276 THE BRAIN OF THE TIGER SALAMANDER 



elsewhere in this work. Further study has yielded additional details 

 about the composition of the ten groups of tegmental fascicles. 

 These and some other fascicles and tracts of this region are here 

 analyzed as far as their composition is now known. Most of these 

 fibers are descending. There are ascending fibers also, but our mate- 

 rial has yielded little information about them. 



During the preliminary study it was anticipated that each group 

 of tegmental fascicles would prove to be composed chiefly or wholly 

 of fibers of a single tract or related group of tracts, as are the 

 lemniscus systems. This proves not to be the case, for most of these 

 bundles are mixtures of fibers of diverse sorts from unexpectedly 

 widely separated sources. The reasons for their fasciculation in the 

 pattern observed are not clear. Some specific tracts, like the mamillo- 

 interpeduncular and strio-tegmental, are fairly clearly segregated (in 

 groups (2) and (9) in the cases mentioned), but most of the bundles 

 are heterogeneous mixtures. The pattern of fasciculation seems to be 

 determined more by the ultimate destination of the fibers than by 

 their nuclei of origin. In Weigert and especially in reduced silver 

 preparations these fascicles, particularly those bordering the gray, 

 are clearly defined for long distances; but Golgi sections show that 

 there is much anastomosis among them and that there are number- 

 less unmyelinated fibers which are not fasciculated but spread dif- 

 fusely in the alba. 



As the lateral forebrain bundles recurve dorsally at the antero- 

 dorsal border of the peduncle (figs. 6, 16, 101, 102; '36, figs. 5, 6), 

 sagittal Weigert sections show that the compact bundles of myeli- 

 nated fibers disintegrate, with diffuse spread of the fibers in the alba 

 of the posterior part of the thalamus, dorsal tegmentum, and pe- 

 duncle. Some of these fibers are reassembled farther spinal ward in 

 fascicles (9) and (10). Golgi sections show a similar dispersal of the 

 thinner unmyelinated fibers. Reduced silver preparations, however, 

 reveal many slender fascicles of unmyelinated fibers which traverse 

 this region without loss of their individuality. It is evident that these 

 bundles, like most of the other tegmental fascicles, are mixtures of 

 fibers of diverse distribution and physiological significance. In trans- 

 verse Golgi sections of adult brains, in which the myelinated fibers 

 of the deeper fascicles are slightly darkened and the unmyelinated 

 fibers are electively impregnated, it is seen that the proportion of 

 unmyelinated fibers is greater in the dorsal fascicles than in the ven- 



