160 JULIA WORTHINGTON. 
the spinal cord is reached the whole ventricle contracts, 
preserving, however, its peculiar form, which is continued in 
the central canal of the spinal cord (fig. 12), the only differ- 
ence being that in the cord the dorsal part is much smaller 
compared to the ventral than in the medulla. 
Regions of the Medulla.—In analysing the medulla of 
Bdellostoma into its component regions it is easily seen that 
the great ascending tract of the sensory trigeminus is the 
dominant structure. ‘This tract, judging from Johnston, 
1902, is relatively much larger in Bdellostoma than it is in 
Petromyzon. Itis the largest and most important tract of 
the hind brain, just as the olfactory tracts are the largest 
and most important tracts of the fore part of the brain, and 
it dominates all the anatomical arrangements in the medulla, 
displacing other tracts. It springs from the nucleus funi- 
culi, a diffuse group of cells lying near the dorsal surface of 
the hind end of the medulla. Whether this nucleus is_in 
Bdellostoma a development of the dorsal horn of the spinal 
cord, as stated by Johnston for Petromyzon, I am not pre- 
pared to say, but its position makes it likely. The nucleus 
extends cephalad for quite a distance, and the fibre tract, 
constantly growing larger and more pronounced, takes its 
way in the dorso-lateral part of the medulla to emerge at its 
cephalo-lateral angle as the greater part of the trigeminus 
nerve (figs. 1, 10, 11). 
The fasciculus communis, the continuation in part of 
the dorsal columns of the spinal cord, hes in the dorsal part 
of the medulla, next to the median line (fig. 1). A little 
cephalad of the commissura infima Halleri, a crescent- 
shaped line of cells, the nucleus of the fasciculus com- 
munis, appears on the ventral surface of the fibre tract, and 
continues as a semicylindrical layer as far forward as the exit 
of the facialisfrom the brain. A little behind this the fibre 
tract begins to spread farther over the surface of the medulla, 
gradually shifting its position to one more removed from the 
median line (fig. 11). While shifting its position it also 
sends fibres laterad over the sensory trigeminus to the 
