186 ZOOLOGY. 



co-ordinating centre of muscular action in keeping its 

 equilibrium. 



16. The Cranial Nerves are exactly like those of the 

 frog when allowance is made for the differences in the other 

 systems of organs. /. and //. are exactly the same in origin 

 and in distribution ; so also are ///., 7F., and VI. : but it may 

 be mentioned that ///. and IV. pass through the cranial wall 

 by foramina of their own (see fig. 93). 



V. has its usual three branches. V.\ emerges by an aper- 

 ture of its own, runs along the upper side of the orbit, and 

 is joined by a parallel branch of VII. The joint nerve thus 

 formed is called the ophthalmic, and is distributed to the 

 sensory tubes (see 3) of the snout. The other two 

 branches are mixed nerves, and pass to upper and lower 

 jaws respectively. 



The branch of VII. mentioned above is not apparently 

 found in higher types, but the other branches are familiar 

 to us a vidian or palatine to the roof of the mouth ; and 

 a hyomandibular branch which forks over the spiracle, 

 which is thus proved homologous with the frog's and rabbit's 

 tympanum. These are all apparently mixed nerves, and 

 supply the skin and muscles of the regions they run to. 



VIII. is the auditory nerve, as usual. 



IX. passes through the otic capsule, ventral to the internal 

 ear, and emerges apparently into the anterior cardinal sinus 

 (see 10). It soon forks over the first gill-slit, and its 

 branches (mixed) are distributed to the same regions of 

 pharynx and tongue as in higher types. 



X. arises from a number of roots as though it represented 

 several nerves run together. It runs through the otic 

 capsule and along the floor of the anterior cardinal sinus. 

 Its branches are (a) a lateral nerve (sensory) to the sense- 

 organs of the lateral line ; (b) four branchial nerves, each 

 forking over a gill-slit, as IX. does over the first ; (c) a visceral 

 nerve to heart and stomach. Of these, the first is unrepre- 

 sented in frog and rabbit, as no lateral -line-organs exist in 

 them ; the branchial branches may be represented by the 

 laryngeal nerves which supply somewhat the same region of 

 the throat ; while the visceral is of course familiar to us. 



