AMPHIBIA. 213 



angle of the mouth, supplying the regions of the tympanic mem- 

 brane and jaw-articulation. It then divides into (a) The man- 

 dibular nerve, running along the inner side of the mandible, giving 

 branches to its superficial muscles ; and (6) The hyoid nerve, 

 which passes along the anterior hyoid cornu and innervates its 

 muscles. VIII. The auditory nerves arise immediately posterior 

 to the facials, and enter the auditory capsules to supply the 

 essential organs of hearing. IX. The glossopharyngeal nerve on 

 each side takes origin a little way behind the root of VIII., and 

 in close connection with that of X. After leaving the skull by 

 the vagus-foramen, it is connected by a commissure with the 

 facial, and then runs down to the floor of the mouth, along 

 which it runs with a tortuous course, supplying the mucous 

 membrane of the pharynx and tongue. X. The vagus (pneumo- 

 gastric) nerve of each side arises in close connection with IX., 

 passes through the vagus-foramen and dilates outside the skull 

 into the vagus-ganglion. It takes a downward and backward 

 course, and gives off branches to the larynx, heart, lungs, and 

 stomach. 



The ten pairs of spinal nerves have this in common, that each 

 arises from the spinal cord by two roots a dorsal, upon which 

 is a ganglion, and a ventral. These unite together to form the 

 oerve-trunk in the corresponding intervertebral foramen, which 

 serves as a point of exit. The roots of the anterior nerves pass 

 directly outwards, but the posterior ones slope more or less 

 backwards within the spinal canal before reaching their foramina. 

 The last few roots thus form with the filum terminale ? a brush- 

 like cauda equina. The ganglia on the roots are covered ventrally 

 by calcareous sacs. Each nerve sends off, near its commencement, 

 a small dorsal ramus to part of the muscles and skin of the back, 

 while the remaining part, the ventral ramus, is connected by a 

 delicate commissure, ramus communicans, with a sympathetic 

 ganglion. 



The first or hypoglossal nerves run forwards in the throat- 

 muscles and innervate the muscles of the tongue. The second 

 and third unite to form a brachial nerve, which supplies the fore- 

 limb. Nerves 4, 5, and 6 supply the body-wall in their region, 

 while nerves 7, 8, and 9 unite to form a network, the sciatic 

 plexus, which gives off branches to the posterior viscera, and 

 a very large sciatic nerve to the hind-limb. The small tenth or 



