740 



NEUROLOGY 



with, the outer surface of the main part of the alar lamina, and so covers in the 

 tractus solitarius and also the spinal root of the trigeminal nerve; the nodulus 

 and flocculus of the cerebellum are developed from the rhombic lip. 



Neuroblasts accumulate in the mantle layer; those in the basal lamina corre- 

 spond with the cells in the anterior gray column of the medulla spinalis, and, like 

 them, give origin to motor nerve fibers; in the medulla oblongata they are, however, 

 arranged in groups or nuclei, instead of forming a continuous column. From the 

 alar lamina and its rhombic lip, neuroblasts migrate into the basal lamina, and 

 become aggregated to form the olivary nuclei, while many send their axis-cylinders 

 through the floor-plate to the opposite side, and thus constitute the rudiment of 

 the raphe of the medulla oblongata. By means of this thickening of the ventral 

 portion, the motor nuclei are buried deeply in the interior, and, in the adult, are 

 found close to the rhomboid fossa. This is still further accentuated: (a) by the 



development of the pyramids, which 

 are formed about the fourth month 

 by the downward growth of the 

 motor fibers from the cerebral cortex ; 

 and (6) by the fibers which pass to 

 and from the cerebellum. On the 

 rhomboid fossa a series of six tem- 

 porary furrows appears; these are 

 termed the rhombic grooves. They 

 bear a definite relationship to certain 

 of the cranial nerves; thus, from 

 before backward the first and second 

 grooves overlie the nucleus of the 

 trigeminal; the third, the nucleus of 

 the facial ; the fourth, that of the ab- 

 ducent; the fifth, that of the glosso- 

 pharyngeal; and the sixth, that of 

 the vagus. 



The pons is developed from the 

 ventro-lateral wall of the meten- 

 cephalon by a process similar to that 

 which has been described for the 

 medulla oblongata. 



The cerebellum is developed in 

 the roof of the anterior part of 

 the hind-brain (Figs. 649 to 654). 

 The alar laminae of this region 

 become thickened to form two 



lateral plates which soon fuse in the middle line and produce a thick lamina which 

 roofs in the upper part of the cavity of the hind-brain vesicle; this constitutes 

 the rudiment of the cerebellum, the outer surface of which is originally smooth 

 and convex. The fissures of the cerebellum appear first in the vermis and floccular 

 region, and traces of them are found during the third month; the fissures on the 

 cerebellar hemispheres do not appear until the fifth month. The primitive fissures 

 are not developed in the order of their relative size in the adult thus the hori- 

 zontal sulcus in the fifth month is merely a shallow groove. The best marked 

 of the early fissures are: (a) the fissura prima between the developing culmen and 

 declive, and (6) the fissura secunda between the future pyramid and uvula. The 

 flocculus and nodule are developed from the rhombic lip, and are therefore recog- 

 nizable as separate portions before any of the other cerebellar lobules. The 

 groove produced by the bending over of the rhombic lip is here known as the 





Fio. 651. Brain of human embryo of four and a half weeks, 

 showing interior of fore-brain. (From model by His.) 



