166 Basic Structure of Vertebrates 



line organs; the trigeminal fibers go to other receptors in the skin but 

 never to neuromasts. In vertebrates which do not possess lateral-line 

 organs, the facial nerve totally lacks anything corresponding to its 

 lateral-line division in fishes and amphibians. 



The facial nerve in all vertebrates has a large geniculate ganglion 

 (Fig. 147 B), whence an important sensory palatine branch goes to 

 the roof of the mouth and a sensory internal mandibular (chorda 

 tympani of mammals) to the lower jaw and floor of the mouth and to 

 taste-receptors on the anterior region of the tongue. The main motor 

 component of the facial is a hyomandibular trunk which distributes 

 to numerous striated voluntary muscles (the branchiomeric mus- 

 cles) in the region of the lower jaw, hyoid, and pharynx. 



The seventh nerve acquires its name from the fact that, in mam- 

 mals — especially man — it comes into prominence as being the chief 

 nerve supplying the greatly expanded superficial musculature of the 

 whole facial region, including some muscles of the external ear. Thess 

 facial muscles apparently had their evolutionary origin in the branchio- 

 meric muscles of fishes (Figs. 95, 96). 



VIII. Acoustic (or auditory), but inaccurately so called because 

 the ear serves not only for hearing but also for equilibration, and it is 

 possible that the ears of some fishes do not mediate a sense of hearing. 

 The nerve is purely sensory. It is virtually two nerves, each having a 

 ganglion. The vestibular ganglion and vestibular trunk are con- 

 cerned with equilibration, the spiral ganglion and cochlear nerve 

 w ith hearing. The fibers of both nerves pass to primary centers in the 

 medulla. The eighth nerve is the only cranial nerve that does not 

 emerge from the cranium, the ear itself being within the cranium. 



IX. Glossopharyngeal, a mixed nerve. Its visceral sensory fibers 

 come from the petrosal ganglion (Fig. 146, pt), the motor fibers 

 from nuclei in the medulla. In some fishes the nerve includes somatic 

 sensory fibers passing to neuromasts of a small dorsal division 

 (supratemporal : Fig. 150. st) of the lateral-line system. 



In gill-breathers the main nerve passes to the roof of the first 

 branchial chamber and there divides into two trunks (Fig. 147C), a 

 purely sensory pretrematic distributing to the anterior wall of the 

 chamber and a mixed post-trematic to the posterior wall. The pre- 

 trematic is, accordingly, associated with the hyoid arch and the 

 post-trematic with the first branchial arch. The sensory fibers, 

 mostly visceral, pass to interoceptors of the pharynx and hind part of 

 the oral cavity. The relatively small somatic division goes to the skin 

 in the region of the first gill-cleft. The post-trematic trunk supplies the 

 striated branchiomeric muscles of the first branchial arch. 



