552 THE CRANIAL NERVES. 



from the interior of this organ. The main anatomical peculiarity, 

 therefore, which distinguishes the central origin of the auditory from 

 that of the other cranial nerves, is its abundant and direct connection 

 with the substance of the cerebellum 



The auditory nerve, formed by the union of these two bundles of root 

 fibres, emerges from the lateral surface of the medulla oblongata, at the 

 inferior edge of the pons Yarolii, and immediately outside the facial 

 nerve. In company with the facial it then passes forward and outward, 

 enters the internal auditory meatus, penetrates through the perforations 

 at the bottom of this canal, and terminates in the nervous expansions 

 of the internal ear. 



Physiological Properties of the Auditory Nerve. The auditory 

 nerve is evidently a nerve of special sense, and serves to communicate 

 to the brain the impression of sonorous vibrations. In the experiments 

 of Magendie upon dogs and rabbits, the auditory nerve, when exposed 

 in the cranial cavity, was found to be insensible to the severest me- 

 chanical irritation, although the roots of the fifth pair exhibited at the 

 same time an acute sensibility. Its exclusive distribution to the inter- 

 nal ear, for which it forms the only nervous connection with the bitf in, 

 leaves no doubt that its function is that of transmitting to the central 

 organ the nervous influences which produce the sensation of sound. 



Behind the situation of the auditory there commences a special divi- 

 sion of the cranial nerves, which differ in great measure from the pre- 

 ceding. All the foregoing nerves, excepting those of special sense, are 

 either distinctly motor or have a highly developed general sensibility ; 

 they are distributed to the integument and to muscles which are con- 

 cerned in the execution of voluntary movements ; and they are all asso- 

 ciated in the production of nervous action in the various regions of the 

 face. 



The second division of the cranial nerves, on the other hand, com- 

 prising the glossopharyngeal, the pneumogastric, and the spinal acces- 

 sory, are distributed to the deeper parts about the commencement of 

 the digestive and respiratory passages, where the general sensibility is 

 comparatively deficient, and the movements are, for the most part, 

 involuntary ; and they exhibit phenomena which have more especially 

 the character of reflex actions. Externally, they show a marked simi- 

 larity of anatomical arrangement, originating one behind the other, in 

 a continuous line, along the lateral furrow of the medulla oblongata 

 and the side of the spinal cord, each by a series of separate filaments ; 

 and in such juxtaposition that it is in some instances difficult to say, 

 from external inspection, where the root fibres of one terminate, and 

 those of the other begin. The two sensitive nerves belonging to this 

 group, namely, the glossopharyngeal and the pneumogastric, have their 

 source in two nuclei which are continuous with each other at the pos- 

 terior surface of the medulla oblongata ; and, according to the observa- 



