CENTERS OF CORRELATION. 267 



which has been described in fishes by various authors lies in the 

 region of junction of thalamus and tectum mesencephali and 

 probably is at least a part of the nucleus of the posterior commissure. 

 After crossing, the limbs of the commissure bend downward and 

 backward through the central gray more or less parallel with the 

 tractus habenulo-peduncularis and are lost among the longitu- 

 dinal and decussating tracts in the region of the III nucleus and 

 the ansulate commissure. Most authors agree that the tract is 

 related to the nucleus which gives rise to the fasciculus longi- 

 tudinalis medialis and to the nuclei of the eye muscle nerves. 

 The commissure contains many fibers which arise from or end hi 

 the tectum opticum and its whole nucleus of origin in fishes is 

 closely related to the tectum. All the evidence goes to show that 

 the system of the posterior commissure is closely related to the 

 optic centers. 



DEMONSTRATION OR LABORATORY WORK. 



1. Trace the lemniscus systems in Weigert sections of the brain of a 

 bony fish, frog and a mammal. Note especially the relations of the 

 thalamic centers. 



2. Study in Golgi sections the structure of the tectum mesencephali 

 (corpora quadrigemina) and the corpus geniculatum laterale in the 

 brain of a selachian, frog and the mouse or rat. 



3. In the Golgi and Weigert sections used above search carefully for 

 the origin and destination of the fibers of the posterior commissure. 



LITERATURE. 



Barker, L. F.: The Nervous System. Chapter LIII. 



Cajal, S. R.: BeitragezurStudium der Medulla oblongata. Leipzig. 1896. 



Cajal, S. R. : Textura del sistema nervioso del Hombre y de los vertebrados. 

 Madrid. 1904. 



Catois, E. H.: Recherches sur Phistologie et 1' anatomic microscopique de 

 1'encephale chez les poissons, Bull, de Sci. de la France, Tome 31. 1901. 



Edinger, L.: Vorlesungen liber den Bau der nervosen Centralorgane- 

 Leipzig. 1904. 



Johnston, J. B.: The Radix mesencephalica trigemini. Ganglion isthmi. 

 Anat. Anz., Bd. 26. 1905. 



Kappers, C. U. A.: Teleostean and Selachian Brain. Jour. Comp. Neur. 

 and Psych., Vol. 16. 1906. 



Koelliker, A.: Gewebelehre, 6 Aufl. Bd. 2. 



