THE SYSTEMS OF FIBERS 283 



the systems just mentioned, traverse the whole length of the peduncle 

 and isthmic tegmentum, and some of them enter the f . longitudinalis 

 medialis. Figure 3^2 cuts these fascicles a few sections ventrally of 

 their most dorsal course as they arch across the peduncle; compare 

 the sagittal section (fig. 103). 



The thickest fibers of tegmental fascicles (4), (6), and (8) decussate 

 in the dorsal and posterodorsal part of the postoptic commissure, 

 those of group (8) being the most dorsal fibers of this complex at their 

 crossing in the mid-plane. Most of the latter enter group (8), but 

 some enter group (6). These are fibers of tr. tecto-thalamicus et 

 hypothalamicus cruciatus posterior of my former descriptions, but 

 these longer fibers are properly called tr. tecto-peduncularis et teg- 

 mentalis cruciatus. Posteriorly and ventrally of these at the decussa- 

 tion are similar coarse fibers of tr. thalamo-tegmen talis ventralis cru- 

 ciatus, which cross in more dispersed arrangement posteriorly and 

 ventrally of the preceding system. These enter the three groups of 

 fascicles, (4), (6), and (8). In the horizontal sections here illustrated 

 the courses of these fibers are in some places not very clearly seen, for 

 they are not separately fasciculated. Their courses are indicated on 

 the drawings as determined (where doubt arises) by comparison with 

 sections in other planes and with the early larvae, where their courses 

 are clear. It should be noted that this crossed system of ventral 

 thalamo-tegmental fibers is accompanied by similar uncrossed fibers, 

 most of which enter fascicles of group (8). These were first described 

 in the larva ('39&, p. 546, figs. 13-16, 81) and in the adult are seen 

 here in figures 30-33 as fibers joining bundle (8) from the thalamus. 

 There is also a broad uncrossed connection from both ventral and 

 dorsal parts of the thalamus to the tegmentum by tr. thalamo- 

 tegmentalis rectus (figs. 30-34, tr.th.teg.r.). These fibers pass from the 

 thalamic gray to the pial surface of the tegmentum and end here in 

 the superficial neuropil and obviously have physiological properties 

 different from the deeper fibers of groups (4), (6), and (8), which 

 make synaptic contacts with the large cells of the tegmentum in the 

 intermediate and deep neuropil. 



Dor ml fascicles (7). — Two systems of fibers have been identified 

 in these fascicles: {a) one from the dorsal thalamus and dorsal teg- 

 mentum and (6) one from the tectum. 



a) Tractus tegmento-isthmialis (fig. 21). — This tract was first de- 

 scribed as the chief component of fascicles of group (7) under the 

 name tr. tegmento-bulbaris ('36, p. 334, figs. 4, 6, f.d.t.{7); '39&, p. 



