MORPHOLOGY OF THE FOREBRAIN 479 



terminate in a special olfacto-hypothalamic correlation center, the 

 nucleus medianus septi, which is the most important component 

 of the precommissural body and which in Amphibia effects asso- 

 ciational connections with all other parts of the hemisphere. The 

 descending tractus olfacto-hypothalamicus and the ascending 

 hypothalamo-septal tract constitute the most important com- 

 ponents of the medial forebrain tract. The whole pars ventro- 

 medialis is also related with the habenula by way of the stria 

 medullaris. The latter is an olfacto-somatic correlation path, 

 while the larger hypothalamic connection is for olfacto-visceral 

 reflexes. 



(3) The dorso-lateral part. This also is primarily a secondary 

 olfactory center, receiving the tractus olfactorius dorso-lateralis. 

 It is connected by associational tracts with the dorso-medial part 

 and much more intimately with the ventro-lateral part, from 

 which it is very imperfectly separable in urodeles and frog tad- 

 poles. It is represented in mammals as one of the components of 

 the pyriform lobe. 



(4) The ventro-lateral part. Secondary olfactory fibers reach 

 this part, but in smaller number than the dorsal-lateral part. 

 In the frog these come chiefly from the bulbulus accessorius and 

 are distributed to a special grey center, laterally of the lamina 

 terminalis, which has by some recent authors called been corpus 

 striatum. The remainder of this part is characterized by the great 

 lateral forebrain tract, which includes descending projection 

 fibers from both lateral parts to the pars ventralis thalami and 

 lower regions and ascending projection fibers from the pars dor- 

 salis thalami, including the general sensory nuclei and the cor- 

 pus genie ulatum laterale, and from the colliculus inferior to both 

 lateral parts. It is the great somatic or exteroceptive projection 

 tract for the hemisphere and passes back directly into the pedun- 

 culus cerebri system; i.e., it is the precursor of the striatal and 

 internal capsule fibers of mammals. The ventro-lateral part 

 includes within it the materials out of which the mammalian 

 corpus striatum is developed, though the striatum as such is not 

 yet differentiated. 



The two lateral parts of the hemisphere, then, are the direct 



