NETJROLOGY 291 



The valve of Vieussens is located at the posterior end of the aque- 

 duct of Sylvius. The posterior part of the fourth ventricle is marked 

 by grooves or furrows. Extending along the central part of the 

 floor of the fourth ventricle there is a sulcus, or groove, called the 

 sulcus centralis, which divides the superior pyramids. On the 

 median lateral sides there are two points of gray substance which 

 form the alae cinereae. The roots of the pneumogastric, the glosso- 

 pharyngeal, and the spinal accessory may be traced to the alae 

 cinereae and the gray matter of the ridges of the medulla oblongata. 

 The sixth pair of the cranial nerves emerge from the medulla near 

 the sulcus centralis and to the side of the auditory nerve. Part 

 of the trigeminus ernerges from the rim of the furrow next to the 

 ridge. At the outer border of the medulla are observed the thick 

 ends of the roots of the pneumogastric, the spinal accessory, the 

 glosso-pharyiigeal, and the auditory facialis. 



On both sides of the sxilcus longitudinahs inferior are found the 

 inferior pyramids, or pyramidal columns. These pyramids become 

 expanded near the origin of the third pair of nerves and merge into 

 the cerebral peduncles, or crura cerebri. 



The pyramidal fibers may also be traced to the optic lobes. The 

 roots of the abducens are found at a point between the crura cerebri 

 and the optic lobes. The third pair lies to the side of this, and the 

 roots of the trigeminus are adjacent to those of the third. A 

 bimdle of fibers from this region pass into the cerebellum and form 

 the crura cerebeUi; others pass into the cerebellum from the side, 

 spreading out in fan-shaped radiation, and forming the white central 

 substances peculiarly arranged, caUed the arbor vitae, or tree of life. 

 A third bundle fuse with the crura cerebelli anteriorly, and pass 

 into the peduncles of the cerebrum, or crura cerebri. 



The gray ganghonic substance forms columns which extend the 

 whole length of the medulla oblongata and into the cerebri. The 

 medulla oblongata contains numerous centers which preside over 

 various visceral functions as deglutition, respiration , thermotactic, 

 secretory, cardiac, and digestion. 



In the medulla as in the spinal cord there are five main groups 

 of cells. First, the posterior or upper horns, from which come the 

 somatic sensory nerves. Second, Clark's cells, located centrally, 

 which are the origin of the gangKonated splanchnic nerves. Third, 

 cell groups of the lateral horns which are the center for the non- 

 ganglionated splanchnic nerves, and certain other nerves for the 



