TRANSITION OF MEDULLA OBLONGATA INTO SPINAL CORD. 523 



apparent in the crusta and tegmentum of the crus cerebri, was 

 still clearly expressed. 



The organization of the crus cerebri had still superadded to 

 it an appendage, the crus cerebelli, by the breaking up of 

 which into its constituent fasciculi, in the interior of the me- 

 dulla oblongata, on the one hand, the most essential portion of 

 the system of arcuate fasciculi, and upon the other the compact 

 grey matter forming the olivary bodies and the scattered grey 

 formations, were introduced into the peduncular organization. 



The most essential differences between the organization of 



O 



this segment of the spinal cord and that of the medulla ob- 

 longata consists in four points : 



(]) Whilst the medulla oblongata consists of a prolongation 

 of the anterior and posterior tract of the crus cerebri, and 

 whilst (2) an additamentum or third tract is found in the crus 

 cerebelli, the transverse section of the spinal cord forms a sym- 

 metrical liont&geneous and continuous medullary investment 

 in which the threefold mode of origin of the medulla oblongata, 

 as well as the double nature of the cerebellar penduncle, is 

 entirely lost. (3) Whilst in the upper half of the oblongata 

 the grey matter of the central cavities is represented by the 

 broad unfolding and expansion of the floor of the fourth ven- 

 tricle, the motor and sensory nuclei of origin being developed 

 side by side as internal and external masses, we find the 

 organization of the spinal cord disposed around a narrow 

 enclosed cavity, the central canal, in relation to which the 

 motor and sensory grey nuclei of origin are arranged in 

 an antero-posterior direction before and behind each other. 

 (4) The so modified grey matter of the central cavity, in op- 

 position to the variously formed and manifold grey substances 

 of the medulla oblongata, constitutes the single and uniform 

 ganglionic substance of the spinal cord. 



These differences are abolished by two principal morphologi- 

 cal processes ; namely, by the closing in of the central canal, 

 occasioned by the formation of the posterior columns, which 

 occurs in the upper half of the medulla oblongata, and by the 

 decussation of the pyramids, which occurs in the lower half. 



1. The inclusion of the central canal. Whilst the fasciculi of 

 the restiform body (M F (7), constituting the external division 



