252 C. JUDSON HERRICK 



fibers of the tractus tecto-habenularis cruciatus (see p. 261 and 

 figs. 5, 6, 28, 37, 62, comi.d). 



2') The commissura posterior. This includes a very compact 

 mass of coarse and hea\ily myelinated fibers crossing at the 

 rostral end of the tectum mesencephali. It is continuous in 

 front with the commissura tecti diencephali and behind with 

 the commissura tecti mesencephali. Fibers enter it from the 

 rostral part of the tectum (tectum opticum or colliculus su- 

 perior). Some of these are probably true commissural fibers; 

 others after their decussation turn abruptly ventralward and 

 slightly caudalward keeping close to the gray layer. These 

 end in the large ventricular eminence dorsally of the tuberculum 

 posterius which contains large neurons of the nucleus of the 

 posterior commissure (figs. 7, 8, 29, 38, 39, 41, 59, 62, 63, 64, 

 68, com.post.). I have not been able to separate these large 

 neurons into distinct nuclei of the posterior commissure and 

 nucleus of the fasciculus longitudinalis medialis, the dendrites of 

 these cells being related to the commissural fibers and their 

 axons entering the longitudinal fasciculus (p. 271). Fibers of 

 the commissura posterior doubtless also effect direct synaptic 

 connection with dendrites of the nucleus of the III nerve, though 

 this has not been directly observed. 



3) The commissura tecti mesencephali. This is a continuation 

 of the posterior commissure backward throughout the length 

 ■of the tectum. It is a thin sheet of myelinated fibers whose 

 connections have not been definitely observed. Some are prob- 

 ably true commissural fibers; others are doubtless tecto-pe- 

 duncular fibers (figs. 9 to 14, 62, com.t.m.). 



In Necturus the tectum mesencephali seems to be the most 

 important correlation center for afferent somatic sensory im- 

 pulses. From it efferent tracts diverge in all directions, for 

 the most part in rather diffuse formation. Some of the more 

 definite collections of these fibers are enumerated below. 



10. Tractus tecto-bulbaris 



These descending myelinated fibers arise from all parts of the 

 tectum, especially the region of the colliculus inferior, and ac- 



