BRAIN AND CRANIAL NERVES OF BDELLOSTOMA DOMBEYI. 157 
fibres of the sensory trigeminus, still ranning caudo-laterad, 
until they near the sides of the medulla. Here they turn 
again, and run caudad on their way to the spinal cord. ‘The 
tractus tecto-bulbaris et spinalis, after leaving the 
mid brain, runs through the floor of the cerebellum, where it 
is fused with the roof of the medulla. These tracts, after 
leaving the cerebellum, are also raised above the surrounding 
dorsal surface of the medulla, and, when the fore part of the 
brain is deflected, may be seen in relief as two longitudinal 
strands, one on either side of the median line (fig. 4, tr. t. b. et 
s.). With the cerebellum in place, both these tracts and the 
tractus cerebello-spinalis are hidden. 
The cerebellar ventricle leaves the fourth ventricle in the 
median line at its anterior dorsal angle, just before the 
beginning of the iter. The ventricle runs dorsad and dilates 
into an elliptical cavity that lies immediately ventrad of the 
glia layer of the cerebellum, and has its distal end projecting 
caudad (fig. 5). 
The Medulla.—The medulla is the most intricate and 
complex of all the divisions of the brain of Bdellostoma. 
Although conformiug in the main to the usual vertebrate 
type, it is, nevertheless, owing to causes as yet undiscovered, 
in some ways decidedly different, and is well worth caretul 
study. It is the largest single division of the Bdellostoma 
brain, and is, in fact, very nearly as large as the cerebellum, 
mid brain, ’tween brain, and fore brain put together (figs. 
1—8). It is bilaterally symmetrical, a slight depression 
marking its median line on both dorsal and ventral surfaces. 
It is about as broad as the two lobes of the mid brain, and 
about as long as mid brain and fore brain combined—for its 
actual measurements see the table on pages 142-3. Onits ven- 
tral surface it is shield shaped, the top of the shield resting 
against the ’tween brain, the point at the base cut off by the 
spinal cord. At the upper angles of the shield the 
trigeminus leaves the medulla, at the lower angles the 
vagus. The tacialis,acusticus, except acusticus a, and 
glosso-pharyngeus leave along the sides. Directly 
