544 THE NERVOUS SYSTEM 



of the nucleus they turn sharply dorsalward along the median 

 raphe", pass between the interolivary fillets, and thus reach the 

 region of the hypoglossal nucleus, where they turn sharply out- 

 ward, to end by arborization around its cells. 



The peripheral neurones of the twelfth cranial nerve begin with 

 the large cells of the hypoglossal nucleus. Their neuraxes pass 

 directly ventralward in large bundles and make their exit from an 

 antero-lateral groove between the pyramid and the inferior olivary 

 body. 



The eleventh or spinal accessory nerve (Fig. 382), like the 

 twelfth, is a centrifugal or motor nerve trunk. Its fibres arise 

 from the nerve cells of the ventral horns of the six upper cervical 

 segments and, in the lower part of the medulla, from that detached 

 portion of the ventral horns which forms the so-called lateral nucleus. 



Its central neurones probably reach these nuclei after the same 

 manner as those of the spinal nerves. Its peripheral neurones take 

 origin from the large cells of these nuclei, leave the grey matter at 

 the dorso-lateral angle of the ventral horns, pass lateralward through 

 the white matter, and make their exit from the lateral surface of 

 the spinal cord and medulla in a vertical plane midway between the 

 ventral and dorsal nerve roots of the spinal cord. Outside of the 7 

 central nervous system they form small root bundles, which pass 

 cephalad and unite to form the spinal accessory nerve trunk. 



The tenth, pneumo gastric, or vagus nerve (Figs. 383 and 384) 

 contains both centrifugal and centripetal fibres. The neuraxes of 

 its centrifugal or motor fibres arise from the large scattered nerve 

 cells of the nucleus ambiguus, which are homologous with the more 

 caudad group forming the lateral nucleus, and also with the nerve 

 cells of the ventral horns of the spinal cord. 



The neuraxes from the scattered cells of the nucleus ambiguus 

 pass at first dorsalward until they reach the neighborhood of the 

 tractus solitarius, where they join the centripetal bundles, pass 

 lateralward and slightly ventralward, and make their exit from the 

 side of the medulla just ventral to the margin of the restiform 

 body. Outside of the medulla the several bundles unite to form 

 the median portion of the vagus. 



The central motor neurones of the vagus reach the medulla ob- 

 longata through the pyramidal tracts, and, having arrived at the 

 proper level, turn dorsalward along the median raphe in which 

 they promptly decussate and terminate by arborization about the 

 cells of the nucleus ambiguus. 



