146 Cincinnati Society of Natural History. 



but it was conjectured that it arises from a tract of the postero- 

 basal portion of the cortex near the ventricle and represents the 

 posterior sensory area. 



In sections still farther back it appears that the posterior fibres 

 from the anterior commissure cross the peduncular bundles dorsally 

 without coming into any relation to their fibres. The system seems 

 to be purely commissural (Plate VIII., Fig. 10). In sections far- 

 ther back (Fig. 12) the taenia thalami reach a point near the summit 

 of the thalamus (habenulae), and the optic tracts can be traced 

 upward along the surface. 



4. Minute structure of the diencephalon and mesencephalon. Sev- 

 eral details of the thalami have already been given, and further 

 description of the region of the infundibulum is added in connec- 

 tion with the optic nerve. 



In transverse sections, behind the anterior commissure (Fig. 11) 

 a considerable accumulation of fibres from the antero-central part 

 of the thalamus collects to form a strong bundle. 



Still farther back fibres from the habenulae converge caudad and 

 ventrad to form Meynert's bundle. In sections caudad to the 

 above A (Fig. 14) the soft commissure, or commissura media, 

 appears. It is simply an adhesion, with few transverse fibres. 

 At this level fibres converge from above to the large oval corpora 

 geniculata (Figs. 14-16). A small but definite fibre tract passes 

 from the summit of the corpus geniculatum to the gray matter 

 about the superior commissure. The superior commissure is 

 seen in this region as a small transverse band at the. 

 very summit of the thalamus. Its relation to the taenia thai- 

 ami and the apparent absence of pineal crura has been already 

 noticed. The posterior commissure is in part buried in the sub- 

 stance of the thalamus at its juncture with the optic lobes, to which 

 latter it properly belongs. Fibres from this region can be traced 

 ventrad near the median line to the ascendmg pyramids, where 

 they seem to turn cephalad (Plate IX., Fig. 8, t. r. p. p.). Back 

 of the posterior commissure a well-marked bundle of fibres descends 

 from the corresponding region or lateral aspects of the colliculi 

 between the optic lobes, on either side the aqueduct, to a decus- 

 sation below the level of the oculomotor nucleus (Plate XII., 

 Fig. 2). 



For additional data, we may examine longitudinal-perpendicular 

 sections near the middle of one of the hemispheres (Figs. 6-8, 

 Plate IX.) 



