822 



THE NERVOUS SYSTEM 



Cell-bodies in the nucleus of the ala cinerea, the nucleus of the solitary tract and in the 

 commissural nucleus of the ala cinerea comprise the so-called respiratory and vaso-motor 

 nuclei ("centres") of the medulla. Some of the caudal branches of the axones given off by 

 the cells of these nuclei descend the spinal cord, not only to the segments giving origin to 

 the phrenic nerve, but also to those supplying the intercostal and levatores costarum muscles. 

 Some of these augment the solitary tract; most of them descend in the reticular formation of 

 the medulla and cord. Further, axones given off by these cells convey vaso-motor impulses 

 which are distributed to visceral efferent neurones throughout the cord. 



Fig. 648. — Diagram illustrating Principal Central Relations of the Vagus Nerve, 



EXCLUSIVE OF RELATIONS TO DESCENDING CeREBRAL OR PYRAMIDAL FiBRES. 



Medial lemniscus — 



Medial longitudinal 

 fasciculus 



Dorsal efferent nucleus 

 of vagus 



Nucleus of hypoglossus '"" 



^, Nucleus of ala cinerea 



~ Nucleus anbiguus 



> -4 Ganglia of vagus 



Solitary tract and nucleus 

 of solitary tract 



The nuclei of origin of the motor fibres of the vagus and glosso-pharyngeus are the dorsal 

 efferent nucleus of the vagus and the nucleus ambiguus. The cells of the dorsal nucleus 

 of the vagus lie somewhat clustered in the ventro-mesial side of the nucleus of the ala cinerea 

 and lateral to the nucleus of the hypoglossus. Their axones pass outward among the entering or 

 afferent vagus fibres, and it is suggested that most of them are visceral efferent fibres of the 

 vagus, i. e., they terminate about sympathetic neurones. The nucleus ambiguus or ventral 

 efferent nucleus of both nerves lies in the lateral half of tlie reticular formation, about mid-way 

 between the olive and the line traversed by the rootlets of the two nerves. Its upper end la 

 larger. Its cells are considerably dispersed by the fibres of the reticular formation. Ihe 

 axones arising from its cells course at first dorsalward and then turn abruptly outward to jom 



:k. 



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