THE NERVOUS SYSTEM. 



523 



mesial formatio reticularis alba (white reticular formation) and correspond in 

 position to the fibers of the medial longitudinal fasciculi and reticulo- spinal 

 tracts in the adult medulla, representing probably the same system as the 

 medial part of the ventro-lateral funiculi of the cord (medial longitudinal 

 fasciculi, reticulo-spinal and ventro-medial ground bundles of the cord). The 

 medial longitudinal fasciculi are in part descending fibers from higher levels 

 described later. 



Taenia 



Marginal layer 



Mantle layer 

 Inner layer 



Tractus solitarius 



N. X. 

 (Medullary XI) 



Internal arcuate fibers 



(in beginning gray 



reticular formation) 



N. XII 



Alar plate 



Sulcus limitans 



Basal plate 



Ventral funiculus Floor plate 



(beginning of form, retic. alba; 



FIG. 449. Half of a transverse section of the medulla of a 10.2 mm human embryo. His. 



In the basal plate, between the medial and lateral efferent nuclei, there are, 

 even at the beginning of the fifth week, not only the efferent neurones and the 

 heteromeric (commissural) neurones already mentioned, but other neuroblasts 

 whose axones have a radial direction, i.e., toward the periphery. (Figs. 449 

 and 452.) The interlacing of these with the arcuate fibers forms the first 

 indication of the formatio reticularis grisea (gray reticular formation) . Later, 

 longitudinal fibers are present here, giving rise to a condition more fully 

 corresponding to that in the adult, analogous also to the condition in the 

 lateral funiculi of the cord, especially in the processus reticularis. 



