NERVE COMPONENTS 23 



In the vagus nerve the general cutaneous fibres compose the 

 rami cutanei dorsales passing upwards behind the skull ; the large 

 acustico-lateralis component forms the ramus lateralis vagi, and a 

 small ramus supratemporalis. The branchial nerves are chiefly 

 formed by the splanchnic or visceral sensory component, and some 

 splanchnic motor fibres, which pass into the post-trematic branches 

 to innervate the branchial muscles ; an intestinal and several 

 pharyngeal mixed branches, with splanchnic sensory and motor 

 components, pass inwards to the alimentary canal. The pharyngeal 

 taste-buds are also supplied by the branchial nerves. 



The glossopharyngeal nerve has a complete set of components 

 distributed in much the same way. The lateralis branch, however, 

 is small or absent. 



The facialis nerve, which is very intimately connected with 

 the trigeminus, has a dorsal ramus oticus, a supraorbital ramus 

 opthalmicus superior, and an infraorbital ramus buccalis composed 

 of general cutaneous and acustico-lateralis elements distributed in 

 the skin. A large mixed ventral hyomandibular trunk gives off 

 cutaneous acustico-lateralis and splanchnic motor fibres passing 

 behind the spiracular cleft to the hyoid region and lower jaw ; and 

 an internal branch, ramus palatirius, carries most of the splanchnic 

 sensory component to the roof of the mouth. Jacobson's anasto- 

 mosis of splanchnic sensory fibres generally unites the facial 

 (geniculate) ganglion with the jugular ganglion of the glosso- 

 pharyngeal. The auditory nerve represents a specialised portion 

 of the acustico-lateralis component. 



The trigeminus nerve divides into a supraorbital branch, the 

 ramus opthalmicus superior, and an infraorbital ramus maxillaris, 

 both composed of general cutaneous fibres ; and a mixed ramus 

 mandibularis with a splanchnic motor component as well. The 

 opthalmicus profundus nerve belongs, as a rule, to the general 

 cutaneous system. 



The three nerves to the eye -muscles represent the somatic 

 motor components corresponding to the three last nerves. 



It is unnecessary here to enter into a detailed description of 

 the paired organs of sense. In front are found the olfactory sacs : 

 i imaginations of the ectoderm which retain their opening to the 

 exterior, the primitive nostrils. Next come the lateral eyes, organs 

 of very complex structure, derived partly from an outgrowth of the 

 fore-brain (p. 16), partly from an ingrowth of the outer ectoderm 

 (lens). Lastly, the auditory organ (concerned with equilibration 

 ;is well as hearing) is developed from a more posterior imagination 

 of the ectoderm (Fig. 1 3) forming a deep sac, which remains in com- 

 munication with the exterior by a narrow ductus endolymphaticus 

 in the adult in the case of some Elasmobranch fish only. By a 



