INTERPEDUNCULAR NUCLEUS 201 



This connection may be present in Amblystoma also, though we have 

 no demonstration of it. 



EFFERENT CONNECTIONS 



The widely branched axons of the interpeduncular neurons ap- 

 parently ramify throughout the entire interpeduncular field, and 

 some of them pass beyond this field. The latter go out in three direc- 

 tions and are designated as three tracts named after their terminal 

 connections — tr. interpedunculo-bulbaris dorsalis and ventralis and 

 tr. interpedunculo-tegmentalis (figs. 19, 83, 84). All these efferent fi- 

 bers go out in a web of mixed neuropil, within which the two bulbar 

 groups are more or less clearly fasciculated in elective impregnations. 

 The tegmental group of efferents are nowhere fasciculated, and we 

 have no Golgi preparations in which they are electively impregnated. 



Sagittal and transverse sections show that the interpeduncular 

 neuropil extends dorsally into the isthmic and trigeminal tegmentum. 

 Most of these fibers trend dorsoventrally, these comprising part of 

 the mixed system, termed "fasciculus tegmentalis profundus." These 

 fibers are marked f .teg. p. in the following figures: 29, 30, 31, 79; '42, 

 figs. 30, 41, 42, 43. Elective impregnations have revealed some of the 

 components of this fasciculus, including the brachium conjunctivum, 

 tertiary visceral tract, and tr. tegmento-interpeduncularis. The lat- 

 ter is shown in figures 61, 62, 63, 65, and 66. These sections show also 

 (though not clearly drawn in the figures) that from the interpedun- 

 cular neuropil a large number of fibers swing dorsolaterally into the 

 alba of the tegmentum, where they engage dendrites of tegmental 

 neurons. These fibers are interpreted as an interpedunculo-tegmental 

 connection. In the light of the mammalian connections of the chief 

 efi^erent tract from the interpeduncular nucleus to the dorsal teg- 

 mental nucleus discussed below, it seems probable that in Amblys- 

 toma the small-celled central nucleus of the isthmic tegmentum con- 

 tains the precursor of the dorsal tegmental nucleus, though this evi- 

 dently is only one of its relationships. 



The interpedunculo-bulbar connections are clearly seen in our ma- 

 terial. These fibers emerge from the diffuse interpeduncular neuropil 

 in dorsal and ventral strands, which are connected with each other 

 and with the surrounding neuropil, as shown in figures 79 and 81. 

 The tr. interpedunculo-bulbaris ventralis descends for an undeter- 

 mined distance at the lateral border of the interpeduncular neuropil 

 and farther spinalward in the lip of the ventral fissure. The dorsal 



