CHAP. IT.] THE BRAIN. 785 



The decussation of fibres which takes place in the chiasma has 

 peculiar characters. At their decussation (we are speaking now 

 of man) the fibres in the optic nerve belonging to the temporal 

 half of the eye in which the nerve ends pass into one optic tract, 

 namely, the optic tract of the same side, while the fibres belong- 

 ing to the nasal half pass into another optic tract, namely, the 

 optic tract of the opposite side. Thus the fibres of the temporal 

 half of the right eye and of the nasal half of the left eye pass 

 into the right optic tract, and the fibres of the nasal half of the 

 right eye and of the temporal half of the left eye pass into the 

 left optic tract. Compare Fig. 138, in which the fibres forming 

 the right optic tract are shaded while those forming the left 

 optic tract are left unshaded. Now, the nasal half of one retina 

 and the temporal half of the other retina are 'corresponding' 

 parts. Hence, while each optic tract contains fibres belonging 

 to half of each eye, the two halves thus represented in each tract 

 are corresponding halves. 



The amount and character of the decussation taking place 

 in the optic chiasma differs in different animal types, the differ- 

 ences having relation to the amount of binocular vision, which 

 in turn depends on the position of the eyes in the head, that is, 

 on the prominence of the face between the eyes. In the fish for 

 instance, with laterally placed eyes, no binocular vision at all is 

 possible, and the decussation is complete ; the whole optic nerve 

 of each eye crosses over to the other optic tract. Between this 

 and the arrangement in man just described, various stages obtain 

 in various animals. 



The chiasma also contains at its hinder part fibres which 

 have no connection with the optic nerves or the eyes, but are 

 simply commissural tracts passing from one side of the brain, 

 namely, from the median corpus geniculatum (this has no con- 

 nection with the optic nerves and is not concerned in vision) 

 along one optic tract, through the chiasma to the other optic 

 tract, and so to the median corpus geniculatum of the other side 

 of the brain. These fibres are spoken of as the inferior or pos- 

 terior (optic) commissure or arcuate commissure, or Gudden's 

 commissure. It was once thought that in a similar way fibres 

 passed from one retina along one optic nerve, through the front 

 part of the chiasma to the other optic nerve, and so to the other 

 retina forming an anterior (optic) commissure ; but this seems 

 to be an error. 



The optic tract also contains fibres taking origin from the 

 grey matter in the floor of the third ventricle and forming what 

 is sometimes spoken of as Meynert's commissure ; these fibres 

 which belong neither to the optic nerves nor to the inferior com- 

 missure, join the optic tracts for a while, but eventually leave 

 them to pass to the pes. 



496. That part of the optic tract which consists exclu- 



50 



