CHAP. IL] THE BRAIN. 955 



and the pyramid and median accessory olive on the median side, gain 

 the surface along the groove which separates the pyramid from 

 the olivary body. 



617. The Ninth or Glossopharyngeal, Tenth or Vagus, and 

 Eleventh or Spinal accessory Nerves. It will be advantageous 

 to consider these three nerves together. 



In the spinal accessory nerve we must distinguish, as we have 

 said, two parts : the " spinal accessory " proper, formed by the 

 roots which come off from the cervical spinal cord, reaching as far 

 down as the sixth or seventh cervical nerve, and the " bulbar 

 accessory," whose roots come off from the bulb just below the 

 vagus. 



The spinal accessory proper takes origin in the group of cells 

 lying in the extreme lateral margin of the anterior horn, from 

 whence the fibres proceed directly outwards through the lateral 

 column, and issue from the cord along a line intermediate between 

 the anterior and posterior roots ; the upper roots undergo, with the 

 portion of the lateral horn from which they spring, the shifting 

 spoken of in 605. 



The bulbar accessory starts from an elongated nucleus in the 

 bulb which is common to it, to the vagus, and to the glosso-pharyn- 

 geal ; hence we have taken these three nerves together. This 

 (Fig. 115) stretches farther forward than the hypoglossal nucleus, 

 reaching the level of the transverse fibres called striae acusticae 

 (str.) t but does not extend so far behind. 



In transverse sections of the bulb, which pass a little below 

 and a little above the point of the calamus scriptorius (Fig. 109, 

 4, 5), two nuclei or collections of cells are seen in the grey 

 matter round the central canal. The more ventral one is the 

 hypoglossal nucleus, the more dorsal one the beginning or hind 

 part of the combined accessory-vago-glossopharyngeal nucleus. 



When a little farther forward the central canal opens out 

 into the fourth ventricle (by which change the hypoglossal nucleus 

 (Fig. 109, 6 n. xn.) is brought nearer to the dorsal surface in the 

 floor of the fourth ventricle) this combined nucleus, increasing in 

 breadth, is thrown to the side and assumes a more lateral position, 

 lying now on the side of, but still somewhat dorsal to, the hypo- 

 glossal nucleus, between it and the now diminishing gracile 

 nucleus. In this position the nucleus appears to consist of two 

 parts, a median and lateral, the median part having conspicuous 

 nerve cells of moderate size, the lateral part having but few cells, 

 and those of small size. From this level the nucleus runs 

 forwards, maintaining nearly the same position in the floor of the 

 fourth ventricle but gradually becoming thinner, and ends as we 

 have said at about the level of the striae acusticae on the dorsal 

 surface corresponding on the ventral surface to a level a little 

 behind the hind margin of the pons. 



From this combined nucleus, but chiefly from the median 



