478 THE NERVOUS SYSTEM. 



cerebellum to the lateral surface of the medulla at the lower border 

 of the pons Varolii. Some of them follow a deeper course, passing 

 obliquely through the substance of the medulla to the same point. 

 They form the superior or external root of the auditory nerve. 



The internal root consists of fibres which, according to Huguenin, 

 may be traced backward from their point of emergence into the inferior 

 peduncle of the cerebellum, where they meet with a second nucleus of 

 gray substance, and continue their course, in company with the longi- 

 tudinal fibres of the peduncle, toward the white substance of the cere- 

 bellum. This connection with the cerebellum is the main anatomical 

 peculiarity by which the auditory is distinguished from the other cranial 

 nerves. 



The auditory nerve, formed by the union of these two roots, after 

 emerging from the lateral surface of the medulla oblongata, passes for- 

 ward and outward, through the internal auditory meatus, and termi- 

 nates in the nervous expansions of the intern*! ear. 



Physiological Properties of the Auditory Nerve. The auditory 

 is a nerve of special sense, serving to communicate the impression of 

 sonorous vibrations. In the experiments of Magendie on dogs and 

 rabbits, the auditory nerve, when exposed in the cranial cavity, was 

 found insensible to mechanical irritation, although the roots of the fifth 

 pair exhibited at the same time an acute sensibility. Its exclusive 

 distribution to the internal ear, for which it forms the only nervous 

 connection with the brain, leaves no doubt that its function is that of 

 transmitting to the central organ the nervous influences which produce 

 the sensation of sound. 



The remaining cranial nerves, comprising the glossopharyngeal, the 

 pneumogastric, and the spinal accessory, are distributed to the deeper 

 parts about the commencement of the digestive and respiratory pas- 

 sages, where general sensibility is but slightly developed, and the 

 movements are, for the most part, involuntary. Externally, they show 

 a marked similarity 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 diffi- 

 cult to say 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 each a 

 distinct ganglion, situated within their point of emergence from the 

 cranium, and originate from two continuous nuclei at the posterior 

 surface of the medulla oblongata. The motor nerve of the group, or 

 the spinal accessory, originates from a nucleus of its own, and sends 

 branches of communication to the other two. While these nerves, 

 therefore, can hardly be regarded as a single pair, they have never- 

 theless a close mutual relation both in anatomical arrangement and in 

 physiological properties. 



