CHAPTER VIII 
THE SEGMENTS BELONGING TO THE TRIGEMINAL 
NERVE-GROUP 
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The prosomatic segments of the vertebrate-——Number of segments belonging 
to the trigeminal nerve-group.—History of cranial segments.—Eye-muscles 
and their nerves.— Comparison with the dorso-ventral somatic muscles of the 
scorpion —Explanation of the oculomotor nerve and its group of muscles. 
—Explanation of the trochlearis nerve and its dorsal crossing.—Explana- 
tion of the abducens nerve——Number of segments supplied by the 
trigeminal nerves.—Evidence of their motor nucleii—Evidence of their 
sensory ganglia.—Summary. 
From the evidence given in the last chapter, combined with that 
given in Chapter IV., the probability of the theory that the trigeminal 
group of nerves of the vertebrate have been derived from the 
prosomatic group of nerves of the invertebrate can be put to the 
test by the answers to the following morphological and anatomical 
questions :— 
1. Do we find in the vertebrate two segmentations in this region 
corresponding to the two segmentations in the branchial region, we. 
a somatic or dorsal series of segments, and a splanchnic or ventral 
series of segments? The latter would not be branchial, but rather 
of the nature of free tactile appendages; so that it is useless to look 
for or talk about gill-slits, although such appendages, being serially 
homologous with the branchial mesosomatic appendages, would 
readily give rise to the conception of branchial segments. 
2. Is there morphological evidence that the trigeminal nerve is 
not the nerve belonging to a single segment, or even to two segments, 
but is really a concentration of at least six, probably seven, segmental 
nerves ? 
3. Is there morphological evidence that the oculomotor and 
trochlear nerves, which on all sides are regarded as belonging to 
the trigeminal segments, are not single nerves corresponding each 
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