OPTIC AND VISUAL-MOTOR SYSTEMS 225 



(p. 268). They are the largest of the efferent tracts, and most of their 

 fibers are well myelinated. There are two crossed tracts (figs. 12, 

 27-36, tr.t.b.c.l. and ir.i.b.c.2.) and an uncrossed tract (tr.t.h.c.r.) from 

 the tectum opticum. The posterior crossed tract itr.t.h.c.2.) decus- 

 sates transversely in the vicinity of the nucleus of the IV nerve. The 

 anterior crossed tract {tr.t.b.c.l.) takes a peculiar course, its fibers 

 entering the ventral medial tegmental fascicles, f.v.t.{l), within 

 which they decussate obliquely. There are about a dozen of these 

 anastomosing fascicles extending from the commissure of the tuber- 

 culum posterius backward to the level of the IV nucleus, where these 

 crossed fibers join those of tr.t.b.c.2. and descend to the medulla 

 oblongata and spinal cord (figs. 6, 12). 



The uncrossed tecto-bulbar fibers — tr. tecto-bulbaris rectus — 

 were seen in my earlier studies to arise only from the posterior part 

 of the tectum, as drawn here in figure 12 (tr.t.b.r.); but later ('42, 

 p. 269) other fibers of this system were found to arise more anteriorly 

 and to pass backward by several pathways to join the posterior 

 group. The most anterior of these fibers enter dorsal tegmental 

 fascicles of group (7) , from which they separate to join the posterior 

 tract. Others pass spinal ward in layers 3 and 4 of the tectum, at the 

 posterior end of which they join the other fascicles. Here they send 

 collateral branches into the isthmic visceral-gustatory nucleus, as 

 shown in figure 23. Below the junction of these three divisions, the 

 tr. tecto-bulbaris rectus descends superficially in the isthmus, then 

 turns spinalward, under the lateral recess of the ventricle, passing 

 through the ventral border of the auricle into the medulla oblongata, 

 where it joins the crossed tecto-bulbar tract. 



5. Brachia of superior and inferior colliculi. — These are large collec- 

 tions of fibers passing in both directions between the tectum and the 

 thalamus. They are widely dispersed, and most of them discharge 

 from tectum to dorsal thalamus. Those from the superior colliculus 

 are spread throughout the alba and form a massive tract (figs. 34, 35, 

 36, tr.t.th.r.). These are joined by some fibers from the inferior col- 

 liculus, and from the latter area other strands of unmyelinated fibers 

 take separate courses, some passing forward in the dorsal part of the 

 tectum and some at its ventral margin; most of these fibers pass from 

 inferior to superior colliculus, but some of them continue into the 

 thalamus, particularly ventrally, to enter the geniculate neuropil 

 (figs. 14, 15, 16, br.col.; '42, p. 264). 



