THE NERVOUS SYSTEM 149 



appears to spring from the angle formed by these two nerves. It is 

 a R. muscularis to the M. depressor mandibulae. 



The main nerve continues ventro-laterally, passing between the 

 above-named muscle and the squamosal to the surface of the muscle. 

 Here it turns posteriorly, passing right across the muscle, between 

 it and the skin. In doing so it gives ofF {h) one or two RR. cutanei 

 to the overlying skin. 



At the posterior border of the M. depressor mandibulae the 

 nerve makes yet another half-turn, and pursues an antero-mesial 

 course, passing dorsal to the M. interhyoideus posterior, and ventral 

 to the M. interhyoideus. A few fibres enter the M. interhyoideus 

 posterior from its dorsal surface, while others turn back over its 

 anterior edge and enter the muscle from the ventral side (i.hy.p.). 

 The larger part of the nerve, however, spreads out fan-wise over the 

 M. interhyoideus and innervates it (i.hy.). A few of the more lateral 

 twigs pass right across the muscle to the space between it and the 

 M. intermandibularis, and then turn dorsalwards to supply the M. 

 subhyoideus, while a few fine fibres may pass right through the M. 

 interhyoideus and enter the M. subhyoideus. 



Driiner (1901) thinks that the glossopharyngeal nerve also contri- 

 butes motor-fibres to these muscles by means of the R. communicans, 

 but, as stated above, it is exceedingly unlikely that the Salamander 

 is so different from all other Urodeles, which have been investigated 

 from the point of view of nerve components, as to possess motor- 

 fibres in the R. communicans IX + X ad VII. 



Besides the motor branches to the interhyoid muscles described 

 above there also arise, from the same fan-like termination of the 

 nerve — the Pes anserinus (Driiner) — a number of sensory branches, 

 which are (<:) RR. cutanei intermandibulares (Drtiner), supplying the 

 skin underlying the muscles. 



Finally a few of the most anterior fibres of the R. jugularis may 

 very occasionally extend to the trigeminus region and enter the 

 posterior edge of the M. intermandibularis. 



VIII. N. Acusticus (n.8). As above described the auditory nerve is 

 indistinguishable from the facial nerve until it separates from the acu- 

 stico-facialis ganglion. It leaves this ganglion posteriorly, and enters 

 the auditory sac by three foramina at the base of its mesial wall. The 

 three foramina correspond to the three divisions of the auditory nerve. 



(i) The R. anterior (n.8. a.) supplies the recessus utriculi and the 

 ampullae of the anterior and horizontal canals. 



(ii) The R. medianus (n.8.m.) is a very fine nerve and supplies 

 only the sacculus. 



