SENSE AREAS AND ASSOCIATION AREAS. 195 



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) An anterior commissure. Several observers have 

 claimed that there is a commissural band along the anterior margin 

 of the chiasma which connects one optic nerve or retina with the 

 other. In accordance with this claim it is said that when a local 

 lesion is made experimentally in one retina the degeneration that 

 extends backward along the corresponding optic nerve is contin- 

 ued in part over into the optic nerve of the other side. The exist- 

 ence of this interesting commissural connection between the retinas 

 is, however, still a matter of much uncertainty. 



There are many points in connection with the course of the 

 optic fibers and the physiology of the different parts of the occip- 

 ital cortex which are unknown and require further investigation. 

 Some of these points may be referred to briefly. 



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 mammalia 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, and it is 

 also possible or probable that the extent of decussation in man 

 shows individual differences. There seems to be no acceptable 

 suggestion regarding the physiological value of this partial decus- 

 sation other than that of a probable relation to binocular vision. It 

 has been used to explain the physiological fact that simultaneous 

 stimulation of symmetrical 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 (Fig. 82) on the 

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

 should be regarded as the true cortical center for vision, the re- 

 * Henschen, " Brain," 1893, 170. 



