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, 

 Vllth, 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 vertebrae, 

 and therefore corresponded to lower arches in the spinal region, 

 i.e. the skull was composed of as many vertebrae as there are 

 branchial arches. These vertebrae were confined to the notochordal 

 part of the skull, the prechordal part having arisen secondarily from 

 the vertebral part, while the number of vertebrae 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. 



