ISTHMUS 185 



ated neuropil comprise one of the most primitive and conservative 

 features of this part of the brain and also of the diencephalon and the 

 mesencephalon. In mammals it contributes fibers to the dorsal lon- 

 gitudinal fasciculus of Schiitz, as described in the opossum by 

 Thompson ('4'-2). 



The second comi)onent, the central nucleus, is clearly delimited in 

 its middle part. Anteriorly, it merges with the small cells of the pos- 

 terior gray of the peduncle (figs. 29, 30, 31) and more dorsally with 

 the ventral border of the dorsal tegmentum (fig. 32). Posteriorly, it 

 is continuous with the small-celled component of the trigeminal teg- 

 mentum (fig. 91), though the boundary between these is clearly 

 marked in many preparations. At its ventral border the gray is con- 

 tinuous with that of the interpeduncular nucleus, with difference in 

 form and arrangement of the cells, though many transitional forms 

 are seen ('42, fig. 43). The boundaries here mentioned are in some 

 preparations quite indeterminate; in some reduced silver prepara- 

 tions they are evident; and in Golgi preparations each of the regions 

 mentioned has characteristic structure and connections. All these 

 areas are intimately connected by a web of interstitial neuropil, the 

 structure of which is characteristically different in each of them. All 

 this gray is connected with the interpeduncular nucleus by fibers 

 passing in both directions, and in chapter xiv, devoted to that nu- 

 cleus, it is suggested that the central nucleus of the isthmus contains, 

 among other constituents, the primordium of the dorsal tegmental 

 nucleus of mammals. 



The large-celled component 3 surrounds the central nucleus on all 

 sides, and many of these larger neurons lie deeper, mingled with 

 those of the small-celled component. These cells have massive den- 

 drites, which spread widely in the overlying alba. Among these are 

 cells of the nucleus of the nervus trochlearis (fig. 70). Figure 61 shows 

 three neurons from this region which probably belong to the IV nu- 

 cleus. They are surrounded by other similar elements of the teg- 

 mentum. Most of the axons fi-om the large tegmental neurons de- 

 scend, crossed or uncrossed, in the tegmento-bulbar tracts, as de- 

 scribed in the larva ('396, p. 590). Some axons from the isthmic teg- 

 mentum, as also from the trigeminal tegmentum, are directed for- 

 ward. So far as observed, these take short courses, though some may 

 extend farther (figs. 43, 46). This may be a precursor of the con- 

 nection from the substantia nigra to the corpus striatum (Kodama, 

 '29) which has recently been confirmed by several students of 



