598 THE NERVOUS SYSTEM. 



that it is carried for some distance downwards into the superior part of the spinal 

 medulla, and, according to Kolliker, to the level of the fourth cervical nerve. Most 

 modern writers, however, limit it to the medulla oblongata. The relations of the 

 tractus solitarius are not the same in all parts of its course. It lies immediately 

 to the lateral side of the dorsal vago-glossopharyngeal nucleus; but, whereas in 

 the superior part of the medulla oblongata it is situated somewhat on the ventral 

 side of that nucleus, in the inferior, closed part of the medulla oblongata it is 

 placed on its dorsal aspect. Throughout its entire length it is intimately associated 

 with a column of gelatinous gray substance called the nucleus tractus solitarii, 

 which constitutes the nucleus of termination in which its fibres end. When 

 traced from above downwards, the tractus solitarius is observed to become gradually 

 smaller owing to the loss of fibres which it thus sustains. The great bulk of the 

 tractus solitarius is formed of fibres derived from the glossopharyngeal nerve ^ only 

 a few of the afferent fibres of the vagus enter it, but fibres of the sensory root 

 (nervus intermedius) of the facial also enter it. As the fibres of the three nerves 

 join the fasciculus they immediately turn downwards, and at different levels come 

 to an end in the associated nucleus tractus solitarii. 



As the afferent root-bundles of the vagus and the glossopharyngeal nerves traverse 

 the substance of the medulla oblongata in a backward and medial direction to reach 

 the tractus solitarius and the dorsal nucleus of termination, they pass through the 

 tractus spinalis of the trigeminal nerve and the nucleus of that tract. As the 

 afferent root of the vagus passes through the trigeminal tractus spinalis and its 

 nucleus, which is somatic sensory in nature, it gives off to this nucleus its own somatic 

 sensory branches, the peripheral ends of which constitute the auricular branch, dis- 

 tributed to the skin on the back of the auricle. The other afferent fibres in the 

 glossopharyngeal and vagus nerves include taste fibres, sensory fibres from the pharynx, 

 larynx, and other parts of the respiratory and alimentary systems, and other splanchnic 

 afferent fibres. Although there is no sharp demarcation between the terminal nuclei of 

 these various components, it is probable that the taste fibres proceed to the nucleus 

 traetus solitarii, the splanchnic afferent fibres to the dorsal nucleus, and the somatic 

 afferent fibres to the nucleus of the spinal trigeminal tract. 



Nervus Acusticus.^As this is a nerve of special sense it will be left for con- 

 sideration after the rest of this series. 



Nervus Facialis (Figs. 530 and 531). The facial nerve is composed of two 

 distinct parts, viz., a large efferent (mainly motor) portion, the facial nerve proper, 

 and a small afferent sensory portion termed the nervus intermedius. 



The facial nerve proper emerges from the brain at the inferior border of the pons, 

 to the medial side of the acoustic nerve, whilst the nervus intermedius sinks into 

 the superior part of the medulla oblongata between the facial and acoustic nerves, 

 but alongside the latter, rather than the former, from which it is separated 

 by the fasciculus obliquus pontis (Fig. 527). The three nerves, therefore, lie in 

 intimate relation with each other, where they are attached to the surface of the 

 brain, and they pass in company into the internal acoustic meatus. 



The fibres of the nervus intermedius arise from the cells of the ganglion geniculi 

 of the facial nerve. These, like the cells of a spinal ganglion, are unipolar, the 

 single process in each case dividing into a peripheral and a central branch. The 

 group of peripheral fibres represent parts of the greater superficial petrosal nerve 

 and chorda tympani branch of the facial nerve, whilst the central fibres form the 

 nervus intermedius. The central fibres penetrate the brain, and, parsing either 

 through or on the dorsal side of the tractus spinalis of the trigeminal nerve, they 

 finally reach the superior part of the column of gray matter in connexion with the 

 tractus solitarius, and in this they end. The nervus intermedius presents, therefore, 

 the same terminal connexions within the brain as the glossopharyngeal nerve. 



The motor nucleus of the facial nerve contains elements serially homologous 

 with both the somatic (nucleus ambiguus) and splanchnic (nucleus dorsalis) 

 efferent nuclei of the glossopharyngeal and vagus. It is composed partly of the 

 larger cells characteristic of the former and the smaller cells distinctive of 

 the latter. The axons of the somatic cells innervate the striated muscles of 

 the face, whereas the splanchnic efferent fibres pass to the spheno-palatine, otic 



