148 ELEMENTS OF HISTOLOGY. [Chap. xvn. 



olivary nucleus. In both nuclei are found numerous 

 multipolar ganglion cells, each with an axis cylinder 

 process. 



192. The grey matter at the floor of the 



fourth ventricle is that from which the nerve roots 

 of the cerebral nerves (facial auditory, glossopharyn- 

 geal, pneumogastric, accessory, and hypoglossal) origi- 

 nate. The ganglion cells in it are of various sizes, 

 and are aggregated into groups which represent the 

 "nuclei" i.e., the origin of the above nerves. The 

 thin layer of grey matter forming the floor of the 

 ventricle, in the strict sense, is neuroglia only, a con- 

 tinuation of the central grey nucleus of the cord. 



The nerve cells in the hypoglossal nucleus are 

 the largest ; they are as large as the large cells 

 of the anterior horns of the cord. The cells of 

 the glossopharyngeal nerves are considerably smaller. 

 The motor nerve-fibres (e.g., those of the hypoglossal 

 and pnoumogastric) originate, as the axis cylinder 

 process of the multipolar ganglion cells, in exactly 

 the same manner as was mentioned in the cord, 

 but the sensory nerve- fibres of these nerves originate 

 from the nerve ground network, into which the 

 processes of the ganglion cells of these nuclei break up. 



193. In the lower part of the medulla, as long as 

 there is still a closed central canal, we find next to 

 this the last outrunners of the groups of ganglion 

 cells representing the nucleus of the spinal accessory 

 and hypoglossal. 



As we pass upwards, and as the central canal 

 opens as the fourth ventricle, the groups of ganglion 

 cells below the floor of the ventricle are so arranged 

 that we find near the median line the group repre- 

 senting the hypoglossal nucleus; then, farther out- 

 wards, several groups representing several sub-divisions 

 of the pneumogastric nucleus ; still farther upwards, 

 but more in the anterior part of the medulla, the nucleus 



