INTERPEDUNCULAR NUCLEUS 197 



Other branches spread widely in the general interpeduncular neuro- 

 pil, of which they form an important component. Most of these fibers 

 are dispersed, but some of them form compact longitudinal tufts 

 within the glomeruli (fig. 84: '396, figs. 41, 61). 



AFFERENT CONNECTIONS 



Fibers from remarkably diversified sources terminate in the inter- 

 peduncular neuropil. The widely branched terminals of some of these 

 fibers spread diffusely ; the endings of others are specialized in diverse 

 ways. The dominant member of this complex evidently is tr. haben- 

 ulo-interpeduncularis, which is the chief component of the f. retro- 

 flexus as described in chapter xviii. The interpeduncular connections 

 of this tract are similar in Amblystoma and Necturus ('34c). 



Tractu.s habenulo-interpediaicularis. — The two bundles of unmye- 

 linated fibers converge immediately below the fovea isthmi, and here 

 most of their fibers decussate close to the ventral surface. Their f ur- 

 thei- course is shown in figure 50. Immediately after crossing, each 

 fiber reverses its course, and this is repeated so that a compact spiral 

 is formed, which extends the entire length of the interpeduncular 

 neuropil, diminishing spinalward. This spiral is coextensive with the 

 specific interpeduncular neuropil and is its most characteristic fea- 

 ture. There are many series of sections cut in various planes in which 

 this is clearly shown. In some of these, only one of the habenulo-in- 

 terpeduncular tracts is impregnated (figs. 51, 52, 56, 57,. 58). 



Figures 53 and 54 are drawn from a specimen in which tr. haben- 

 ulo-interpeduncularis is impregnated on both sides at the origin in 

 the habenula. The impregnation fails on the left side from the level 

 of the dorsal thalamus downward. On the right side it is well stained 

 as far as the decussation, below which only a few fibers are blackened, 

 thus revealing clearly the courses of individual fibers, one of which is 

 drawn in figure 54 through several turns of the spiral (cf. fig. 

 55). These fibers are slightly varicose and thorny. No specialized 

 endings of any sort have been seen, and they have not been observed 

 to branch except for occasional short forked terminals. The spiral 

 penetrates the glomeruli, and its fibers are in synaptic connection 

 with the glomerular dendrites. 



Transverse sections show that the spiral is flattened dorsoventrally 

 ('42, figs. 40-43). In the specimen from which these figures were 

 drawn the spiral is fully impregnated, and few other fibers of the in- 

 terpeduncular neuropil are stained so that the structure of the spiral 



