TRANSITION OF MEDULLA OBLONGATA INTO SPINAL CORD. 527 



nuclei of the slender and cuneate fasciculi, the posterior cornu 

 reaches the position in the spinal masses from the cord it sub- 

 sequently retains, by the elimination of these posterior columns 

 which remain imbedded in them only as far as to the com- 

 pletion of their origin by the additions they receive from the 

 pyramids. . 



Coincidently, however, with the difference in the form of 

 the grey substance of the central cavities, which takes place 

 between the medulla oblongata and the spinal cord, and with 

 the exhaustion of the corpus restiforme, the peduncle of ike 

 cerebellum ceases to exist, becoming continuous with the forma- 

 tion of the posterior column ; and because, with the conclusion 

 of this process, the system of the fibrce arcuatce, by which it 

 is accomplished, disappears, so do the formations of the olivary 

 nuclei and the scattered formations proper to the medulla 

 oblongata, continuous with the fibrse arcuatse, also disappear. 

 The formation of the posterior column, however, conditions 

 the inclosure of the central canal, so that it is impossible to 

 consider the two processes separately. 



Thus, after the closure of the central canal, the area of the 

 transverse section of the medulla oblongata presents the follow- 

 ing segments : 



(1) The pyramids (P) forming a continuation of the crusta, 

 and still large and complete, lying near the median line. (2) 

 The outer portion of the posterior tract of the peduncle, 

 forming the continuation of the tegmentum of the cms, and 

 destined to pass directly into the antero-lateral column of the 

 spinal cord (from Oito 6?). Behind this runs the triple forma- 

 tion of the posterior column. (3) Adjoining the antero-lateral 

 column of the posterior division of the medulla oblongata, 

 the grey tubercle of Rolando projects, shimmering through its 

 medullary investment as the grey caput of the posterior 

 cornu, which here attains its greatest development. Partly in 

 its interior, and partly surrounding it, the transverse sections 

 of the ascending roots of the fifth nerve, springing from a still 

 lower plane, may be recognized. (4) Behind the tubercle of 

 Rolando, the triangular nucleus of the fasciculus cuneatus; and 

 (5) internal to it, near the posterior fissure, the clavate nucleus 

 of the fasciculus gracilis, enclosed in the posterior columns, 



