MIDBRAIN AND THALAMUS OF NECTURUS 245 



mesencephali. This area of distribution is the true tectum 

 opticum, or colhculus superior; apparently it does not extend 

 backward to the caudal end of the tectum (figs. 9 to 12, 21 to 

 24, 32, 33, 36, 37, 65, 67 col.swp.). 



3. The spinal lemniscus 



This tract has been referred to in my paper on the cerebellum 

 of Necturus ('14, pp. 7, 11) under the name tractus spino- 

 tectalis and more fully described in larval Amblystoma ('14 a, 

 p. 374) with the same designation. In Necturus, as in larval 

 Amblystoma, fibers ascend in the lateral funiculus of the spinal 

 cord and traverse the entire length of the medulla oblongata 

 as a large fascicle containing spino-bulbar, spino-cerebellar, 

 spino-tectal, and spino-thalamic fibers. The fibers of this 

 system which ascend beyond the isthmus (i.e., the tractus spino- 

 tectalis and tractus spino-thalamicus) are here termed spinal 

 lemniscus. 



For the relations of the spinal lemniscus in the medulla oblon- 

 gata of Necturus and Amblystoma, see the papers last cited. 

 In the region of the auricular lobes just caudad of the isthmus 

 the spinal lemniscus fibers are intimately associated with those 

 of the tractus thalamo-peduncularis cruciatus which descends 

 from the postoptic commissure (see p. 259). At the level of 

 the isthmus the spinal lemniscus fibers ascend sharply and take 

 a position as a series of small fascicles at the boundary be- 

 tween the gray and white layers immediately ventrally of the 

 fibers of the mesencephalic V root and dorso-medially of the 

 much larger acoustico-lateral lemniscus (figs. 6 to 14, tr.sp.t. 

 and tr.sp.th.). In this relation they continue forward through- 

 out the entire length of the tectum, most of them ending in its 

 caudo-lateral part (colliculus inferior, figs. 21, 24, 32, 33, 35, 

 56, tr.sp.t.) ; but a smaller number clearly continue forward 

 to terminate in the caudal part of the pars dorsalis thalami 

 (figs. 6, 66, tr.sp.th.). These fibers are coarser than those of 

 the bulbar lemniscus systems and can readily be distinguished 

 from them. 



