MIDBRAIN AND THALAMUS OF NECTURUS 273 



the ventro-medial part of the brain in this region. Their direc- 

 tion of conduction is uncertain, but their course and arrange- 

 ment suggest that they arise from cells of the subcerebellar 

 eminence. They seem to end in the nucleus of Edinger-West- 

 phal of the III nerve (fig. 68, tr.teg.i.). 



V. THE COMMISSURES OF THE MIDBRAIN AND THALAMUS 



Under this head the commissures and decussations of these 

 regions will be enumerated, and so far as their component tracts 

 have been described in the preceding pages they will be sum- 

 marized here. These commissures fall into a dorsal and a 

 ventral series. The dorsal series extends continuously forward 

 from the cerebellar commissures in the decusssatio veli to the 

 commissura habenularum, with a short interruption in the reces- 

 sus posterior mesencephali and a more extensive interruption 

 at the pineal recess. The ventral series of decussations extends 

 continuously forward from the ventral commissure of the me- 

 dulla oblongata to the optic chiasma except for a very short in- 

 terruption at the fovea isthmi and a more extensive break at the 

 wide infundibulum. 



Commissura habenularum {superior commissure) 



This contains chiefly decussations of various components of 

 the stria medullaris, which are not considered in this paper. As 

 we have seen, a decussation of fibers of the parietal nerve in 

 this commissure, which has been described in some other verte- 

 brates, does not occur in Necturus. 



Commissura tecti diencephali 



This is Gaupp's name for the fibers crossing in the dience- 

 phalic roof between the epiphysis and the commissura posterior, 

 i.e., those contained in the pars intercalaris diencephali. Some 

 of these are probably true commissural fibers between the ad- 

 jacent regions. There is also included the decussation of the 

 tractus tecto-habenularis cruciatus (see p. 261). 



