THE SYSTEMS OF FIBERS 275 



lateral forebrain bundle. The connection with the interpeduncular 

 nucleus is extensive and physiologically distinctive. This tract is the 

 most direct connection between the olfactory area and the primary 

 motor field of the peduncle and the interpeduncular nucleus. It is an 

 internuncial of intermediate-zone type, with the olfactory influence 

 predominant. 



THE TEGMENTAL FASCICLES 



The strio-thalamic and strio-peduncular components of the lateral 

 forebrain bundle activate large neurons of the ventral thalamus and 

 peduncle, and the axons of these cells transmit impulses downward 

 to the tegmentum from the isthmus to the spinal cord. These de- 

 scending fibers form the most conspicuous components of a series of 

 ventral tegmental fascicles, some of which at lower levels are as- 

 sembled in the f. longitudinalis medialis (figs. 6, 18, 91). Ventrally 

 and dorsally of these are many other tegmental fascicles, composed 

 chiefly of descending fibers; and still farther dorsally are the ascend- 

 ing lemniscus systems (fig. 14). The analysis of the composition of 

 these longitudinal fascicles has been very diflficult, yet this knowledge 

 is essential for an understanding of the brain stem. 



The topographical analysis of the brain stem of 1935 was followed 

 in 1936 by a reconnaissance survey of the related fibers in the pe- 

 duncle and tegmentum. The more obvious and constant bundles of 

 longitudinal fibers were enumerated as ten fascicles or groups of 

 fascicles ('36, p. 303), and the specific tracts represented in these and 

 some other bundles, so far as then known, were Hsted ('36, pp. 334- 

 46). These fascicles are divided into a ventral and a dorsal series, the 

 former including fascicles (1) to (6) and part of (10); the latter, (7) 

 to (10). Figures 91, 92, 94, and 102-4 are here reproduced from the 

 paper of 1936; compare figures 30-33 and 101. 



This classification was arbitrary for descriptive purposes only and 

 included only the most clearly defined and constant fascicles border- 

 ing the gray substance. The symbol for each group is an Arabic num- 

 ber inclosed in parenthesis. Externally of these ten groups, in the 

 intermediate alba, there are other less well-defined fascicles with a 

 larger proportion of unmyelinated fibers. These are imbedded in 

 much neuropil and are more variable than the deeper fascicles. Super- 

 ficially in the subpial neuropil there is another series of fascicles, 

 chiefly of unmyelinated fibers, some of which form recognizable 

 tracts. Dorsally of all these fascicles are the lemniscus systems; these 

 and the ascending secondary visceral-gustatory tract are described 



