MEDULLA OBLONGATA OF AMBLYSTOMA 377 



gata the fibers of the bulbar lemniscus arise (tractus octavo- 

 tectalis et thalamicus). These fibers decussate in the ventral 

 commissure and terminate in the tectum mesencephali and 

 thalamus. Accompanying the latter tract ventrally and laterally 

 is a smaller fascicle which I have termed the tractus bulbo-tectalis ; 

 this arises (in part at least) from the secondary trigeminal neu- 

 rones of the vagus region, these same neurones being also con- 

 nected with the visceral sensory root fibers of the fasciculus 

 solitarius. These fibers cross in the ventral commissure and 

 terminate in the nucleus posterior tecti of the midbrain. Their 

 physiological significance is not wholly clear, but they seem to 

 share the functions of a trigeminal lemniscus and a secondary 

 visceral tract. Between the tract last mentioned and the spinal 

 \' root is a smaller bundle of non-myelinated fibers which are 

 derived from the neurones of the visceral sensory lobe of the same 

 side and which terminate in a small secondary visceral nucleus in 

 the isthmus, this tract being clearly comparable with the secondary 

 \'agus bundle of ]\Iayser, or the ascending secondary gustatory 

 tract of Herrick, in teleosts. 



THE MOTOR NUCLEI AND TRACTS 



The cells of the ventral gray column of the spinal cord which 

 give rise to ventral root fibers in larval Salamandra have been 

 figured by Van Gehuchten ('97, pi. 29, fig. 1). Neurones of the 

 same type are seen in our figures 17 and 18, col.v., though I have 

 not seen root fibers arising from these cells. 



The approximate boundaries of the motor nuclei of the V, VI, 

 Ml, IX and X cranial nerves are indicated in figure 1. It is diffi- 

 cult to determine the precise limits of these nuclei, for their neu- 

 rones are mingled with those of the motor tegmentum and in 

 none of our Golgi preparations are the axones of the motor roots 

 impregnated. Neurones which probably belong to the nucleus 

 ambiguus (nuc.amb.) are shown in figures 37, 39 and 41. Den- 

 drites of these cells spread out widely in the tegmentum and also 

 in the secondary visceral tract {tr.v.a.). Mingled with these are 

 other similar neurones whose axones are directed ventrad into the 

 ventral funiculi of the same side and of the opposite side. The 



