Houser, Zhe Neurones of a Selachian. 159 
The viscero-sensory system has had annexed to it a com- 
plex of peripheral sense-organs. The communis system which 
results from this union is represented by components of the 
VII, IX, and X nerves. The lobus vagi is the centre of the 
system. Fewer fibres enter the fasciculus communis than in 
either the teleosts or the amphibians. 
The viscero-motor nucleus gives origin to the motor fibres 
of the V, VII, IX, and X nerves. The axones enter their 
nerves chiefly through the medium of the fasciculus longitudi- 
nalis dorsalis. The nucleus receives impressions radiated from 
the lobus vagi. 
The general cutaneous nucleus is the homologue of the 
dorsal cornu of the spinal cord. General cutaneous fibres, com- 
ponents of the V, IX, and X nerves, terminate in both the sub- 
stantia gelatinosa and the deeper part of the nucleus. Many 
fibres of the system enter the spinal V tract for ultimate distri- 
bution in the spinal cord. 
The tuberculum acusticum is phylogenetically young. It 
may have been derived from the structures of the dorsal cornu. 
Fibres of the acustico-lateral system terminate in the tubercu- 
lum acusticum. These are components of the VII, VIII, and 
X nerves. Neurones are present in the tuberculum acusticum 
of the molecular, granular, and PURKINJE types, equivalent 
morphologically to those of the cerebellum. 
The Cerebellum is relatively large in Mustelus. This fact is 
to be interpreted from the strong development of the sense of 
equilibrium in the animal. The structural plan of the cerebel- 
lum is the same as that characteristic of higher vertebrates, the 
differences being due to greater simplicity of detail. 
The evidence is for the origin of the cerebellum, in the 
phylogeny of the vertebrates, as a fused outgrowth of the pair 
of tubercula acustica. The organ represents a specialization of 
that part of the oblongata forming the original terminal station 
for the acustico-lateral system. 
The Midbrain appears to have its organization arranged 
contributory to the roof-nucleus. This is a group of giant neu- 
rones, the axones of which enter into and very largely compose 
