PKMBErrroN, Structiirc of the Optic Thalami. 141 



of the chiasma, the lateral thalamus fibres that belong to the 

 peduncular tract and come from the region of the inter- 

 peduncular nucleus. The optic tract, accompanied by fibres 

 of the inferior commissure, takes a direction toward the 

 geniculate bodies; a few leave it, but not until it has reached 

 a position just under the internal geniculate, do the fibres of 

 the commissure separate from the tract and enter the internal 

 geniculate body. Most of the optic fibres remain on the 

 surface; a few enter, but only to pass through to the surface 

 of the thalamus. Fibres from the hemispheres, fibres from 

 the " red nucleus," also fibres coming from the corona radiata 

 and passing transversely through the dorsal portion of the 

 thalamus — all these enter the internal geniculate body and by 

 their presence render the identification of the optic tract 

 fibres more difficult. The lower caudal root of the tract 

 passes laterally from the geniculate body to the corpora 

 quadrigemina. Especially worthy of notice is the connection 

 of the thalami with the corpora quadrigemina. This con'es- 

 ponds to the outer anterior portion of the tectum opticum of 

 the lower vertebrates. Near the posterior commissure this 

 tract commences to give oft' branches, all of which sink into 

 the grey substance of the nates and probably terminate there. 

 Posterior Cofiimissure. — One probable function of this is 

 to connect the two thalami; in fact, Mayser says [', p. 357] 

 that this is the only true commissure in the brain, connect- 

 ing, as it does, the two sidewalls of the third ventricle. 

 There is no such structure as the pons in the teleosts, and 

 Rohon thinks that this posterior commissure takes its place 

 and performs the same function as the pons of the higher 

 vertebrates. Osborn tells us ['", p. 79] that the posterior 

 commissure in the amphibians has a three-fold relationship: 

 I. To the oculo - motor nidulus, and perhaps to the main 

 sensory tract. 2. To the pale ganglion cells behind this 

 nucleus. 3. To the tectum opticum. As the fibres of the 

 posterior commissure descend, they divide into two bundles; 

 the anterior surrounds the superior processes of the ganglion 



