336 



THE BRAIN OF THE TIGER SALAMANDER 



In this specimen the cell bodies are not blackened, and no dendrites are visible. All nuclei 

 of cells are stained, and the gray pattern is clearly shown. There is scanty impregnation of 

 the neuropil. In the cerebral hemispheres only the thickest axons are impregnated; elsewhere 

 the thick and medium fibers are brilliantly differentiated. In some places where the boundaries 

 of fascicles and tracts are obscure, interpretation has been aided by comparison with other 

 specimens by methods of Cajal, Rogers, Golgi, and Weigert. 



Horizontal sections are not so favorable for analysis of the tegmental fascicles (chap, xx) 

 as are those cut in transverse and sagittal planes, and it is difficult to follow individual bundles 

 as they recurve around the tuberculum posterius in the peduncle and tegmentum; but, by 

 comparison with sections prepared by other methods and cut in various planes, the courses 

 of most of the tracts and of the tegmental fascicles of groups (1) to (10) can be followed. The 

 limits of the numbered groups of fascicles are not always clear, but their identification by num- 

 ber on the drawings is believed to be substantially correct. 



tr. op. m 



24 



Fig. 25. — Through the ventral part of the anterior commissure ridge and the dorsal border 

 of the chiasma ridge. Only a few of the thicker fibers of the medial forebrain bundle are im- 

 pregnated. These thread their way through the decussating fascicles of the chiasma ridge and 

 spread out in the ventral part of the hypothalamus. The dense neuropil of these regions is not 

 impregnated. All the optic fibers decussate ventrally of this level, except the most dorsal fibers 

 of the axial tract (tr.op.ax.). The lightly stippled area in the postoptic commissure, marked 

 tr.th.h.d.c, contains thin fibers from the dorsal thalamus, which decussate more ventrally 

 (fig. 2C, tr.th.h.d.c). Farther dorsally (fig. 26) these fibers, after crossing, separate into fascicles 

 A and B (p. 299). 



Fig. 26. — Ten sections more dorsally, the section passes through the dorsal fascicles of the 

 medial forebrain bundle, tr. olfacto-peduncularis, and above the chiasma ridge. The mixed 

 system of thalamo-hypothalamic and tegmental fibers (fig. 25, tr.th.h.d.c.) has separated into 

 superficial (.4) and deep {B) tracts for the tegmentum, as described on page 299. The course 

 of the superficial tract (A) can be followed in figures 27-34; most of the deeper fibers (B) 

 join the dorsal tegmental fascicles of group (8). 



Fig. 27.— This cuts the decussation of the dorsal fascicles of the lateral forebrain bundle in 

 the anterior commissure and the motor roots and nucleus of the V cranial nerve. The fibers 

 descending from the dorsal thalamus to the postoptic commissure {tr.th.h.d.c.) lie deep in the 



