MIDBRAIN AND THALAMUS OF NECTURUS 251 



8. Brachiwn conjunctivum 



These fibers are unmyelinated. Their origin in Necturus 

 I have previously described ('14, p. 8). They are axons of 

 neurons in the ventral part of the body of the cerebellum and 

 floor of the lateral recess ('14, figs. 4, 18, 19); these neurons are, 

 therefore, to be compared with the dentate nucleus of mammals. 

 The fibers of the brachium conjunctivum are directed forward 

 and inward in rather compact formation in immediate contact 

 with the stratum griseum. In the region of the eminentia sub- 

 cerebellaris tegmenti they drop ventralward and decussate in 

 the ventral commissure as the most dorsal fibers of this complex 

 decussation. 



I have no clear Golgi impregnations of the further course of 

 these fibers and they take a very pale color in Cajal prepara- 

 tions. In horizontal Cajal preparations they can be followed, 

 mingled mth other pale fibers derived from the tectum and the 

 motor tegmentum under the cerebellum, to their decussation 

 at a level near the fovea isthmi. Their course beyond the 

 decussation is more obscure. 



9. The dorsal commissures 



A complex series of dorsal commissural and decussating fibers 

 extends from the habenular bodies back to the decussatio 

 veli at the level of the cerebellum without interruption except 

 in the regions of the pineal recess and recessus posterior mes- 

 encephali. The first of these is the habenular commissure, or 

 commissura superior, which will not be further considered in 

 this communication. Then follow (1) the commissura tecti 

 diencephali, (2) the commissura posterior, and (3) the commis- 

 sura tecti mesencephali. 



1) The commissura tecti diencephali. This includes the mye- 

 linated and unmyelinated fibers which cross through the roof 

 of the pars intercalaris diencephali (Schaltsttick) between the 

 pineal recess -and the posterior commissure. Part of tjie mye- 

 linated fibers appear to be truly commissural between the ad- 

 jacent centers in the pars intercalaris. Others are decussating 



