THE HABENULA AND ITS COXNECTIOXS 263 



is seen in the series of sagittal Cajal sections from which figures 74-78 

 were drawn. The fasciculus is well impregnated on both sides, and on 

 one side it takes the typical course. At the decussation the thinnest 

 fibers cross in the usual way, and some fibers of thicker caliber 

 descend uncrossed at the lateral border of the interpeduncular neu- 

 ropil. Whether they ultimately decussate and enter the interpeduncu- 

 lar spiral is unknown, because the impregnation fails below the decus- 

 sation. On the opposite side the f . retroflexus divides as it enters the 

 peduncle. The larger medial bundle takes the txT^ical course. A small 

 compact fascicle separates from it and passes more laterally to reach 

 the area ventrolateralis pedunculi, where its fibers spread in the 

 neuropil and disappear. Modifications of the arrangements just de- 

 scribed have been seen in several other series of Golgi sections of late 

 larv^ae and adults. The lateral bundle may be larger or smaller than 

 the medial bundle, and in the latter instances it may end in the 

 peduncle or rejoin the medial bundle at the decussation. 



From these observations it is concluded that in Amblystoma the 

 f. retroflexus is a mixture of fibers of different physiological charac- 

 teristics. The main axial bundle of fine unmyelinated fibers is the 

 habenulo-interpeduncular tract. Most of these fibers decussate below 

 the fovea isthmi and enter a peculiar elongated spiral in the inter- 

 peduncular neuropil within which they end. A smaller number, in- 

 cluding some thicker axons, descend uncrossed along the lateral 

 border of the interpeduncular neuropil. These endings are in isthmic 

 territory. Rostrally of the fovea many thin unmyelinated fibers, evi- 

 dently of habenular origin, end uncrossed in the posteroventral 

 border of the cerebral peduncle near the level of the nucleus of the 

 III nerve. This area may be regarded as a mesencephalic sector of the 

 interpeduncular nucleus, though there is no cellular differentiation 

 here, like that of this nucleus below the fovea. Other similar fibers, 

 doubtless also from the habenula, leave the f. retroflexus farther for- 

 ward and terminate in the superficial neuropil termed "area ventro- 

 lateralis pedunculi." In some specimens these are very numerous. 

 The few myelinated fibers of f . retroflexus derived from the habenula 

 end in the cerebral peduncle. This fasciculus receives accessions of 

 fibers, a few of which are myelinated, from the dorsal thalamus, 

 pretectal nucleus, and eminence of the posterior commissure. All 

 these fibers probably end in the cerebral peduncle; there is no evi- 

 dence that any of them reach the interpeduncular nucleus. In a series 



