198 THE BRAIN OF THE TIGER SALAMANDER 



is clear. No fibers leave the spiral to pass beyond the specialized in- 

 terpeduncular neuropil. There is a small fascicle of uncrossed fibers 

 at each lateral border of the sjjiral, and these a})parently enter the 

 spiral at successive levels. Another series of transverse sections (no. 

 'i'iB'i) from the same lot of young adult s])ecimens shows almost iden- 

 tically the same structure. 



Not all the habenulo-intcrpeduncular fibers enter the initial decus- 

 sation. As mentioned above, a considerable fraction of them, includ- 

 ing some of thicker caliber, descend uncrossed along the lateral bor- 

 ders of the interpeduncular neuroi)ih which they enter farther spinal- 

 ward. In some preparations both crossed antl uncrossed fibers are 

 imi)regnated, in some only the spiral fibers (fig. 50), and in one sagit- 

 tal series only the uncrossed fibers are blackened (fig. S'i) . In the latter 

 case the f. retroflexus is partially impregnated on each side for its 

 entire length from the habenula to the interpeduncular nucleus. At 

 the decussation none of the crossing fibers or the spiral fibers below 

 it are darkened, but there is a large fascicle of uncrossed fibers, which 

 descends laterally of the decussation and the spiral for almost the 

 entire length of the interpeduncular nucleus. These fibers terminate 

 within the interpeduncular neuropil among those of the unimpreg- 

 nated spiral. 



In 1894 van (iehuchten described thef. retroflexus of the trout. The 

 origin in the habenula is similar to that of Amblystoma, and the 

 course is the same. These fibers decussate in the interpeduncular nu- 

 cleus and recross with much branching, but they do not form a com- 

 pact spiral. They are varicose and have no specialized terminals. 

 Simpler spiral terminals of mammals have been many times de- 

 scribed (rabbit, von Gudden, '81; mole, Ganser, '82, p. 682; mouse, 

 Ramon y Cajal, '11,2, 27-1; Calderon, '28; and many others). 



Tractuff mamillo-i)ifi'rpeduncuIaris. — These fibers comprise group 

 (2) of the tegmental fascicles ('3G, pp. 303, 338, figs, 3, 8). They arise 

 in the dorsal part of the hypothalamus in company with similar fibers 

 for the peduncle and tegmentum (figs. 19, 71, 79). The mixed bundle 

 decussates partially in the retroinfundibular commissure, which is 

 component 1 of the commissure of the tuberculum posterior (p. 302). 

 The interpeduncular fibers, crossed and uncrossed, pass the fovea 

 isthmi at the ventral surface close to the mid-plane and then descend 

 along the lateral border of the specific interpeduncular neuropil (figs. 

 27-30, 60; '396, figs. 22, 41, 42, 57-61; '42, fig. 3), within which they 

 end in open arborizations. They do not join the spiral or participate 



