492 THE NERVOUS SYSTEM 



or inferior peduncle of the cerebellum. At the mid-level of the 

 medulla the gelatinous substance of Rolando is so increased in 

 amount as to form a slight protuberance, tubercle of Rolando, 

 which is seen on the ventral border of the restiform body. 



The Nucleus of the Twelfth Cranial Nerve. The four lower cra- 

 nial nerves are given off from this portion of the rhombencepha- 

 lon. The twelfth, a motor nerve, arises from a grey nucleus on 

 either side of the raph6 just ventral to the grey commissure. Its 

 nucleus extends from a point just above the motor decussation 

 upward to the level of the caudal margin of the pons Varolii. 

 Fibres pass ventralward from this nucleus and emerge from the 

 medulla oblongata through a groove between the pyramid and the 

 olivary body. 



The eleventh cranial nerve (motor) arises from the intermedio- 

 lateral cell group in the ventral horns of the six upper cervical 

 segments and from the lateral nucleus of the medulla. Fibres pass 

 from these nuclei through the lateral columns, from which they 

 make their exit, and passing upward unite with each other to form 

 the trunk of the spinal accessory. 



The tenth cranial nerve arises in two main divisions, the motor 

 and the sensory. The motor portion takes its origin from the 

 nucleus ambiguus which, toward the spinal cord, is continuous 

 with the lateral nucleus and the ventral horns. Higher in the 

 medulla the nucleus ambiguus also gives origin to the peripheral 

 neurones of the ninth nerve. The motor fibres of the tenth 'and 

 ninth nerves make their exit from the lateral surface of the medulla 

 oblongata, ventral to the restiform body. 



The sensory roots of these nerves enter the medulla in com- 

 pany with the outgoing motor fibres; they pass to a triangular 

 area of grey matter in the floor of the fourth ventricle. This is 

 the chief nucleus of the vagus. It is continuous above with the 

 similar nucleus of the ninth nerve. 



Descending branches from the fibres of the ninth and tenth 

 nerves form a small but prominent tract which lies in the margin 

 of the grey matter, and ventro-lateral from the chief nuclei of these 

 nerves. This is the tractus solitarius or spinal root of the ninth 

 and tenth nerves. It is continued downward as far as the cervical 

 region, and its path is surrounded by a thin shell of grey matter 

 (nucleus of the tractus solitarius) in which its fibres terminate. 



The ninth or glossopharyngeal nerve is very similar to the tenth 

 in its origin and its exit from the medulla. The root bundles 



