CHAPTER XVIII 

 THE HABENULA AND ITS CONNECTIONS 



IN ALL vertebrates with fully evaginated cerebral hemispheres the 

 deep transverse stem-hemisphere fissure separates the dorsal 

 parts of the hemisphere from those of the diencephalon. Ventrally of 

 the floor of this fissure there are two great systems of fore-and-aft 

 conduction — the basal forebrain bundles and the stria medullaris 

 thalami. These form a superficial ventrolateral eminence, which is 

 more conspicuous in the frog, where it was termed by Gaupp the 

 "prominentia fascicularis." Attention has been called to the funda- 

 mental difference between these two great systems of fibers. The 

 basal bundles are concerned primarily with the patterning of be- 

 havior, and, accordingly, their composition and connections vary 

 widely from species to species in conformity with diverse modes of 

 life. The composition of the stria medullaris, on the other hand, is 

 remarkably constant throughout the vertebrate series, and its func- 

 tional role is obscure. This chapter is devoted to these fibers and their 

 widely dispersed connections, together with a few other systems of 

 fibers which are associated with them. 



THE DI-TELENCEPIIALIC JUNCTION 



All afferent fibers to the habenula, with the exception of the tecto- 

 habenular tract, enter the stria medullaris thalami for longer or 

 shorter parts of their courses. These fibers are assembled from all 

 parts of the cerebral hemisphere and preoptic nucleus and, in smaller 

 number, from the thalamus. They form a massive fasciculus, which 

 ascends vertically in the posterior lip of the stem -hemisphere fissure; 

 and associated with them is a series of bed-nuclei at the di-telence- 

 phalic junction, which will next be described. These nuclei have 

 diverse and complicated connections, and all have this in common, 

 that they are in functional relations with the stria medullaris. The 

 following are listed in this series; for their arrangement see figure 2B. 



1. Preoptic nucleus. — Among the connections of this nucleus, as 

 described in the preceding chapter, are terminals and collaterals of 

 descending fibers of the nervus terminalis, medial forebrain bundle, 



247 



