THE COMMISSURES 301 



middle and anterior parts of the ventral thalamus reach the teg- 

 mentum through fascicles of groups (6) and (8) . These crossed fibers 

 are accompanied by many others that take similar courses without 

 decussation. 



The hypothalamic components of the postoptic commissure in- 

 clude, in addition to the terminals of extrinsic tracts already de- 

 scribed, a few well-defined tracts and several other less-well-known 

 connections. 



12. Tractus hypothalamo-peduncularis et tegmentalis (figs. 18, 23, 

 tr.hy.ped.; fig. 21, tr.hy.ieg.). — This is the most noteworthy com- 

 ponent originating within the hypothalamus. Its fibers assemble from 

 the whole of the ventral part of the hypothalamus and comprise the 

 main pathway from this region to the motor field of the peduncle and 

 tegmentum. Some of them decussate in the posterior part of the 

 chiasma ridge ; others are uncrossed ; still others decussate in the com- 

 missure of the tuberculum posterius. They connect by terminals or 

 collaterals with the dorsal part of the hypothalamus, ventral part of 

 the peduncle (including the neuropil of the area ventrolateralis 

 pedunculi), and isthmic tegmentum. The last-mentioned group in- 

 cludes thick fibers, some of which are well myelinated, which enter 

 ventral tegmental fascicles (4); and some of these may take long 

 courses in the f. longitudinalis medialis (for further description see 

 p. 280, and '42, p. 226). 



13. Olfactory projection tract (fig. 19, oLp.tr.). — This name has been 

 given to a thin strand of unmyelinated fibers which pass in both 

 directions between the strio-amygdaloid area and a specific nucleus 

 at the posterior border of the chiasma ridge ('21, p. 247; '27, p. 304; 

 '36, fig. 5). Some of these fibers decussate here. 



14. Tractus pedunculo-hypothalamicus. — A large component in the 

 posterodorsal part of the postoptic commissure was provisionally 

 given this name, though the exact connections of its fibers could not 

 be determined ('42, p. 227). 



15. Medial forebrain bundle. — The fibers of the medial forebrain 

 bundle are interlaced with all components of the postoptic commis- 

 sure laterally of the chiasma ridge. Many of these fibers of both 

 descending and ascending systems enter the postoptic neuropil and 

 participate in its formation. This implies a transfer of more or less of 

 this activity to the opposite side of the brain, but no details of specific 

 decussational or commissural pathways have been revealed. 



