SCYLLIUM CANICULA 261 



mucous membrane and muscles of the hyoid arch, and the third, a 

 lateralis nerve, distributed as the external mandibular to the sensory 

 canals and ampullae in the hyoid and mandibular regions. 



The eighth nerve, the auditory, passes directly to the membranous 

 labyrinth of the ear, over which it spreads. The ninth, or gloss o- 

 pharyngeal, nerve leaves the cranium by an aperture in the post- 

 orbital groove, and has three main branches. The first is a small 

 dorsal factor to the skin ; the second is a palatine branch running 

 forward to join with the similar branch from the facial. The third 

 and largest branch soon divides into two, one passing in front of, and 

 the other behind, the first gill cleft to the structures surrounding 

 which they are related. 



The tenth or vagus nerve, like the seventh, contains also an 

 admixture of lateralis fibres. In the same way, too, the lateralis 

 nerve, a large branch, arises from the medulla by its own root, and, 

 although joining the main trunk for a short distance, soon leaves it 

 again to pass backwards fairly superficially in the myoseptum 

 between the epiaxial and hypaxial portions of the myomeres just 

 under the lateral line canal whose sense organs it supplies. The 

 second branch of the tenth is the visceralis nerve, consisting of true 

 vagus fibres, passes back to the heart and other viscera. The third 

 branch sends a factor to each of the four posterior gill clefts, over 

 which they split into a pre- and a post-branchial portion. This 

 third branch also is not found typically developed in air-breathing 

 vertebrates where, of course, the gills are absent, but is probably 

 represented in them by the pulmonary branch of the vagus. 



When we consider the functions of the cranial nerves we 

 see that they can be divided into three groups. The first is com- 

 posed of the oculomotorius, the patheticus and the abducens, all 

 of which are entirely motor in function, and as they go solely to 

 muscles, in this case eye muscles, they are termed myomeric nerves. 

 The second group, consisting of the olfactorius, the opticus, the 

 auditorius, the lateralis nerves and, perhaps, also the nervus 

 terminalis, is solely concerned with conveying impressions to the 

 brain, and hence its constituents are termed sensory nerves. The 

 remainder of the cranial nerves are both sensory and motor in 

 function, and so spoken of as Mixed nerves ; viz. the trigeminus 

 (of which, however, the ophthalmic branch is entirely sensory), 

 the glosso-pharyngeus and the branches of both the facialis and the 

 vagus that are not lateralis nerves. 



All the lateralis nerves, namely, the ophthalmic, the buccal and 

 the external mandibular of the facialis, and the lateral line nerve 

 of the vagus, in spite of the way in which they leave the cranium, all 

 originate in the lobus lineae laterals in the medulla. This large nerve 



