72 
BULLETIN OF THE LABORATORIES 
izontal section at the level of the third and fourth nuclei. A dense 
nucleus, (JV. interpatheticus,) occupying.the region of the raphe caudad 
to the fourth nucleus is conspicuous. In higher sections the emerg- 
ence of the fourth root tracts by a number of small bundles describing 
an abrupt outward curve, is well shown. The. interpathetic nucleus 
above described now lies in the median plane between the nuclei. 
While no decussation was observed, we are convinced that fibres from 
the third are given off to tracts passing caudad and laterad. 
The relations of the tegmentum with the corpora quadrigemina 
may be best studied in longitudinal vertical sections. (Plate XIX, 
Fig. I.) The nates are nearly quadrangular and inclined posteriorly 
while the testes lie nearly at right angles to the former a,pd do not reach 
one-half the size. Fibres from the optic tracts spread out upon the 
surface of both anterior and posterior 'corpora and are followed with 
difficulty. 
In Didelphys the testes are much more prominent than the nates, 
and contain relatively more gray matter. The tectum proper is not 
developed below the lateral plane passing through the third nerve nu- 
clei. In this plane there appear to be five lateral nuclei, three in the 
thalamus and two in the mesencephalon. The median part of the 
section is filled with cells, constituting one unseggregated nucleus. 
We have been able to convince ourselves that the fibres from the optic 
tracts do not spread out upon the surface of the tectum, but rather are 
over-capped, by a cortex similar to the cerebral. Thus the portion 
above the aqueduct is separable into four gray layers with three white 
zones^ bounding them. The innermost gray zone is immediately sur- 
rounding the ventricle and is filled with small cells. (Plate IX, Fig. 
3, etc.) This zone is limited by the decussating tract of the nates, in 
which are found the bladder-cells connected with the superior nucleus 
of the trigeminal. 
The second gray zone seems to partake of the structure of the 
formatio reticularis and contains scattered nuclei and is bounded per- 
ipherally by a discontinuous zone of fibres, apparently extending ob- 
liquely caudad. In connection with these fibres are numerous large 
multipolar cells, whose processes can be traced in various directions, 
especially peripherad, for long distances. The space intervening be- 
tween this tract and the tract formed by the fibres from the optic tract 
is a reticulum of fibres and cells in concentric and radiating chains. A 
few of the large multipolar cells are likewise sparsely scatffired through. 
