THE OBLONGATA AND THE NUCLEI -OF THE CHANIAL NERVES. 87 



also motor fibers. Eecall that Fig. 34 shows fibers from the cells of the 

 ajuterior horn to the sensory roots. When that figure was under discussion 

 your attention was called to the fact that we probably have to deal here 

 with motor elements to the visceral tracts. To Gaskell is due the credit of 

 having shown how the motor tracts for the Vagus differ in no way from 

 those already described for the spinal nerves. (Compare Fig. 34 with 

 Pig. 46.) 



Just as in the chicken the multipolar cell near the anterior horn sends 

 its neuraxon out in the posterior root, so the similarly located motor nucleus 

 of the vagus sends its neuraxons to the sensory vagus root, which is the 

 homologiie of a posterior root. The motor vagus fibers are essentially 

 equivalent to the motor fibers in the posterior root. They arise from a 

 nucleus which, in fishes, lies dorsal to the nucleus of the Hypoglossus, but 



Pig. 46.- — Motor and sensory vagus nucleus of Alligator mississippiensis. 



in amphibians, reptiles, and birds lies ventrolateral to this. To be sure, 

 the origin of the vagus fibers is frequently not sharply difEerentiated from 

 that of the hypoglossus fibers. The nucleus is the motor nucleus of the 

 Vagus. It sends root-fibers to the same and to the opposite sides. The 

 decussation takes place very near the fioor of the fourth ventricle. 



That a part of the vagus root as a descending fascicle ends in the gray 

 matter somewhat posterior to its entrance has been mentioned. 



In the region of the Vagus a great number of root-fibers enter the 

 medulla. It is not with certainty decided whether we are dealing here with 

 a single nerve or with a fused combination. Only the most anterior of the 

 sensory roots of the Vagus, always somewhat separated from the others at 

 the surface of the medulla, has it been thought necessary to separate out as 

 a distinct nerve: the Glosso-pharyngeus. This nerve, in all vertebrates, arises 

 only in a small part direct from the gray matter of the spinal cord. A 



