CENTERS OF CORRELATION. 261 



anterior corpus quadrigeminum, has chiefly shifted its place of 

 ending to the dorsal part of the thalamus. There is evidence 

 that a few fibers of both auditory and cutaneous tracts may go 

 directly to the cerebral hemisphere. 



2. THE VISUAL APPARATUS. 



It was seen in the last section that optic tract fibers in all verte- 

 brates end in the tectum opticum. Although this is apparently 

 the earliest place of ending of optic tract fibers a second important 

 end nucleus appears as low as the selachians. This is the corpus 

 geniculatum later ale situated in the lateral wall of the thalamus. 

 This nucleus increases in importance in the series of vertebrates 

 and forms the largest optic center in the mammalian and human 

 brain. In addition, the pulvinar of the thalamus receives optic 

 tract fibers in mammals. Some authors have described as places 

 of ending of optic tract fibers a nucleus named by Edinger the 

 corpus ectomammillare (reptiles, Edinger; teleosts, Goldstein) 

 and the ganglion isthmi in birds. The relations of the ganglion 

 isthmi of birds require further study. The nucleus in amphibia 

 and fishes to which Edinger has given the same name is of a very 

 different character. The identity of the corpus ectomammillare 

 and the proof that optic fibers end in it are not clear from the 

 descriptions that have been given. 



In selachians the optic tracts after their decussation in the 

 chiasma pass up on the lateral surface of the thalamus where they 

 cover superficially a layer of cells which is homologous with the 

 corpus geniculatum laterale of higher forms. Only collaterals 

 of the optic fibers are said to end in this nucleus. The main body 

 of the optic tract ends in the tectum opticum. In some bony 

 fishes the corpus geniculatum is more highly developed and 

 receives a larger number of optic fibers. This is true also in 

 Atnia (Fig. 141). In some bony fishes and in the sturgeon the 

 corresponding cells are closely related to the nucleus anterior 

 thalami and receive the endings of a small part of the optic tract. 

 In amphibia, reptiles and birds a well developed corpus genicula- 

 tum laterale is found and a large part of the optic tract ends in 



