278 THE BRAIN OF THE TIGER SALAMANDER 



c) Tractus pedunculo-tegmentalis cruciatus ('36, figs. 2, 7, 

 fjn.t.{l)c.). — These fibers enter the ventral fascicles from the pe- 

 duncle and perhaps also from the ventral thalamus. At the posterior 

 end of these fascicles they do not turn laterally with the others but 

 continue posteriorly and dorsally as one of the components of the 

 f. tegmentalis profundus. They arborize in the deeper layers of neu- 

 ropil of the isthmic tegmentum. 



V entromedian fascicles {2) . — These fascicles of thin unmyelinated 

 fibers comprise tr. mamillo-interpeduncularis, lying laterally and 

 ventrally of those of group (1). These are shown in figure 19 and 

 separating from tr.mam.teg. in figure 21. They assemble in the peri- 

 ventricular neuropil of the ventral lobe of the dorsal part of the 

 hypothalamus. Figure 27 {tr.mam.inp.{2)) shows them converging 

 into a ventricular protuberance of this lobe, which, immediately 

 dorsally of this level, extends across the mid-plane to join the corre- 

 sponding structure of the opposite side at the attenuated ventral 

 border of the commissure of the tuberculum posterius (compare the 

 median section, fig. 2C). Here some of these fibers decussate as com- 

 ponent 1 of this commissure, as shown in figure 28. The crossed and 

 uncrossed fibers continue dorsalward and then spinalward, recurving 

 around the cerebral flexure at the extreme ventral surface (figs. 29, 

 30), to end in open arborizations within the interpeduncular neuropil. 

 Their entire course in the larva is shown in a published diagram 

 ('396, fig. 22; other details are also illustrated in that paper — figs. 

 6-9, 35, 41, 42, 57-61). These fibers comprise the whole of group (2) 

 of the tegmental fascicles, described in 1936. They do not form a 

 compact bundle but are rather loosely spread, and their courses can 

 be followed only in favorable Golgi sections. Unlike the other con- 

 nections of the dorsal part of the hypothalamus, these fibers are ag- 

 gregated in the deep periventricular neuropil, and evidently their 

 physiological properties are radically different, from those of the 

 mamillo-peduncular and mamillo-tegmental tracts. 



V entral fascicles {3). — The chief component is tr. mamillo-tegmen- 

 talis (fig. 21; '36, figs. 3, 8; '42, fig. 3). These thin myelinated and 

 unmyelinated fibers pass, probably in both directions, between the 

 dorsal hypothalamus and the tegmentum in close association with 

 similar fibers related with the peduncle and thalamus. The mam- 

 malian equivalents of the complex are found in the mamillo-thalamic 

 and mamillo-peduncular tracts and the mamillary peduncle. 



