THE GRAY MATTER. 603 



of the fourth ventricle, brought into an apparently more dorsal position 

 (Fig. 132, 6). 



The nucleus consists mainly of large nerve-cells with distinct axis-cylinder 

 processes, which though pursuing a somewhat irregular course may be traced 

 into the fibres of the nerve. These, starting from the ventral surface of the 

 nucleus along its length, run ventrally through the reticular formation, and 

 making their way in a series of bundles, between the olivary nucleus on the 

 lateral side 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. 



530. The ninth or glosso-pharyngeal, tenth or vagus, and eleventh or 

 spinal accessory nerves. It will be advantageous to consider these three 

 nerves together. 



In the spinal accessory nerves 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 outward through the lateral column, and issue from the cord along 

 a line immediate 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 518. 



The bulbar accessory starts from an elongated nucleus in the bulb which 

 is common to it, to the vagus, and to the glosso-pharyngeal ; hence we have 

 taken these three nerves together. This (Fig. 138) stretches further for- 

 ward than the hypoglossal nucleus, reaching the level of the transverse fibres 

 called striae acusticse (str.), 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. 132, 4, 5), two nuclei or 

 collections of cells are seen in the gray 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-glosso-pharyngeal 

 nucleus. 



When a little further forward the central canal opens out into the fourth 

 ventricle (by which change the hypoglossal nucleus (Fig. 132, 6 n. xii.) 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 hypoglossal 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 forward, 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 acusticse on the dorsal surface correspond- 

 ing 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 part, fibres 

 sweep in a ventral and lateral direction through the dorsal part of the retic- 

 ular formation, pass ventral to, or in some cases through, the gelatinous sub- 

 stance and the strand of fibres connected with the fifth nerve (Fig. 132, V. 

 a), and reach the surface of the bulb on its lateral aspect in a line between 

 the olivary and restiform bodies (Fig. 131, C). Along this line may be seen 



