48 Anatomy of the Nervous System 



pass in a dorso-medial direction towards the position whence 

 the cells have migrated. Close to the floor of the ventricle, 

 they turn forwards and group themselves into a compact 

 bundle, the genu or ascending portion of the motor VII root. 

 After ascending a short distance to a point just in front of 

 the nucleus VI, the root turns sharply at about a right angle 

 and runs ventro-laterally to emerge at the ventral edge of 

 the spinal V root (Pis. X., XL). In the human brain, the 

 facial nucleus is relatively rather smaller than in the rat, and 

 is not quite so far ventral in position, while its rostral end lies 

 dorso-lateral to the superior olive instead of practically 

 directly behind it, as in the rat. This is probably due partly 

 to the more dorsal position in man of the reduced spinal V 

 nucleus, from which the motor VII nucleus receives impulses, 

 and partly to the enormous development of the pons. The 

 latter factor has pushed back the emerging motor VII root 

 in man until it lies beside the front part of its nucleus, ventral 

 to which the posterior portion of the pons itself extends. 



The special motor cells of the IX and X nerves form a 

 continuous column of very much smaller size than the motor 

 facial nucleus. This is the nucleus amhiguus, which is found in 

 a position similar to that of the motor VII nucleus, but not 

 so near the surface, being medial to the ventral edge of the 

 spinal V nucleus (PI. VIII.). Its anterior extremity is about 

 on a level with that of the dorsal motor X nucleus, but 

 posteriorly it gradually thins out until it is represented by 

 a few isolated cells so irregularly scattered that its posterior 

 end cannot be exactly determined. 



The root fibres from this nucleus run dorso-medially and 

 join those from the dorsal nucleus, passing out with them in 

 small groups in a lateral direction. 



The special motor fibres of the XI nerve arise from the 

 nucleus accessorius, which may be regarded also as part of 

 the visceral motor column which has changed its position, 

 migrating in a posterolateral direction and being joined by 



