THE BRAIN 335 



growth into it of fibers from neuroblasts in the spinal cord and in other parts of 

 the brain. 



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 hypo- 

 glossal nerves, nuclei corresponding to the ventral and lateral gray columns of the 

 spinal cord. The basal plate likewise produces part of 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 fasciculus corresponding to the ventral ground 

 bundles of the spinal cord. Other axons grow into the marginal zone of the same 

 side and form intersegmental fiber tracts. The reticular formation is thus differ- 

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

 located 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 solitarius, 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 proUferate, migrate into the marginal zone, and surround the tracts. 

 These neuroblasts of the alar plate form groups of cells alon^ 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, 

 eighth, ninth, and tenth cerebral nerves. Caudally, the nucleus gracilis and 

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

 afferent fibers which ascend from the dorsal funiculi of the spinal cord. The 

 axons of the neuroblasts forming these receptive nuclei decussate through the 

 reticul.ir formation chiefly as internal arcuate fibers and ascend 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 



