82 THE NERVOUS SYSTEM OF THE FROG 



with it, and trace it back over the columella and down to the angle 



of the mouth. 



a. The ramus mandibularis runs forwards in the 

 floor of the mouth, lying along the inner edge 

 of the lower jaw and between the mylohyoid 

 muscle and the skin, as far forward as the 

 chin. 



Dissect from the ventral surface : remove the skin from the 

 under surface of the floor of the mouth, and find the nerve running 

 along the inner border of the mandible. 



/3. The ramus hyoideus is the posterior and larger 

 of the two branches : it runs forwards in the 

 floor of the mouth along the anterior cornu 

 of the hyoid, supplying its muscles. 



8. The auditory nerve, the nerve of hearing, arises from the 



side of the medulla immediately behind and in close 

 contact with the root of the facial nerve : it enters the 

 auditory capsule and ends in the internal ear. 



9. The glossopharyngeal nerve arises from the side of the 



medulla behind the auditory nerve, by a root common 

 to it and the tenth nerve : it leaves the skull by an 

 aperture immediately behind the auditory capsule, and 

 divides behind the capsule into two branches. 



i. The ramus anterior runs downwards and forwards 

 round the hinder border of the auditory capsule 

 and beneath the depressor mandibulse muscle to 

 join the facial nerve just after it has crossed over 

 the columella. 



The dissection already made for the ramus hyomandibularis of 

 the facial nerve will show also the above branch of the glosso- 

 pharyngeal. 



ii. The ramus posterior runs downwards and for- 

 wards to the ventral wall of the pharynx, passing 

 beneath the fourth division of the petrohyoid 

 muscle but superficial to the others ; it runs 

 just behind and parallel to the anterior cornu of 

 the hyoid. On reaching the floor of the mouth 

 it crosses the hypoglossal nerve obliquely, lying 

 dorsal to it and then runs forwards in a 



