OF DENISON UNIVERSITY. 
71 
the opossum, one of us has found these cells widely distributed through 
the corpora quadrigemina and thalamus about the roof of the third 
ventricle and aqueduct, as well as the tectum opticum. We have 
thought these cells indicative of a low or primitive condition, especial- 
ly as these cells seem iti. 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 e:5cisting 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 ped- 
iincularis 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 posterior margin 
of the tuber cinereum. The few cells occurring in the nucleus are 
small flask-cells. At a slightly higher level there appear two distinct 
nuclei, one seeming like the forward continuation of the pons 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 b,e 
rq,erely the cross section of Meynert’s bundle. 
At the ventral leyel of the peduncular nucleus the eleven fibre 
bundles of the third root have nearly reached the median line. The 
nucleus is disperse and extends cephalad 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 topography of a hor- 
