MIDBRAIN AND THALAMUS OF NECTURUS 247 



enter also the optic part of the tectuni, or colhculus superior 

 (figs. 21, 24, 39, Im.). A smaller number of its fibers continues 

 forward to terminate in the caudal part of the pars dorsalis 

 thalami (fig. 66, Im.). 



5. Tractus bulbo-tectalis 



Under this name I have described in larval Amblystoma 

 ('14 a, p. 372) a companion tract to the acoustico-lateral lem- 

 niscus which follows the ventral border of the latter tract 

 throughout the medulla oblongata. A portion of its course in 

 the upper part of the oblongata is figured in my former paper 

 ('14, figs. 4 to 9, tr.b.t.). In the isthmus region it turns dorsal- 

 ward along the lateral side of the acoustico-lateral lemniscus 

 and its fibers end by free arborizations in the nucleus posterior 

 tecti. The origin of these fibers is imperfectly known. In 

 larval Amblystoma some of its axons have been seen to arise 

 from neurons related to both the spinal V tract and the fascicu- 

 lus solitarius of the opposite side of the medulla oblongata, 

 and it is provisionally regarded as an imperfectly differentiated 

 crossed secondary ascending trigeminal tract, probably with 

 visceral connections also. 



In this contribution I am able to add nothing further regard- 

 ing the origin and significance of this tract. In the upper levels 

 of the medulla oblongata of Necturus it is easily recognized in 

 the same position as in Amblystoma, and its feebly myelinated 

 fibers distribute in the same way in the tectum. Golgi prepara- 

 tions show these fibers ending in wide arborizations throughout 

 the stratum album of the nucleus posterior tecti (figs. 14, 50 to 

 53, 66, tr.b.t.). The area of distribution of these fibers is a 

 rather large and poorly defined region of neuropil filled with 

 the coarse and contorted dendrites of the underlying neurons 

 and the widely branched fine terminals of the bulbar tract (figs. 

 36, 37, 50 to 53, 66, nuc.p.t.). This nucleus gives rise to a 

 strong component of the tractus tecto-thalamicus et hypothal- 

 amicus cruciatus posterior for the postoptic commissure com- 

 plex (p. 257), and also to the superficial tractus tecto-pedun- 

 cularis posterior (p. 254). 



THE JOCHNAL OF COMPARATIVE NECROLOGY, VOL. 28, NO. 2 



