THE MEDULLA OBLONGATA. 



1073 



which the lower and middle are continuous and the upper is unconnected, and four 

 small isolated masses of gray matter along the dorsal border of the nucleus. The 

 inferior or spinal end of the nucleus is thickened and bent outward, so that its plane 

 is oblique and parallel with the ventral surface of the chief olive. Higher, when the 

 latter is well established, the mesial accessory nucleus is represented by a narrow 

 broken tract, that corresponds more closely with the sagittal plane. In this situa- 

 tion the nucleus lies between the fillet and the inner end of the chief olive and across 



Dorsal nucleus of vagus 



FIG. 927. 



Ventricular roof 



Fuuiculus cunea- 



tus, overlaid by 



restiform body 



Fascicu]us 

 solitarius 



Substantia gela- 



tinosa overlaid 



by root of V 



Nucleus ambiguus- 

 Nucleus lateralis 



Nucleus cuneatus 



Post, longitudinal 

 fasciculus 



Root-fibres of XII 



Inferior olivary 

 'nucleus 



Tract of mesial fillet 



Pyramidal tract 



Anterior superficial arcuate fibres 



Transverse section of medulla at level E, Fig. gig ; central canal has opened into fourth ventricle ; restiform 

 body appearing. X 5- Preparation by Professor Spiller. 



its hilum. The dorsal accessory olivary nucleus (nucleus olivaris accessorius dorsalis) 

 is less extensive than the median, measuring about 9 mm. in length, and lies close to 

 and behind the posterior lip of the hilum of the inferior olive. 



The Central Gray Matter. As pointed out, within the closed part of the 

 medulla the central canal and the surrounding gray matter are gradually displaced 

 dorsally in consequence of the increasing space required by the pyramid, the fillet 

 tract and the posterior longitudinal fasciculus, three paired tracts of longitudinally 

 coursing fibres that lie close to the median raphe and enlarge as they are followed 

 upward. When the central canal opens out into the fourth ventricle, the sur- 

 rounding gray matter is correspondingly spread out and forms the lining of the 

 ventricular floor. Within this gray sheet and near the mid-line, on each side, is seen 

 the group of cells constituting the hypoglossal nucleus from which the fibres of the 

 twelfth cranial nerve arise. These strands take a direct ventro-lateral course through 

 the medulla and emerge on the surface in the groove between the pyramid and 

 olivary eminence. Slightly more lateral, and to the outer side of the hypoglossal 

 nucleus, another group of cells marks the position of the elongated vago-ghsso- 

 pharyngeal nucleus, partly sensory and partly motor, belonging to the tenth and 

 ninth cranial nerves. The fibres of the vagus traverse the medulla laterally and 

 meet the surface at the junction of the lateral and posterior areas. In this way 

 the diverging fibres of the tenth and twelfth nerves subdivide each half of the medulla 

 into three triangular areas a mesial, a lateral and a posterior (Flechsig). 



Viewed in transverse sections through the upper third of the medulla, the poste- 

 rior area the space between the vagus fibres and the dorsal surface of the medulla 

 is seen to contain a number of important fibre-tracts. ( i ) The restiform body appears 



