CH. XLVI.] THE ELEVENTH AND TWELFTH NERVES 



679 



glosso-pharyngeal). This approaches the middle line as it descends, 

 and finally joins that of the opposite side over the central canal 

 (commissural nucleus). 



The eleventh nerve (spinal accessory) is wholly efferent : it arises 

 by two distinct origins one from a centre in the floor of the fourth 

 ventricle, and connected with the glosso-pharyngeal-vagus-nucleus ; 

 the other, from the outer side of the anterior cornu of the spinal cord 

 as low down as the fourth cervical nerve. The fibres from the two 

 origins come together at the jugular foramen, but separate again into 

 two branches, outer and inner. The outer, consisting of large 

 medullated fibres from the spinal origin, supplies the trapezius and 



cf.n.X X/. 



Fio. 418. The tenth and twelfth nerves, pyr, Pyramid ; n.XIL, nucleus of hypoglossal ; XII., fibre of 

 hypoglossal ; d.n.X.XL, combined nucleus of vagus and spinal accessory; n.amb., nucleus 

 ambiguus ; /.s., fasciculus solitarius, descending fibres of vagus and glosso-pharyngeal ; f.s.n., its 

 nucleus; X., motor fibre of vagus ; g, ganglion cell in vagus trunk giving rise to a sensory fibre; 

 d. V., descending root of the fifth nerve ; r, restiform body. (Schiifer.) 



sterno-mastoid muscles. The inner branch, consisting of small 

 medullated fibres from the medulla, supplies chiefly viscero-motor 

 and cardio-inhibitory filaments to the vagus. The muscles of the 

 larynx, all of which are supplied by branches of the vagus, derive 

 their motor nerves from the accessory; Vrolik states that in the 

 chimpanzee the internal branch of the accessory does not join the 

 vagus at all, but goes direct to the larynx. The crico-thyroid, how- 

 ever, receives fibres which leave the bulb by glosso-pharyngeal 

 rootlets. 



The twelfth nerve (hypoglossal) is also entirely efferent. It sup- 

 plies the muscles of the tongue. It arises from a large celled and long 

 nucleus in the bulb, close to the middle line (see figs. 408 and 418). 



