Houser, Zhe Neurones of a Selachian. 85 
and they are homologous with the somatic sensory spinal fibres. 
The cell-bodies of these neurones lie in sensory ganglia, and 
the central termination is comparable to that of the somatic 
sensory spinal fibres. Part of the general cutaneous fibres ter- 
minate in the general cutaneous nucleus, the homologue in the 
head of the dorsal cornu; see Subsection 6; but others turn 
into the spinal V tract and take a course posteriorly for ultimate 
termination in the dorsal cornu of the cord. There can be no 
question as to the identity of this system with the somatic sen- 
sory system in its simpler condition. 
c. The Viscero-Motor System.—Fibres of this system form 
the motor roots of the V, VII, IX, and X nerves. The cell- 
bodies from which the axones arise form a column of cells lat- 
eral to the fourth ventricle, known, according to the level, as 
the nucleus ambiguus, and the motor nuclei of the VII and V, 
respectively. This column of cells is the cranial continuation 
of the paracentral nucleus of the cord, and the homology is 
rendered complete by the distribution of the fibres to the vis- 
ceral musculature. 
a. The Communits System.—This term is used in the sense 
defined by HERRICK (’99, p. 208), and as the equivalent of the 
fasciculus communis of STRONG (’95) as applied to the system 
in the tadpole, and of Kincspury (’97) as used for various 
fishes. Communis fibres are components of the VII, IX, and 
X nerves. They are wholly sensory. They innervate visceral 
and mucous surfaces, and also taste-buds and those specialized 
sense-organs of the skin (end-buds) not referable to the lateral 
line system. The fibres are characterized by their small size, 
and they are distinguishable from other components of the 
same nerves by this feature. They terminate in the lobus vagi. 
The greater number of them in Mustelus pass directly to their 
central termination without entering the fasciculus communis, 
using this term in the sense as originally applied by OsBorn 
(88) to a definite longitudinal tract. 
The communis system is homologous with the viscero-sen- 
sory system, the lobus vagi in which its central termination occurs 
being the continuation into the brain of the lateral or interme- 
