746 



THE NERVOUS SYSTEM. 



either crossed or uncrossed, the former being the axons of the cells of the right 

 accessory nucleus or of the cells of the right tuberculum acusticum, while the latter 

 are derived from the same cells of the left side. In the upper part of the fillet 

 there is a collection of nerve-cells, the nucleus of the fillet, around the cells of 

 which some of the fibres arborize, and from the cells of which axons originate to 

 continue upward the tract of the lateral fillet. The ultimate ending of the left lat- 

 eral fillet is partly in the quadrigeminal bodies of the same or opposite side, while 

 the remainder of the fibres ascend in the posterior limb of the internal capsule to 

 reach the first and perhaps the second left temporal convolution. 



The auditory nerve contains a few afferent fibres which arise in the quadrigemi- 

 nal bodies, the nucleus of the lateral fillet, trapezoid nucleus, and superior olive. 



The auditory nerve after leaving the medulla passes forward across the poste- 

 rior border of the middle peduncle of the cerebellum, in company with the facial 

 nerve, from which it is partially separated by a small artery (auditory). It then 

 enters the internal auditory meatus with the facial nerve. At the bottom of the 

 meatus it receives one or two filaments from the facial nerve, and then divides into 

 its two branches, cochlear and vestibular. The auditory nerve is soft in texture 

 (hence the name, portio mollis), and is destitute of neurilemma. The distribution 

 of the auditory nerve in the internal ear will be found described along with the 

 anatomy of that organ in a subsequent page. 



Surgical Anatomy. The auditory nerve is frequently injured, together with the facial 

 nerve, in fractures of the middle fossa of the base of the skull implicating the internal auditory 

 meatus. The nerve may be either torn across, producing permanent deafness, or it may be 

 bruised or pressed upon by extravasated blood or inflammatory exudation, when the deafness 

 will in all probability be temporary. The nerve may also be injured by violent blows on the 

 head without fracture, and deafness may arise from loud explosions from dynamite, etc., prob- 

 ably from some lesion of this nerve, which is more liable to be injured than the other cranial 

 nerves on account of its structure. The test that the nerve is destroyed and that the deafness is 

 not due to some lesion of the auditory apparatus is obtained by placing a vibrating tuning-fork 

 on the head. The vibrations will be heard in cases where the auditory apparatus is at fault, but 

 not in cases of destruction of the auditory nerve. 



The Ninth Pair (Figs. 102, 403, 404). 



The Ninth or Glosso-pharyngeal Nerve is distributed, as its name implies, to 



the tongue and pharynx, being the nerve of ordinary sensation to the mucous 



membrane of the pharynx, fauces, and tonsil ; and the nerve of taste to all parts 



of the tongue to which it is distributed. 



Its superficial origin is by three or four filaments, closely connected together, 



from the upper part of the medulla oblongata, in the groove between the olivary 



and restiform body. 



Its deep origin may be traced through the fasciculi of the lateral tract, to three 



different sources: (1) some of the fibres may be traced to a nucleus of gray matter 



at the lower part of the floor of the fourth ventricle 

 beneath the inferior fovea ; (2) others may be traced 

 downward into the funiculus solitarius, a rounded 

 bundle of fibres in the lower part of the medulla, com- 

 mencing immediately above the decussatiori of the pyr- 

 amids (these fibres have not been distinctly traced to 

 cells) ; (3) a third set of fibres take origin from the cells 

 of the nucleus ambiguus. This nucleus is situated some 

 distance from the floor of the fourth ventricle and lies 

 slightly internal to the inferior fovea. It gives origin 

 to the motor branches of the glosso-pharyngeal and 

 vagus, and to the bulbar part of the spinal accessory. 

 The real origin of the sensory fibres of the glosso- 

 pharyngeal must be looked for in the jugular and pe- 



trosal ganglia which are developed from the neural crest. 



From its superficial origin it passes outward across the flocculus, and leaves the 



Jugular Gangl. 



Petrous Gangl. 

 Tympanic br. 



fneumo- 



Fic. 402. Origin, ganglia, and 

 communications of the ninth, 

 tenth, and eleventh cranial 

 nerves. 



