THE SYSTEMS OF FIBERS 273 



tal fascicles of group (9)) extend farther spinalward with different 

 distribution. They give numberless collaterals to the thalamus, the 

 dorsal part of the peduncle, and the dorsal and isthmic tegmentum; 

 and posteriorly they turn laterally into the trigeminal tegmentum 

 dorsally of its motor zone (figs. 29, 30). The chief synaptic connec- 

 tions of these fibers are not with the primary motor column but with 

 areas of intermediate-zone type. They appear later in embryonic de- 

 velopment than do the ventral fascicles, and their functions are be- 

 lieved to be cerebral control and conditioning of bulbar reflexes. 



Another important descending component of these dorsal fascicles 

 is the recently described tr. strio-tectalis (figs. 11, 101, tr.st.tec; '42, 

 p. 262) and the associated tr. strio-pretectalis (figs. 14, 101, tr.st.pt.). 

 These fibers probably arise from the corpus striatum, though this has 

 not been demonstrated. They end by wide arborizations in the optic 

 tectum, pretectal nucleus, and geniculate neuropil of the thalamus. 



Some fibers descend from the dorsal striatal nucleus in company 

 with more from the amygdala in that component of the stria ter- 

 minalis complex known as the dorsal olfactory projection tract 

 (ol.p.tr.), as described on page 242. 



The most noteworthy ascending system of fibers of the lateral 

 bundle is tr. thalamo-frontalis {tr.th.f.). These slender unmyelinated 

 axons arise from cells of the middle sector of the dorsal thalamus and 

 descend in several small compact strands to the dorsal fascicles of the 

 lateral forebrain bundle, within which they turn forward. As in Nec- 

 turus ('336, p. 170), they probably all end in the striatal neuropil, 

 though their terminals in Ambly stoma have not been described. 

 These are precursors of the ascending thalamic radiations of mam- 

 mals. They arise from the undifferentiated nucleus sensitivus of the 

 thalamus, and no evidence has been seen of any separation among 

 them of projection tracts related with different functional systems. 

 The course of these fibers, so far as known, has been fully illustrated 

 (figs. 3, 15, 30-34, 75, 95, 101, 102, 103; '396, figs. 1, 7, 8, 13-17). 



Medial forebrain bundle. — Most of the fibers of this large system 

 are unmyelinated, passing in both directions between the olfactory 

 bulb and the medioventral parts of the olfactory area of the hemi- 

 sphere and the preoptic nucleus and hypothalamus. Many of these 

 fibers decussate in the ventral part of the anterior commissure ridge. 

 Analysis of this complex is possible only with the aid of elective 

 Golgi impregnations, and there are few of these in our collection of 

 Amblystoma sections. The Necturus material has been more instruc- 



