THE NERVOUS SYSTEM 1461 



efferent nerve-fibres. The alar lamina, ventral to the rhombic lip, receives 

 the centripetal or afferent fibres of the glosso-pharyngeal and vagus nerves, 

 which terminate in a prominent oval area, representing the fasiculus solitarius, 

 at first placed superficially. Subsequently, the two laminae spread outwards 

 and then occupy a horizontal plane, the roof-plate meanwhile expanding. 

 The basal and alar laminae now form, as stated, the floor of the bulbar part 

 of the fourth ventricle, the basal lamina being mesial to the dorsed lamina. 

 The two basal laminae, right and left, are connected by the floor-plate. The 

 trigonum hypoglossi in the bulbar part of the floor of the fourth ventricle 

 pertains to the basal lamina, whilst the trigonum vagi and trigonum acusticum 

 belong to the alar lamina. 



When the rhombic hp has fused with the contiguous part of the alar lamina, 

 a proUfic nest of neuroblasts is formed in this region. Many of these neuro- 

 blasts pass ventralwards, embracing the fasciculus solitarius, which now hes 

 deeply. These neuroblasts, on reaching the basal lamina, become differ- 

 entiated into the olivary body. Other neuroblasts pass into the alar lamina, 

 where they give rise to the nucleus gracilis and nucleus cuveatus. 



Each pyramid is a ventral bulging of that part of the basal lamina which is 

 on the mesial side of the olivary body, and it is produced by the motor tracts 

 as they descend from the Rolandic area of the cerebral cortex through the 

 pons Varolii. 



The arcuate nucleus is developed from neuroblasts derived from the rhombic- 

 lip-area of the alar lamina. 



The formatio reticularis, which lies on the dorsal aspects of the pyramid 

 and oUvary body, is developed in connection with the mantle-layer of the 

 basal lamina prior to the development of the pyramid. 



As the pyramids, right and left, bulge ventralwards, the floor-plate, which 

 connects the basal laminae, sinks, and the anterior median fissure is formed, 

 as in the development of the spinal cord. The spongioblastic floor-plate is 

 now invaded by nerve-fibres, most of which cross from one side to the other, 

 these fibres being (i) the anterior superficial arcuate fibres, (2) the deep 

 arcuate fibres, and (3) the cerebello-ohvary fibres. In this manner the raphe 

 of the bulb is formed, as in the development of the anterior or white commis- 

 sure of the spinal cord. 



The cavity of the medullary or bulbar portion of the neural tube extends 

 in early life as low as the cervical flexure, where it is continuous with the 

 cavity of the spinal portion of the neural tube, and the calamus scriptorius 

 of the fourth ventricle is, at this stage, on a level with the cervical flexure. As 

 development proceeds, the basal and alar laminae, which had spread outwards 

 so as to he horizontally on the ventral wall, resume their original position 

 in the lateral wall of the lower half of the medullary portion of the neural 

 tube on either side. In this manner the lower half of the bulb becomes 

 closed dorsalwards, and so the lower half contains a prolongation of the 

 central canal of the spinal cord. 



Metencephalon. — The metencephalon, which represents the fourth secondary 

 cerebral vesicle, is the cephalic subdivision of the rhombencephalon, hind- 

 brain, or posterior primary cerebral vesicle. From it are developed (i) the 

 pons Varolii, (2) the cerebellum, (3) the superior and middle peduncles of the 

 cerebellum, and (4) the superior medullary velum or valve of Vieussens and the 

 inferior medullary velum. The cavity of the metencephalon forms the upper 

 or pontine part of the fourth ventricle. 



The pons Varolii is developed from the ventral wall of the metencephalon. 



The cerebellum is developed from the dorso-lateral walls of the meten- 

 cephalon. 



The rudiments of the eerebellum appear as thickenings of the right and left 

 dorsal or alar lamines of the lateral walls of the metencephalon. These 

 thickened alar laminae constitute the two lateral cerebellar plates, the cere- 

 bellum being originally composed of two symmetrical halves, separated by a 

 median furrow of the roof-plate of the metencephalon. The two cerebellar 

 plates subsequently fuse across the median dorsal line, and, being now trans- 

 verse, they form one transverse cerebellar plate. The central part of this plate 



