70 
BULLETIN OF THE LABORATORIES 
-led with small cells, except along the raphe, where is the usual nucleus, 
and a short distance on either side, where a disperse nucleus in the 
position of the fourth still remains. In the lateral parts of the area about 
the aqueduct are a few of the large, nearly spherical or polygonal cells. 
c(“ Bladder cells ” ol descending tract of the fifth. ) The tracts of the 
fourth nerve appear as two or three transversely cut bundles at some 
•distance laterad of the aqueduct. The dorsal longitudinal fasciculus 
lies about one fifth the way from the aqueduct to the ventral surface 
and its lateral portions curve ventrad to almost unite with the ventral 
tract. 
Passing cephalad a short distance, the nucleus of the fourth nerve 
■•appears immediately dorsad to the dorsal longitudinal fascicle. It 
consists here of very irregular polygonal, multipolar cells measuring 
about .027 mm., while the nucleus of the raphe consists of a dense 
aggregate of fusiform elements immediately ventrad to the aqueduct, 
which at this point is a transverse slit. The dorso-lateral tract now 
forms a conspicuous prominence ventrad to the nates fissure. The 
nates posteriorly exhibit a conspicuous radiating structure of -th.e cortex 
which appearance is heightened by the very numerous blood vessels 
which jyass toward the centre in a direction parallel to that of the nerve 
•chains. Medianly, sections (VI c-20) display a number of fibre tracts 
in section, which are doubtless continued from the optic tract and enter 
obliquely from the antero-median aspect. Proceeding cephalad these 
bundles increase in number. 
A comparison of the nates with the optic lobes of lower animals 
reveals what at first seems a fundamental difference. While in rep- 
tiles and birds the fibres of the optic tracts pass directly to the j.eriph- 
ery of the tectum opticum, in mammals these fibres appear to pass into 
the deeper portions. In reptiles pretty definite chains can be traced 
from periphery to base of the tectum with insulating, columnar con- 
nective-tissue apparatus which forcibly suggests the arrangement in the 
retina. Nothing of this sort distinctly appears in the mammalian nates. 
The reptilian tectum is characterized by the presence of a few scattered 
cells of great size which resemble greatly Purkinje’s cells of the cere- 
bellum and are especially abundant in the deeper layers of the interval 
between the two tecti. Such cells have not been carefully described 
in mammals, hitherto, to our knowledge, though figured by Bellonci 
in birds and by Herrick in the alligator, and discovered in great abun- 
dance in the same partion of the brain of the turtle, by Coppock. In 
