150 THE BRAIN OF THE TIGER SALAMANDER 



those of the motor tegmentum and are usually indistinguishable 

 from them except in cases where their axons can be followed into the 

 nerve roots. The cells of the nuclei of the eye-muscle nerves are 

 fairly clearly segregated, and in some reduced silver preparations 

 they react specifically to the chemical treatment (fig. 104) ; but even 

 here their dendrites are widely spread and intertwined with those of 

 tegmental cells, so that both kinds of neurons would appear to be 

 similarly activated by the neuropil within which they are imbedded. 

 The oculomotor nucleus lies in the posteroventral part of the pedun- 

 cular gray (figs. 6, 18, 22, 24, 30, 31, 104). The nucleus of the IV 

 cranial nerve lies about midway of the longitudinal extent of the 

 isthmic tegmentum and far removed from the oculomotor nucleus 

 (figs. 61, 104). The thick IV root fibers (most of them myelinated) 

 ascend along the outer border of the gray to decussate in the anterior 

 medullary velum in the usual way. 



As mentioned in chapter xiii, the sensory zone of the isthmus con- 

 tains cells of the mesencephalic V nucleus and others which send 

 axons peripherally to meninges and chorioid plexus. Some of the 

 latter go out with the IV nerve roots to unknown destinations. It is 

 possible that some of these cells are secondarily displaced neurons of 

 the motor IV nucleus, similar to those described by Larsell ('476) in 

 cyclostomes. There is no definite evidence that this condition exists 

 in urodeles; and, indeed, all connections of the cells lying within and 

 adjoining the superior medullary velum require further study. 



The floor plate of Amblystoma throughout its length contains a 

 special type of ependymal elements and the cell bodies of some neu- 

 rons. These neurons do not invade the floor plate from the basal 

 plate, but they develop within this plate intrinsically, as was first 

 pointed out by Coghill ('24, Paper III). In the adult medulla ob- 

 longata there are few of them at any one level, but they constitute a 

 definite nucleus raphis, which is enlarged in some places, notably so 

 in the interpeduncular nucleus. Most of the cells of the nucleus of the 

 VI nerve are median, as in Necturus ('30, p. 14). These cells in 

 ordinary preparations cannot be distinguished from others of the 

 nucleus raphis except by observation of axons emerging in the VI 

 nerve root. They are distributed sparsely in the ventral raphe and 

 adjacent to it between the levels of the VII and IX nerve roots ('445, 

 fig. 2). They emerge usually by two widely separated roots, though 

 more rootlets are sometimes seen. 



