DiENCEPHALON 81 



internal capsule. The radiation contains descending as well 

 as ascending fibres. 



In man, some optic fibres end in the pulvinar, where they 

 meet fibres from the cerebellum, and whence impulses are 

 transmitted to the cerebral cortex. This centre, however, 

 which forms a large mass projecting back over the antero- 

 lateral aspect of the anterior quadrigeminal bodies in the 

 human brain, is not developed in the lower mammals. It is 

 a part of the lateral nucleus of the thalamus (vide infra 

 p. 85). 



The optic chiasma itself belongs to the telencephalon. 

 The optic tracts and the nervous portions of the eye originate 

 developmentally from the wall of the diencephalon, however, 

 and the fibres end, as we have seen, either in the diencephalon 

 or in the midbrain, so that the chiasma also may reasonably 

 be considered at this point. It lies immediately in front of 

 the di-telencephalic boundary on the ventral surface of the 

 brain, and is composed essentially of the decussating fibres 

 of the optic nerves. These arise in the retina and most of 

 them cross over and pass to their terminal centres on the 

 other side. Some of the fibres, however, coming from the 

 lateral (in man) or posterior (in animals with the eyes in the 

 sides of the head) side of the retina, do not decussate, but run 

 through the homolateral optic tract to their termination. 

 The exact proportion of the fibres which cross over varies in 

 different animals according to the degree of overlapping of 

 the fields of vision of the two eyes. In man, where there is a 

 maximum of overlapping, about one quarter of the optic 

 fibres are uncrossed, but in the rat the proportion is much 

 smaller.^ The uncrossed fibres in the rat are scattered 

 among the crossed ones in the optic tract except near the 

 chiasma, where they are grouped chiefly in the dorsal part. 



The optic fibres are accompanied in the chiasma by the 

 commissure of v. Gudden, which was discussed above, and 



^But larger than in the rabbit. 



