716 THE NERVOUS SYSTEM. 



restiform bodies, and mesially the lower half of the floor of the fourth ventricle. 

 It is therefore more convenient to regard these nuclei in their relations to the 

 floor of the fourth ventricle and the restiform body, and especially to the former, 

 as there is practically but one nucleus in relation with the latter viz. the fol- 

 lowing : Just ventral to the latter is the end of the gray matter of the tubercle 

 of Rolando, showing somewhat indistinctly as a rounded mass traversed by the 

 root-bundles of the vagus (Fig. 417). 



Nuclei in Relation to Floor of Fourth Ventricle. As before stated, in the closed 

 portion of the medulla the base of the anterior cornua is found close to the cen- 

 tral canal, on its ventro-lateral aspect. As the floor of the canal becomes the 

 floor of the fourth ventricle in passing into the upper part of the medulla, it 

 necessarily follows that this gray matter is shifted still more dorsally arid comes 

 to lie beneath (ventral to) the floor of the ventricle on each side of the median 

 groove. In this gray matter is a column of large nerve-cells from which the 

 roots of the hypoglossal nerve arise. Hence these cells are called the hypoglossal 

 nucleus. This nucleus extends upward to the pons, and is covered dorsally by 

 white fibres, which are known as the funieulus teres (see below, floor of fourth 

 ventricle). In these fibres, dorsal and mesial to the hypoglossal nucleus, there 

 is also a smaller group of cells, the nucleus of the funieulus teres, from which fibres 

 are traceable to the vago-glossopharyngeal roots. 



The remaining nuclei in this region are those of the auditory, glossopharyn- 

 geal, vagus, and spinal accessory nerves. 



The Nucleus of the Spinal Accessory Nerve. This group of cells begins in the 

 closed part of the medulla, close to the base of the posterior cornu, and extends 

 upward, lying beneath the beginning of the floor of the fourth ventricle, and 

 lateral to the hypoglossal nucleus. Its upper extremity reaches to the eminentia 

 cinerea (see below, floor of fourth ventricle). This is the nucleus of the accessory 

 part of the nerve. 



Nuclei of the Vagus and G-lossopharyngeal Nerves. These are known as prin- 

 cipal and accessory. The principal nuclei of both these nerves are groups of 

 cells practically in continuity upward with the nucleus of the spinal accessory 

 nerve. These cells lie beneath (ventral to) the ala cinerea and inferior fovea in 

 the floor of the fourth ventricle (which see), that of the ninth being above the 

 tenth. 



The accessory nuclei are the upper and lower portions respectively of a small, 

 detached pear-shaped mass of gray matter (nucleus ambiguus) containing nerve- 

 cells, which is found in the reticular formation of the posterior area at some dis- 

 tance from the floor of the ventricle, and about on a line, ventrally, with the ala 

 cinerea. Its stalk is seen to extend mesially and dorsally, and fibres run in this, 

 and then turn outward and forward to join the main bundles of their respective 

 nerves. The nucleus of the funieulus teres (see above) is also an accessory 

 nucleus of these nerves. 



Nuclei of the Auditory Nerve. These are two, dorsal and ventral. The dorsal 

 nucleus lies external to the vago-glossopharyngeal nucleus and underneath the 

 trigonum acoustici, which is on the floor of the ventricle just lateral to the inferior 

 fovea. The ventral or accessory nucleus lies between the two roots of the auditory 

 nerve (which see), ventral and close to the restiform body ; above, in the pons, 

 it unites with the ganglion of the lateral root, which in this region is found mixed 

 in with the fibres of this root as it passes around the restiform body. 



The white matter of the upper part of the medulla is, like that of the lower, 

 found on the surface in comparatively large bundles of fibres and, as smaller 

 bundles or even as individual fibres, in the formatio reticularis of the various 

 " areas " of the deep portion. 



The surface fibres are those of the pyramid, the olivary body, and restiform 

 body, together with small bundles in the ventro-lateral and dorso-lateral grooves. 



The pyramid has already been described in discussing the ventral surface of 

 the medulla. It only remains to state here that its fibres all proceed directly 



