrossed in man than in any of the mammalia. 

 There seems to be no acceptable suggestion regarding the phys- 

 iological value of this partial decussation other than that of a 

 probable relation to binocular vision. It has been used to explain 

 the physiological fact that simultaneous stimulation of symmet- 

 rical points in the two retinas gives us a single visual sensation. 



The Projection or Localization of the Retina on the 

 Occipital Cortex. — It would seem most probable that the paths 

 from each spot in the retina terminate in a definite region of 

 the occipital cortex, and attempts have been made by various 

 methods to determine this relation. According to Henschen,* the 

 visual paths in man end around the calcarine fissure on the mesial 

 surface of the brain, and this portion of the occipital lobe should 

 be regarded as the true cortical terminus of the optic fibers. 

 There seems to be much evidence, indeed, that the immedi- 

 *Henschen, "Brain," 1893, 170. 



