THE BRAIN 



335 



development and growth into it of fibers from neuroblasts in other parts 

 of the brain and spinal cord. 



The neuroblasts of the basal plates early give rise chiefly to the efferent 

 fibers of the cerebral nerves (Fig. 335). They thus constitute motor 

 nuclei of origin of the trigeminal, abducens, facial, glossopharyngeal, vagus 

 complex, and hypoglossal nerves, nuclei corresponding to the ventral and 

 lateral gray columns of the spinal cord. The basal plate likewise produces 

 the reticular formation, which is derived in part also from the neuroblasts 

 of the alar plate (Fig. 336). The axons partly cross as external and internal 

 arcuate fibers and form a portion of the median longitudinal bundle, a fasci- 

 culus corresponding to the ventral ground bundles of the spinal cord. 

 Other axons grow into the marginal zone of the same side and form inter - 

 segmental fiber tracts. The reticular formation is thus differentiated into 

 a gray portion, situated in the mantle zone, and into a white portion lo- 

 cated in the marginal zone (Fig. 336) . The marginal zone is further added 

 to by the ascending fiber tracts from the spinal cord and the descending 

 pyramidal tracts from the brain. As in the cord, the marginal layers of 

 each side remain distinct, being separated by the cells of the floor plate. 



The alar plates differentiate later than the basal plates. The afferent 

 fibers of the cerebral nerves first enter the mantle layer of the alar plates, 

 and, coursing upward and downward, form definite tracts (tractus soli- 

 tarius, descending tract of fifth nerve). To these are added tracts from the 

 spinal cord, so that an inner gray and an outer white substance is formed. 

 Soon, however, the cells of the mantle layer proliferate, migrate into the 

 marginal zone, and surround the tracts. These neuroblasts of the alar 

 plate form groups of cells along the terminal tracts of the afferent cerebral 

 nerves (which correspond to the dorsal root fibers of the spinal nerves) 

 and constitute the receptive, or terminal nuclei of the fifth, seventh, eight, 

 ninth, and tenth cerebral nerves. Caudally, the nucleus gracilis and nuc- 

 leus cuneatus are developed from the alar plates as the terminal nuclei for 

 the afferent fibers which ascend from the dorsal funculi of the spinal cord. 

 The axons of the neuroblast forming these receptive nuclei decussate 

 through the reticular formation, chiefly as internal arcuate fibers, and as- 

 cend to the thalamus as the median lemniscus. 



There are developed from neuroblasts of the alar plate other nuclei, 

 the axons of which connect the brain stem, cerebellum, and fore-brain. 

 Of these the most conspicuous is the inferior olivary nucleus. 



The characteristic form of the adult myelencephalon is determined by 

 the further growth of the above-mentioned structures. The nuclei of 

 origin of the cerebral nerves, derived from the basal plate, produce swell- 

 ings in the floor of the fourth ventricle that are bounded laterally by the 

 sulcus limitans. The terminal nuclei of the mixed and sensory cerebral 



