THE MEDULLA OBLONGATA AND TEGMENTUM OF THE PONS. 201 



structure resembles that of the olivary bodies, and which, like 

 them, are traversed by the arciform fibres. As is shown in the 

 figure, the internal accessory olivary body is traversed principally 

 by the fibres passing from one olivary body to another, while 

 the posterior body of that name gives passage mainly to the 

 fibres of the posterior columns. The internal accessory olivary 

 nucleus is also called the nucleus pyramidalis. 



Dorsad of the olivary body, in the vicinity of the posterior 

 accessory olivary nucleus, is a medullary tract, which we shall 

 see from now on occupying a position in the midst of the teg- 

 mentum, and which can be traced to a certainty above the 

 origin of the trigeminus. Taken together, these fibres the 

 central tegmental tract probably connect the olivary body with 

 the mid-brain. 



The nerve-nuclei are situated along the dorsal periphery of 

 the section. 



Next to the median line is the nucleus of the hypoglossal 

 nerve, whose fibres, after passing through the vicinity of the 

 olivary body, run ven trad. (See Fig. 116.) It receives numerous 

 fibres from the raplie. External to this comes the common 

 sensory nucleus of the accessory, the vagus, and the glosso- 

 pharyngeal nerve. As a rule, the fibres of the accessory do not 

 issue from the nucleus at this level. The majority of them arise 

 lower down from the accessory nucleus proper, which is situated 

 in the upward prolongation of the lateral horns. A remnant of 

 this nucleus, the anterior or motor nucleus of the vagus and 

 glosso-pharyngeal, lies just in front of the posterior horn. Its 

 fibres, before emerging, bend at an angle to join those from the 

 sensory nucleus. 



The thin bundle of transversely-divided fibres lying exter- 

 nal to the last-named nucleus is the common ascending root of 

 the vagus and glosso-pharyngeal. External to the common 

 nucleus of these nerves lie the nuclei of the posterior columns, 

 covered over by a few fibres. Ventrad of them you can see the 

 substantia gelatinosa of the head of the posterior horn of the 



