286 



NORMAL HISTOLOGY. 



The central canal and surrounding gray matter are gradually dis- 

 placed dorsally within the closed lower half of the medulla in consequence 

 of the increasing space required by the pyramids, the fillet-tract and the 

 posterior longitudinal fasciculi tracts that lie close to the median raphe and 

 enlarge as they are followed upwards. Where the central canal opens out 

 into the fourth ventricle, the surrounding gray matter is correspondingly 

 spread apart and becomes the lining of the ventricular floor (Fig. 333). 

 Within this gray sheet and near the mid-line, on each side, lies the group of 

 large richly branched nerve-cells (40-70 /*) constituting the hypoglossal 

 nucleus, from which arise the fibres of the twelfth cranial nerve. These 

 strands take a direct ventro-lateral course through the medulla and emerge 

 on the anterior surface, in the groove between the pyramid and the olivary 

 eminence or olive. Slightly lateral, and to the outer side of the hypoglossal 



FIG. 333. Section across medulla at level B, Fig. 329; i, pyramidal tracts; 2, inferior olivary nucleus, 

 3, median fillet in ventral area ; 4, formatio reticularis alba ; 5, formatio reticularis grisea ; 6, 7,trigeminal 

 nucleus and root; 8, restiform body; 9, torn ventricular roof; 10, posterior longitudinal bundle; n, 

 nucleus of XII and (12) of X nerve; 13, fasciculus sol itarius; 14, dorsal area ; 15, fibres of X nerve; 16. 

 lateral area ; 17, fibres of XII nerve. X 5. (Preparation by Professor Spiller.) 



nucleus, another group of cells marks the position of the dorsal va go-accessory 

 nucleus. These neurones are of small size (30-40 ji) and irregularly stellate 

 or fusiform, and give origin to part of the motor fibres of the tenth and 

 bulbar portion of the eleventh cranial nerves. The root-fibres of the vagus 

 traverse the medulla obliquely and meet the surface in the dorso-lateral 

 groove marking the junction of the posterior and lateral divisions of the 

 medulla. 



In this way, the diverging fibres of the tenth and twelfth nerves sub- 

 divide each half of the medulla, as seen in transverse sections, into three 

 triangular areas the dorsal, the lateral, and the ventral. 



The dorsal area, between the dorsal surface of the medulla and the 

 vagus-fibres, contains a number of important fibre- tracts: (i) The restiform 

 body, or inferior cerebellar peduncle, appears as a large crescentic tract of 

 transversely cut fibres that occupies the greater part of the periphery. (2) 



