THE HABENULA AND ITS CONNECTIONS 261 



rable with the pallido-habenular connection described in man by 

 Marburg ('44). 



Number 12 of figure 20 is an afferent tract to the habenula from 

 the tectum and nucleus pretectaHs, which has no connection with the 

 stria medullaris — tr. tecto-habenularis. These fibers are accompanied 

 by others which pass in the reverse direction — tr. habenulo-tectaUs. 



FASCICULUS RETROFLEXUS 



Efferent fibers leave the habenula, so far as is known, in three 

 tracts: (1) from the dorsal nucleus to the tectum by the tr. habenulo- 

 tectalis, (2) from the ventral nucleus to the dorsal thalamus by the 

 tr. habenulo-thalamicus accompanying the tr. thalamo-habenularis, 

 and (3) between these from both nuclei in the much larger f. retro- 

 flexus of Meynert. There may be other efferent fibers, e.g., to the 

 cerebral hemispheres accompanying those of com. superior telen- 

 cephah and com. pallii posterior, but these have not been observed. 

 One habenula of the newborn rabbit was destroyed by von Gudden 

 ('81), and subsequently Meynert's bundle of the same side was found 

 to be atrophied. 



The chief component of the f. retroflexus is the habenulo-inter- 

 peduncular tract, the fibers of which arise from both dorsal and ven- 

 tral habenular nuclei (figs. 20, 71, 73, 77, 103). This tract is composed 

 chiefly of thin unmyelinated fibers, which form the central core of 

 Meynert's bundle. Surrounding this core, other thicker fibers are 

 loosely arranged, some of these being myelinated. Weigert sections 

 show that a few myelinated fibers arise from both dorsal and ventral 

 habenular nuclei and that these are joined by a few others from the 

 dorsal thalamus, pretectal nucleus, and eminence of the posterior 

 commissure. This fasciculus is also accompanied for part of its course 

 by the few myelinated fibers of the parietal nerve (p. 235). No fibers 

 have been seen to enter it from the tectum. The myelinated fibers 

 leave the fasciculus and scatter in the alba of the nucleus of the 

 tuberculum posterius; none of them enter the interpeduncular nu- 

 cleus. In addition to these myelinated and other thick axons which 

 terminate in the alba of the peduncle, there are many fine fibers from 

 the axial core which take similar courses ; some of these are probably 

 of habenular origin. 



The fasciculus descends across the thalamus at the outer border of 

 the gray and is partly imbedded within it. Its course as seen in hori- 

 zontal sections is shown in figures 30-35 (for the course in transverse 



