MIDBEAIN AND THALAMUS OF NECTURUS 253 



cumulate on the ventro-lateral bordei: of the motor tegmentum 

 below the fovea isthmi, a large proportion of them first decus- 

 sating in the underlying ventral commissure. They continue into 

 the ventral tracts of the medulla oblongata, and some of them 

 may extend farther into the spinal cord as a tractus tecto- 

 spinalis. In larval Amblystoma I have figured their course 

 through the medulla oblongata ('14 a, figs. 4 to 17), as they 

 descend near the median plane ventrally of the fasciculus longi- 

 tudinalis medialis. The relations of the tractus tecto-bulbaris 

 rectus et cruciatus are in Necturus essentially the same as in 

 fishes (figs. 11 to 14, 21, 24, 68, trA.h.). 



11. Tractus tecto-peduncularis 



Fibers of the same type (myelinated and unmyelinated) as 

 the tractus tecto-bulbaris arise from the entire length of the 

 tectum and pass ventralward to reach the underlying motor 

 tegmentum. These are for the most part directed rostrad as. 

 well as ventrad and are, therefore, readily distinguished from 

 those of the tractus tecto-bulbaris (fig. 68, tr.t.ped.). They are 

 arranged in superficial, deep, and intermediate groups. 



The superficial group {tractus tecto-peduncularis superficialis) 

 arise from the entire length of the tectum and form the 

 most superficial fibers of the stratum album (figs. 10, 11, tr. 

 t.ped.s.). Having reached a point dorsally and caudally of 

 the superficial origin of the III nerve, these fibers turn inward, 

 still directed ventralward, and efTect various connections with 

 the motor centers of the tegmentum, partly of the same side and 

 partly after decussation in the ventral commissure. Some 

 reach the nuclei of the commissura posterior and III nerve of 

 the same side. Some appear to enter the fasciculus medialis 

 longitudinalis directly "\\ithout decussation. Others cross in 

 the ventral commissure, some immediately rostrally of the fovea 

 isthmi and some diverging around this recess close to the ven- 

 tricle to decussate below it. After decussation these fibers may 

 effect connection with all of the motor nuclei mentioned above, 

 while others turn caudad and dorsad to reach the neurons of the 



