OF DENISOX UNIVEkSITV. 



71 



the opossum, one of us has found these cells widely distributed through 

 the corpora ([uadrigemina and thalamus about the roof of the third 

 ventricle and a(]ueduct, as well as the tectum opticum. We have 

 thought these cells indicative of a low or primitive condition, especial- 

 ly as these cells seem in Didelphy to be devoid of processes. There 

 seems to be no doubt that the cells of this sort lying behind the roots 

 of the fourth nerve constitute a nucleus of the fifth, and the connection 

 of these cells with the entirely similar cells of the tectum opticum is 

 clearly suggested by the course of the fibres as already pointed out. 

 What the reason for the dispersion of these cells so irregularly through 

 the higher regions may be cannot be determined, nor yet the definite 

 connections existing between these large cells and the adjacent struc- 

 tures. See Fig. 3, Plate IX, where such cells should be indicated in 

 the outer lateral border of the gray matter surrounding the aqueduct. 

 (See Osborn, Journ. Morph., Vol. II.) 



Horizontal sections through the mesencephalon, etc. 



The lowest (most ventrad) sections passing through the pars i)ed- 

 imcularis exhibit the lateral fibre tracts formed by the pyramids, within 

 which lie the cross sections of the 8-10 bundles of the oculomotor. 

 Immediately in front of the pons medianly is a circular prominence 

 occupied by the interpeduncular nucleus and transverse decussating 

 fibres. This body seems to be merely a nodular prominence of the 

 raphe. The body in question lies 4 mm. behind the j)osterior margin 

 of the tuber cinereum. The few cells occurring in the nucleus are 

 small flask-cells. At a slightly higher level there aj^pear two distinct 

 nuclei, one seeming like the forward continuation of the i)ons nucleus 

 and lying caudad to the tracts of the third, the other slightly laterad 

 and cephalad to these tracts. The latter consists of multipolar cells 

 of medium size imbedded in a dense reticulum which seems to be 

 merely the cro.ss section of Meynert's bundle. 



At the ventral level of the peduncular nucleus the eleven fibre 

 bundles of the third root have nearly reached the median line. The 

 nucleus is disperse and extends cephMad some distance beyond the 

 nerve roots. At this level there is a marked decussation of the longi- 

 tudinal fibre bundles immediately behind the third roots. The decus- 

 sating fibres curve laterad and caudad, passing in this section to the 

 nucleus at the base of the peduncles of the cerebellum. 



Fig. 2, Plate XIX, sufficiently illustrates the toi.ograi)hv of a hor- 



