MIDBRAIN AND THALAMUS OF NECTURUS 277 



also be true commissural fibers present connecting any or all 

 of these regions, as well as other elements. Its individual 

 components are not as sharply isolated as in teleostean and 

 higher brains, and a determination of the ■ homologies of its 

 various components in different vertebrates cannot be definitely 

 made at this time. It should be frankly stated, moreover, 

 that the conclusions summarized in the immediately follo^\ing 

 paragraphs must be regarded as tentative rather than as fully 

 demonstrated. An enormous amount of urodele material has 

 been faithfully studied, but the complete unravelling of this 

 fiber complex has not been sucessfuUy accomplished. 



1. Tractus tecto-thalamicus ei hypothalamicus cruciatus. This 

 complex may be described in general as a connection between 

 the tectum mesencephali of one side and the ventral part of 

 the thalamus, the hypothalamus, and to a less extent the cere- 

 bral peduncle of the opposite side. Many of these fibers, more- 

 over, or collaterals from them, connect with the homolateral 

 pars ventralis thalami. 



This system of fibers is divisible into two parts depending 

 upon the part of the tectum "\\dth which its fibers are connected. 

 The pars anterior is related with the optic tectum (colliculus 

 superior). The number of these fibers is relatively small and 

 they are unmyelinated save for a few myelinated fibers of very 

 large size. After decussation in the most rostral part of the 

 chiasma ridge in close association with the optic tracts, they 

 distribute to the hypothalamus, pars ventralis thalami, and 

 nucleus of the tuberculum posterius. There is evidence that 

 this part is functionally complex, part of its fibers carrying 

 chiefly optic impulses derived from the optic tracts, and part 

 carrying chiefly non-optic impulses derived from the terminals 

 of the lemniscus systems in the colliculus superior (see p." 255). 

 The pars posterior is related with the non-optic part of the 

 tectum (colhculus inferior) and especially with the nucleus 

 posterior tecti. This contains a large number of myelinated 

 fibers and many which are unmyelinated; its distribution is 

 similar to that of the pars anterior. This part will probably 

 be found to be a complex containing at least two components 



