SEGMENTS OF TRIGEMINAL NERVE-GROUP 259 
were segmental, with special reference to branchial arches and clefts, 
the facial, glossopharyngeal, and separate vagus branches supplying 
the walls of the various branchial pouches. In a similar manner, 
the supra- and infra-maxillary branches of the trigeminal were 
arranged on each side of the mouth, and the inner and outer twigs of 
the first (ophthalmic) branch of the trigeminal on each side of the 
orbito-nasal cleft, the trabecular and the supra-maxillary arches being 
those on each side of this cleft. Thus Huxley considered that there 
was evidence of a series of pairs of ventral arches belonging to the 
skull, viz. the trabecular and maxillary in front of the mouth, the 
mandibular, hyoid, and branchial arches behind, and that the Vth, 
VIlIth, IXth, and Xth nerves were segmental in relation to these 
arches and clefts. Gegenbaur, in 1871 and 1872, considered that the 
branchial arches represented the lower arches of cranial vertebre, 
and therefore corresponded to lower arches in the spinal region, 
ue. the skull was composed of as many vertebrae as there are 
branchial arches. These vertebree were confined to the notochordal 
part of the skull, the prechordal part having arisen secondarily from 
the vertebral part, while the number of vertebre are at least nine, 
possibly more. The nerves which could be homologized with spinal 
nerves were, he thought, divisible into two great groups—(1) the 
trigeminal group, which included the eye-muscle nerves, the facial, 
and its dorsal branch, the auditory; (2) the vagus group, which 
included the glossopharyngeal and vagus. 
Such was the outcome of the purely comparative anatomical 
work of Huxley and Gegenbaur—work that has profoundly influenced 
all the views of segmentation up to the present day. 
Now came the investigations of the embryologists, of whom I 
will take, in the first instance, Balfour, whose observations on the 
embryology of the Selachians led him to the conclusion that besides 
the evidence of segmentation to be found in the cranial nerves and 
in the branchial clefts, further evidence was afforded by the existence 
of head-cavities, the walls of which formed muscles just as they do 
in the spinal region. He came to the conclusion that the first head- 
cavity belonged to one or more pre-oral segments, of which the nerves 
were the oculomotor, trochlearis, and possibly abducens; while there 
were seven post-oral segments, each with its head-cavity and its 
visceral arch, of which the trigeminal, facial, glossopharyngeal, and 
the four parts of the vagus were the respective nerves. 
