THE NERVOUS SYSTEM 179 



IV. Trochlear nerve, also motor, arising from the dorsal surface 

 of the hinder margin of the mesencephalon and passing to the 

 superior oblique muscle of the eye. 



V. Trigeminal nerve, a large nerve arising from the antero- 

 lateral angle of the medulla and dividing in the higher verte- 

 brates into three main trunks : the ophthalmic, the maxillary, and 

 the mandibular. The first two are sensory in function and supply 

 the skin in the region of the eye, the roof of the mouth, the teeth 

 of the upper jaw, etc., while the mandibular branch, composed 

 of both sensory and motor fibers is distributed to the jaw muscles, 

 the tongue, teeth of the lower jaw, and adjacent parts. Near the 

 point where it leaves the brain the trigeminal nerve has a semi- 

 lunar (Gasserian) ganglion. 



VI. Abducens nerve, a motor nerve arising from the ventral 

 surface of the medulla and innervating the external rectus muscle 

 of the eye. 



VII. Facial nerve, a mixed motor and sensory nerve, arising 

 from the medulla posterior to the fifth nerve. In the lower 

 vertebrates, it has two ganglia, the geniculate and the lateralis, 

 but the latter in lung-breathing forms may disappear or unite 

 with the geniculate or with the semilunar ganglion of nerve V. 



VIII. Auditory nerve, sensory in function, consists in mammals 

 of a vestibular branch from the utriculus and a cochlear branch 

 from the cochlea, each from a separate ganglion, which unite to 

 form a single trunk to the medulla. 



IX. Glossopharyngeal nerve, a mixed motor and sensory nerve, 

 extends from the medulla to the pharynx and tongue principally. 

 It has a large petrosal ganglion near its junction with the brain. 



X. Vagus nerve, also a mixed motor and sensory nerve, extend- 

 ing from the medulla to the esophagus, stomach, heart and other 

 viscera. The jugular ganglion and, in gill breathers, a lateralis 

 ganglion, occur near its origin. In higher forms the intestinal 

 branch of the vagus has a nodose ganglion. 



XI. Spinal accessory nerve, a motor nerve from the medulla, 

 innervates the muscles of the pectoral region. 



XII. Hypoglossal nerve, also a motor nerve from the medulla, 

 is distributed in the muscles of the neck and tongue. 



Long after the 12 cranial nerves were described and named, 

 another pair was discovered at the anterior end of the brain. 

 This nerve which should really be first in the series is known as 



