556 THE NERVOUS SYSTEM 



gustatory or lingual branch of the fifth. Streeter has just published a 

 study of the development of the seventh and eighth nerves in which he 

 traces the chorda tympani through the pars intermedia, as shown in figure 

 388, thus settling this oft-disputed question. 



The Eighth Nerve, or Auditory. The eighth nerve consists of two 

 divisions, anatomically distinct and functionally independent. These are the 

 vestibular and the cochlear divisions of the auditory nerve. 



The cochlear division arises in the spiral ganglion and passes to the medulla 

 to establish immediate connections with the ventral cochlear nucleus and the 

 tuberculum acusticum. The central relations of these nuclei are established 

 by the striae acusticae, the trapezoideus, and the lateral fillet with the internal 

 corpus geniculatum and the inferior corpus quadrigeminum of the opposite 

 side, as told by figure 389. These latter nuclei send tracts to the auditory 

 center in the superior temporal gyrus. 



The vestibular division arises in the vestibular ganglion, which is entirely dis- 

 tinct from the cochlear ganglion, and enters the medulla, passing to the lateral or 

 chief auditory nucleus. From this point the relations are not fully established, 

 but apparently fibers pass to the nucleus fastigii of the opposite side and to the 

 vermis, where they are brought into relations with motor descending paths. 



Functions. The cochlear branch is the auditory nerve proper, and the 

 vestibular is the nerve of equilibrium. 



The Ninth Nerve, or Glosso-pharyngeal. Origin. The glosso-phar- 

 yngeal nerves, figure 364, IX, arise by nuclei intimately associated with 

 those of the vagus and spinal accessory nerves. The union of the nuclei is 

 indeed so intimate that it will be as well to consider the origins of the ninth, 

 tenth, and eleventh nerves together. 



These three nerves emerge from the bulb and spinal cord in their numerical 

 order from above downward, the bulbar portion from the lateral aspect of 

 the bulb in a line between the olivary and restiform bodies; and the spinal 

 portion from a line intermediate between the anterior and posterior nerve roots 

 as far down as the sixth or seventh cervical spinal nerves. 



The combined glosso-pharyngeal-accessory-vagus nucleus appears to con- 

 sist of two parts, viz., one median or common origin, having conspicuous 

 nerve cells of moderate size, and three lateral origins, having but few cells of 

 small size. These are: i, the nucleus ambiguus, which lies on the lateral side 

 of the reticular formation and is the motor origin of the glosso-pharyngeal, 

 the vagus, and the spinal accessory; 2, the fasciculus solitarius, situated in 

 the bulb, ventral and a little lateral to the combined nucleus, is also called the 

 ascending root of the glosso-pharyngeal nerve or the respiratory bundle; and 

 3, the spinal portion, which takes origin from a group of cells lying in the ex- 

 treme lateral margin of the anterior cornu. This is the origin of the spinal 

 accessory; it corresponds to the antero-lateral nucleus of the bulb, and the 

 lateral part of the gray matter of the spinal cord. 



