SEGMENTS OF TRIGEMINAL NERVE-GROUP 281 
The anatomy of the Vth nerve reveals, then, three most striking 
facts :— 
1. The motor nucleus of the Vth extends from the very commence- 
ment of the infra-infundibular region to nearly the commencement 
of the nucleus of the VIIth; in other words, the motor nucleus of the 
Vth extends through the whole prosomatic region, just as it must 
have done originally if its motor nerves supplied the muscles of 
the prosomatic appendages. Such an extended range of origin is 
indicative of the remains of an equally extended series of segmental 
centres or ganglia, 
2. Of these centres the caudalmost have alone remained large and 
vigorous, constituting the nucleus masticatorius, which in the fish is 
divided into an anterior and posterior group, thus indicating a 
double rather than a single nucleus; while the foremost ones have 
dwindled away until they are represented only by the cells of the 
descending root, the muscles of these segments being still represented 
by possibly the tensor velit palati and the other muscles innervated 
from these cells. 
3. The headmost of these cells takes up actually a position dorso- 
lateral to the central canal, so that the groups on each side nearly 
come together in the mid-dorsal line; a very unique and extra- 
ordinary position for a motor cell-group, but not improbable when we 
recall to mind Brauer’s assertion as to the shifting of the foremost 
prosomatic ganglion-cells of the scorpion from the ventral to the 
dorsal side of the alimentary canal. 
On the sensory side the evidence is also suggestive, the question 
here being not so much the distribution of the sensory nerves as the 
number of ganglia belonging to each of the cranial nerves. 
With respect to this question, morphologists have come to the 
conclusion that there is a marked difference between spinal and 
cranial nerves, in that whereas the posterior root-ganglia of the 
spinal nerves arise from the central nervous system itself, 7.c. from 
the neural crest, the ganglia of the cranial nerves arise partly from 
the neural crest, partly from the proliferation of cells on the surface 
of the animal; and because of the situation of these proliferating 
epidermal patches over the gill-clefts in the case of the vagus and 
glossopharyngeal nerves, they have been called by Froriep and Beard 
branchial sense-organs. Beard divides the cranial ganglia into two 
sets, one connected with the neural ridges, called the neural ganglia, 
