88 ANATOMY OF THE RABBIT 



The cranial nerves, those arising from the brain and making 

 exit through the walls of the skull, are comparable in some respects 

 to the spinal nerves, but in many ways are different in nature in 

 addition to being in some cases highly specialized. Three pairs, 

 respectively, olfactory, optic, and acoustic, or first, second, and 

 eighth of the series are afferent nerves from the special sense organs 

 of smell, sight, and hearing, the function of the acoustic nerve 

 including also transmission of afferent impulses of equilibrium. 

 The optic nerve differs from all others both structurally and de- 

 velopmentally, being really an outlying part of the brain itself. 

 The third, fourth, and sixth nerves, respectively, oculomotor, 

 trochlear, and abducent, are distributed as somatic motor nerves 

 to the muscles of the eyeball, but also contain fibres of muscle sense. 



Of the remaining cranial nerves the fifth, seventh, ninth, and 

 tenth are branchiomeric (p. 41). Although the connections of these 

 nerves are not fully considered In the dissection as here outlined, 

 their chief characteristics as branchiomeric structures may be 

 indicated. The fifth, or trigeminal nerve Is the nerve of the man- 

 dibular arch and its branches are related to this arch with its 

 associated structures and to the mouth In a manner comparable 

 with the relations of a typical branchial nerve to Its gill arch and 

 the gill cleft in front of it when these structures are present. It 

 arises in two parts, one of which, the portio major, is sensory, the 

 other, the portio minor, motor. The portio major splits Into three 

 main branches, the ophthalmic (Fig. 45), maxillary, and mandibu- 

 lar nerves, and the portio minor unites with the last of these. 

 Thus, while the terminal branches of all three divisions are distri- 

 buted as somatic sensory nerves to the skin of the head, the man- 

 dibular nerve carries in addition visceral motor fibres for certain 

 muscles (masticatory group, mylohyoid, and digastric) regarded 

 as belonging to this, the first visceral arch. Visceral sensory fibres 

 are carried from the anterior part of the mouth by the lingual 

 branch of the mandibular nerve and by the palatine branches of 

 the spheno-palatine ganglion, but both of these, despite their 

 close peripheral association with the trigeminal, really belong to 

 the seventh nerve, the connection of the former being through the 

 chorda tympani, that of the latter through the great superficial 

 petrosal. 



