Houser, Zhe Neurones of a Selachian. 99 
cutaneous nucleus may pass into their arborizations in relation 
to each of several groups of neurones. What is probably the 
chief mode of termination is shown in Fig. 5, g. c.f, where 
the fibre is seen entering the tangle of the substantia gelatinosa. 
At no other point in Mustelus is there such a bewildering maze 
of nervous processes as that presented by the neurones here. 
So far as structural features may be interpreted, it would seem 
that the substantia gelatinosa is well adapted for the reception 
of the most delicate sensory impressions. The large number 
of arborizations found here indicates the real importance of 
the group. 
Terminations are also to be traced in connection with the 
deeper neurones of the nucleus. In Fig. 6 there is shown an 
incoming nerve-fibre, ¢. c. f., breaking up into its arborization 
near one of these neurones. The arborization is of the broadly 
digitate variety, spreading the disturbance over some slight 
area. The scattered distribution of the neurones at this point 
is doubtless a correlative of this fact. 
Finally, the molecular layer which caps the whole nucleus 
contains many fibres of quite minute size. This level, with its 
small branching neurones, and the dendrites from deeper zones, 
may serve to distribute impressions superficially. 
Turning to the spinal V tract, this is a series of bundles 
of fibres which run posteriorly to the spinal cord. The bun- 
dles are scattered through the deeper part of the general cuta- 
neous nucleus; refer to Fig. 48, sf. V. General cutaneous 
fibres may enter the nucleus directly from the nerve of the same 
side, or through the medium of the arcuate bundles from the 
opposite side. They may terminate at once in the nucleus, as 
described above; or they may turn into the spinal V tract for ter- 
mination farther posteriorly. Some of those present in the tract 
are doubtless branches of fibres which have terminated in part 
in the nucleus. The spinal V tract, then, is a means for carry- 
ing great numbers of sensory nerve-fibres from cranial nerves 
to the dorsal cornu of the spinal cord, giving them a second 
and far wider hold. 
The manifold central terminations of the general cutaneous 
