1074 VISUAL SENSATIONS. [BOOK m. 



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

 the optic tract of the same side, while the fibres belonging 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 differences 

 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 ( 630) 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 posterior 

 (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. 



668. The optic vesicle is as we have seen budded off from 

 the fore-brain or forerunner of the third ventricle, and the optic 

 chiasma is attached to and forms part of the floor or ventral wall 

 of that ventricle. In a view of the basal or ventral surface of the 

 brain the diverging optic tracts are seen to separate the anterior 

 perforated space and lamina cinerea in front from the posterior 

 perforated space, tuber cinereum with the infundibulum, and 

 corpora albicantia behind, all these being parts of the floor of the 

 third ventricle. From the grey matter in this floor fibres, forming 

 what is sometimes spoken of as Meynert's commissure, belonging 

 neither to the optic nerves nor to the inferior commissure, join 

 the optic tracts, eventually leaving them to pass to the pes. 

 Hence the whole of the optic tract is by no means derived from 



