MIDBRAIN AND THALAMUS OP NECTURUS 263 



18. Tractus thalamo-peduncularis 



This name is given to an extensive and very complex connec- 

 tion between the pars intercalaris of the diencephalon and the 

 dorsal and ventral parts of the thalamus, on one hand, and the 

 cerebral pedmicle in the vicinity of the tuberculum posterius, 

 on the other hand. This system of fibers is quite distinct from 

 that described above under the name tractus thalamo-hypo- 

 thalamicus et peduncularis cruciatus and decussating in the 

 postoptic commissure. The fibers here under consideration are 

 partly myelinated, but chiefly unmyelinated. They are directed 

 caudad and ventrad and run in diffuse formation, some in the 

 deepest level of the stratum album, some very superficially, 

 and some at intermediate levels. Many of the superficial and 

 intermediate fibers form compact fascicles of a few fibers each 

 which retain their individuality for long distances. They are 

 for the most part uncrossed, but some of the more ventral fibers 

 of the complex decussate in the rostral part of the commissure 

 of the tuberculum posterius. These crossed fibers are distin- 

 guished from those of the tractus thalamo-peduncularis cruciatus 

 already described (p. 259) by the fact that the latter tract de- 

 cussates in the postoptic commissure. 



The fibers from the pars ventralis thalami may be termed 

 tractus thalamo-peduncularis ventralis (fig. 68, trth.p.v.), and 

 they form three groups, deep, superficial, and intermediate. 

 The deep fibers (tractus thalamo-peduncularis ventralis pro- 

 fundus) form a thin sheet of both myelinated and unmyelinated 

 fibers at the boundary between the gray and white layers. 

 They extend backward and ventralward into the tuberculum 

 posterius, some decussating in the commissure of the tuberculum 

 posterius to enter the opposite motor tegmentum laterally of 

 the fasciculus longitudinalis medialis (figs. 30, 41, 43, 44, 58, 

 59, tr.th.p.v.p.). The superficial fibers (tractus thalamo-pe- 

 duncularis ventralis superficialis) pass directly lateralward from 

 the pars ventralis thalami in slender wisps, some myelinated 

 and some unmyelinated. They usually accompany the blood 

 vessels to the surface" of the brain and here spread out. All 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 28, NO. 2 



