CHAPTEE XXIV. 

 The Medulla Oblongata and the Tegmentum of the Pons. 



Hating seen how, through the rearrangement of fibers, and the ap- 

 pearance of new ganglion-gronps and the disappearance of the posterior 

 columns, the oblongata is formed, there remain still a number of fibrous 

 tracts to be followed upward from the cord. The posterior columns are con- 

 tinued upward indirectly by the lemniscus, and in it also are to be foimd 

 those sensory fibers of the second order which ascend in the antero-lateral 

 columns of the cord. The pyramidal tracts from both the anterior and 

 lateral columns lie now united, ventral, forming the thick pyramid of the 

 bulb. The lateral cerebellar tract retains its position at the periphery as high 

 as the olivary body. There its dorsal fibrils begin to turn toward the cere- 

 bellum, ascending dorsally. They soon afterward form the nucleus of a 

 large bundle, which first appears here: the inferior cerebellar peduncle, 

 corpus restiforme. Its ventral portion occupies its original position as far as 

 the pons, and then turns backward toward the vermis superior. 



The corpus restiforme arises laterally from the upper end of the pos- 

 terior columns, at first because, as just stated, the lateral cerebellar tract 

 there passes upward toward the cerebellum. To it pass also fibers from the 

 posterior columns of the cord, which, as shown in Fig. 233 and Fig. 334 

 (above, to the left), curve around the postero-lateral periphery of the bulb 

 to join it, fibrcB arcuatoe externce posteriores. Other fibers come to them from 

 in front. These, the fihrce arcuatce externm anteriores, come apparently from 

 the lemniscus between the olives, — hence from the opposite posterior 

 columns, — pass near the median line anteriorly along the periphery, and 

 extend partly ventrally to the pyramids, partly behind them, partly also 

 through them, around laterally to the corpus restiforme. The latter fibers 

 have also been called the fibras arciformes of the pyramids (Fig. 337). 

 Among them lies a nucleus of varying size, the nucleus arcuatus (Fig. 241). 

 Accordingly, to the corpus restiforme there pass from the spinal cord (1) the 

 lateral cerebellar tract, (2) fibers from the corresponding posterior column, 

 and (3) fibers probably from the opposite posterior column.^ 



'■ The fibers under No. 3 receive their medullary sheaths months before those of 

 the pyramids and olives, probably at the same time as the posterior columns. 



(379) 



