276 C. JUDSON HEERICK 



this region wholly or in part are as follows: Tractus tecto- 

 peduncularis superficiahs (p. 253), (2) tractus tecto-peduncularis 

 profundus (p. 254), (3) the fasciculus longitudinalis medialis 

 (p. 271), (4) tractus habenulo-peduncularis (p. 262), (5) tractus 

 thalamo-peduncularis ventralis (p. 263), (6) nervus terminalis 

 (p. 236), (7) various hypothalamic connections which have not 

 been fully analyzed (decussatio hypothalamica posterior). This 

 group includes some crossed fibers of the tractus mamillo- 

 peduncularis. Some Golgi preparations indicate the presence of 

 true commissural fibers here, connecting the dorsal parts of 

 the hypothalamus, or of the pars ventralis thalami farther for- 

 ward, of the two sides. 



Optic chiasma and commissura postoptica 



The optic chiasma is described on page 241. The other 

 fibers which cross in the chiasma ridge in the aggregate constitute 

 the postoptic commissure, which is much more extensive in 

 Necturus than the optic chiasma (fig. 67). There are doubtless 

 some true commissural fibers in this complex, but the most im- 

 portant components appear to be decussations. Of these I have 

 especially distinguished three important systems: (1) tractus 

 tecto-thalamicus et hypothalamicus cruciatus (p. 255), (2) 

 tractus thalamo-hypothalamicus et peduncularis cruciatus (p. 

 259), (3) decussation of the tractus pallii (p. 269). My obser- 

 vations upon these decussations may be summarized as follows. 



This aggregation of fibers is exceedingly complex and it has 

 not been fully analyzed in any vertebrate. It is very extensive 

 in all Amphibia and in Nectm-us it is more easily isolated from 

 the associated optic tracts than in most other species because 

 the optic tracts are here wholly unmyehnated, while the post- 

 optic commissure complex is partly myelinated. Its analysis 

 is, however, even here very difficult and our studies are still 

 incomplete. 



The postoptic commissure system as a whole evidently puts 

 the entire tectum mesencephali and pars dorsalis thalami into 

 functional relation with the pars ventralis thalami, hypothal- 

 amus, and cerebral peduncle of the opposite side. There may 



