176 



THE NERVOUS SYSTEM 



as the genu. The second part of the root of the facial nerve is directed ventro- 

 laterally and at the same time somewhat caudally, passing close to the lateral 

 side of its own nucleus, to make its exit from the lateral part of the caudal 

 border of the pons (Fig. 108). 



Fibers from many sources terminate in the facial nucleus in synaptic rela- 

 tion with its constituent cells. Those from the corticobulbar tract place the 

 facial muscles under voluntary control. Others are collaterals from the sec- 

 ondary sensory paths in the reticular formation and are concerned with bulbar 

 reflexes. Some of these collaterals are given off by fibers arising in the trapezoid 

 body and carry auditory impulses. Others are collaterals of fibers arising in 

 the nucleus of the spinal tract of the fifth nerve; and still others are given off by 

 ascending sensory fibers from the spinal cord (Cajal, 1909). 



Bulbar rootlets of accessory nerve ^-^ 

 Foramen magnum-\. 



Spinal root of accessory nerve 



-. Vagus nerve 



-'Jugular foramen 



Internal ramus \ * 

 77 , ; } Accessory nerve 



External ramus / * 



1 



Fig. 125. Diagram of the roots of the vagus and accessory nerves. 



The nucleus ambiguus is a long slender column of nerve-cells, extending 

 through the greater part of the length of the medulla oblongata in the ventro- 

 lateral part of the reticular formation (Figs. 103, 121, 122). Its constituent 

 cells give rise to the special visceral efferent fibers that run through the glosso- 

 pharyngeal, vagus, and accessory nerves to supply the musculature of the 

 pharynx and larynx. It reaches from the border of the pons to the motor de- 

 cussation, but is most evident in transverse sections through the caudal part of 

 the rhomboid fossa. Here it can be found in the reticular formation ventral to 

 the nucleus of the spinal root of the trigeminal nerve. The fibers arising from 

 its cells are at first directed dorsally; then curving laterally and ventrally they 

 join the root bundles of the ninth, tenth, and eleventh nerves with which they 



