BRAIN AND CRANIAL NERVES OF BDELLOSTOMA DOMBEYI. 161 
sides of the medulla, and, shortly after the level of the 
posterior end of the cerebellum is reached, the fasciculus 
communis is entirely displaced from the dorsal to the 
lateral surface. As soon as its fibres reach the lateral 
surface they turn caudad, running backwards to the facialis, 
glosso-pharyngeus, and vagus respectively. 
The acusticus nucleus makes its appearance as a small 
clump of cells a little caudad of the level of the facialis, 
and hes not far from the median line between the fasci- 
culus communis and the sensory trigeminus. It 
increases in diameter as it proceeds cephalad, and stains 
more deeply in hematoxylin than the surrounding tissue. 
As the fasciculus communis passes over to the side the 
acusticus comes to lie on the extreme dorso-mesial surface 
of the medulla (fig.10). The fibres of the two sets of nerves 
arising from it, those of the lateral line and those of the ear, 
leave it in entirely separate groups. ‘lhe most cephalic set 
of fibres is that of Acusticusa. These leave the dorsal 
surface of the acusticus in a well-defined group ‘57 mm. from 
its anterior end, and pass laterad to the cranium above the 
dorsal surface of the medulla just cephalad of the acusticus 
ganglion. ‘lhe fibres of the ear nerves are given off from 
just behind those of Acusticus @ to within about ‘6 mm. 
from the caudal end of the nucleus. Most of them pass 
laterad over the sensory trigeminus, leaving the medulla 
along its dorso-lateral edge, but some are deflected by the 
sensory trigeminus, pass ventrad of it, aud leave the 
medulla in separate fibre groups along its mid-lateral surface, 
entering the ganglion uear its ventral border. ‘I'he fibres of 
Acusticus 6 arise in the ventral portion of the acusticus 
about three fifths of its length behind its cephalic end (fig. 
11), and pass latero-dorsad to emerge upon the dorsal 
surface of the medulla. In the specimen from which the 
photograph of fig. 11 was taken ganglion cells lie among the 
fibres immediately outside the brain. If fig. 11 be examined 
with a magnifying glass a few can be seen in the nerve of 
the left side. 
