VII 



THE MEDULLA OBLONGATA 



393 



single cerebral nerve, in which the vagus represents the dorsal or 



sensory root, and the accessory the ventral or motor root. Certainly 



the mode of origin of the vagus presents analogies with the origin 



of the dorsal roots of the spinal nerves. Its primary root possesses 



a ganglion (root or jugular 



ganglion) which is connected 



with the bulb by a series 



of rootlets, and recalls the 



ganglion of a dorsal spinal 



root. The vagus contains 



both sensory and motor 



fibres. Fig. 209 shows that 



it has two nuclei in the 



bulb, a larger sensory 



nucleus lying under the ala 



cinerea, and a smaller motor 



nucleus which it shares with 



the 9th cerebral nerve, and 



which is known as the 



nucleus ambiguus. 



On the other hand, we 

 learn from histology and 

 embryology that the spinal 

 accessory nerve arises with 

 ventral spinal roots. Its 

 internal or bulbar portion 

 (accessory properly so- 

 called) unites with the vagus 

 beyond the ganglion, as if 

 it were a motor root (Fig. 

 211). 



Whatever the morpho- 

 logical value of the theory 

 which assumes the vagus- 

 accessory to be a single 

 nerve, it is very convenient 

 from the physiological point 

 of view to consider the llth 

 and 10th cerebral nerves 

 together. 



The accessory of Willis 

 is an exclusively motor nerve, 

 which originates (Fig. 209) 

 from a column of cells placed dorso-laterally to the hypoglossal 

 nucleus, and extends into the spinal cord to the 5th cervical 

 segment, in which it forms part of the grey matter of the 

 lateral horn. From this nucleus the fibres emerge in a series 



Fio. 211. Diagram of roots and communicating branches 

 of vagus and neighbouring nerves. (Sappey, after 

 Hirschfeld and Leveille.) 1, facial nerve ; 2, glosso- 

 pharyrigeal with petrosal ganglion ; 2', connection of 

 digastric branch of facial with glossopharyngeal 

 nerve ; 3, vagus, with its two ganglia ; 4, accessory ; 

 5, hypoglossal ; 6, superior cervical ganglion of sym- 

 pathetic ; 7, 7, loop of union between first two 

 cervical nerves ; 8, carotid branch of sympathetic ; 

 it, tympanic nerve given oft' from petrosal ganglion; 10, 

 its carotid-tympanic filaments; 11, twig to Eustachian 

 tube ; 12, twig to fenestra vestibuli ; 13, branch to 

 fenestra cochleae; 14, small, 15, large, superficial 

 petrosal nerve ; 16, optic ganglion ; 17, auricular 

 branch of vagus ; 18, connection of accessory with 

 vagus ; 19, union of hypoglossal with 1st cervical 

 nerve ; 20, union between sterno-mastoid branch of 

 accessory and that of 2nd cervical nerve ; 21, pharyn- 

 geal plexus; 22, superior laryngeal nerve; 23, ex- 

 ternal laryngeal ; 24, middle cervical ganglion of 

 sympathetic. 



