CRANIAL NERVES. 173 



ularis passes behind it and hence is a post-trematic ramus. In some 

 cases a small twig bends down from the palatine and represents the 

 pretrematic branch. 



Three of these ophthalmicus superficialis. buccalis and mandib- 

 ularis externus belong to the lateralis system, which is unrepre- 

 sented in the spinal nerves. This has its own ganglion, which may unite 

 with geniculate or semilunar, and it supplies the lateral line system of 

 the head (see sense organs). The superficial ophthalmic innervates the 

 supraorbital line of these organs, and in the elasmobranchs, breaks 

 up distally to go to the modified organs (ampullae of Savi and Loren- 

 zini) at the tip of the snout. In the same way the buccalis supplies 

 the infraorbital line and the mandibularis externus those of the lower 

 jaw and the hyoid and spiracular regions. 



As there are no myotomic muscles in the region supplied by the 

 facial nerve, there are no somatic motor components The general 

 cutaneous elements run in the hyoideus to the skin of the hyoid region, 

 but in other vertebrates this territory is supplied by branches of the fifth, 

 which spread to the operculum and to the dorsal surface of the head. 

 The visceral motor components occur in the hyoid nerve and, in the 

 aquatic forms, they supply the muscles of the hyoid region and the 

 posterior belly of the depressor mandibulae. In the mammals, with 

 the development of the muscles of expression (p. 134), the same 

 branch (known in human anatomy as the main branch of the facial) has 

 a much greater extension, the result of the migration of the muscles 

 from the hyoid region to their definitive position. 



The geniculate ganglion belongs to the visceral sensory system, 

 the fibres of which run in the hyoid and palatine nerves to reach the 

 sense organs in the oral cavity and in the spiracular gill when this is 

 present. In those fishes where there are taste organs on the outer 

 surface of the body there is a 'nerve of Weber' (ramus lateralis ac- 

 cessorius) which goes to the dorsal surface of the head and then to back, 

 fins and tail, wherever the gustatory organs occur. It is frequently 

 accompanied by fibres of the tenth nerve. In the mammals the visce- 

 ral sensory fibres occur in the main trunk of the seventh, in the great 

 superficial petrosal and in the chorda tympani nerves. This last is a 

 post-trematic nerve which passes through the middle ear and thence on 

 the medial side of the lower jaw to join the lingualis. 



In the adult anura and in the amniotes, where gills and lateral line 

 organs are lacking, the facialis undergoes a corresponding reduction, 



