308 VERTEBRATE ZOOLOGY 



The large trigeminus nerve, the fifth cranial, springs from the 

 posterolateral border of the pons by two roots: a large lateral 

 root, which is sensory in function, and a small medial one, which 

 is motor. The large root soon branches, giving rise to three 

 nerves, the ophthalmic, maxillary, and mandibular; the small 

 medial root joins the last-named nerve. The ophthalmic nerve 

 passes into the orbit, where it gives off the frontal nerve, with 

 which it passes out of the orbit and innervates the upper eyelid 

 and the skin of the nose and the snout. The maxillary and mandib- 

 ular nerves emerge from the cranial cavity back of the orbit, 

 the former being distributed to the lower eyelid, the upper lip and 

 upper jaw, the palate, and the face, and the latter to the external 

 ear, the muscles of the jaw, the lower jaw, the lips, and the tongue. 

 The mandibular nerve is both sensory and motor in function, in 

 that it receives the motor root of the trigeminus. 



Just back of the pons and near the midventral line of the brain 

 is the small sixth pair of cranial nerves, the abducens, which inner- 

 vate the external rectus muscles of the eyeball ; they will be seen 

 when the ventral surface of the brain is studied. Immediately 

 posterior to the root of the trigeminus nerve are the facial nerve, or 

 seventh cranial nerve, which innervates the muscles of the face, 

 and the auditory nerve, or eighth cranial nerve, which is the larger 

 of the two and passes directly to the inner ear ; the auditory is 

 the only cranial nerve which does not leave the cranial cavity. 



The glossopharyngeal, vagus, and spinal accessory nerves, 

 which are the ninth, tenth, and eleventh cranial nerves, spring 

 near together by several small roots from the lateral border of the 

 medulla oblongata. The glossopharyngeal innervates the muscles 

 of the pharynx and the taste buds of the tongue ; it is the sensory 

 nerve of the tongue. The vagus is closely joined with the sym- 

 pathetic nerve and passes posteriorly, first giving off branches to 

 the pharynx and the larynx, alongside the carotid artery and 

 trachea to the anterior end of the lung, where it breaks into 

 branches which innervate the lungs, the heart, the stomach, and 

 other organs. The spinal accessory is formed by the union of 

 about ten roots which spring from the medulla and the spinal 

 cord, and innervates the muscles of the neck. 



