CHAPTEE VIII 



THE SEGMENTS BELONGING TO THE TRIGEMINAL 



NERVE-GROUP 



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 nuclei. — 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, i.e. 

 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 



