494 THE NERVOUS SYSTEM 



of the inferior olivary body. The mesial accessory olivary nucleus 

 is transversely flattened, and lies between the mesial surface of the 

 olivary body and the median raphe. 



Just below the spinal border of the pons Varolii the anterior 

 pyramids present a thin layer of grey matter on their ventral sur- 

 face ; this is the arcuate nucleus (nucleus of the external arcuate 

 fibres, nucleus arcuatus). 



Opposite the upper end of the chief glossopharyngeal nucleus 

 the medulla comes into relation ventrally with the pons Varolii, 

 but is covered dorsally by the overhanging cerebellum. At the 

 lower border of the pons Varolii the eighth, seventh, and sixth 

 cranial nerves are given off, the sixth from the groove between 

 the pyramid and the cephalic end of the inferior olivary body, the 

 seventh lateral to the olivary body, and the eighth still farther 

 lateral and somewhat dorsal from the seventh. 



The twelfth, sixth, and later the third cranial nerves may be 

 collectively considered as a ventral group which are given off in 

 a ventral plane near the median line ; the eleventh, tenth, ninth, 

 eighth, seventh, and fifth form a lateral group, having their super- 

 ficial origin from the lateral surface of the nervous system and in 

 nearly the same vertical plane. The fourth nerve, since it makes 

 its exit from the dorsal surface, is without a homologue. The sec- 

 ond and first cranial nerves are for many reasons considered as 

 diminutive lobes of the cerebrum itself, and as such are not homolo- 

 gous with the other cranial nerves. 



THE PONS VAROLII 



The pons Varolii or metencephalon occupies a region corre- 

 sponding to the upper half of the fourth ventricle. The lateral 

 walls of the ventricle approach each other in this region, finally 

 uniting in the median line and thus surrounding a central canal, 

 the aqueduct of Sylvius, which extends cephalward through the 

 mesencephalon to the third ventricle. These cavities are not only 

 homologous, but at the spinal end of the fourth ventricle are also 

 continuous with the central canal of the spinal cord. 



In the metencephalon the morphological structures of the 

 medulla oblongata or myelencephalon are continued upward in the 

 dorsal half of the organ, known as its tegmentum, with the single 

 exception of the pyramids, which, as longitudinally disposed fibre 

 bundles, enter the ventral portion to interlace with the transverse 

 fibre bundles of the pons Varolii. This reticular mass of nerve 



