MEDULLA OBLONGATA OF THE RABBIT. 



7 



One which grows outward and which is joined by the 

 anterior nerve root — the two forming a spinal nerve. The 

 second process grows inward, and enters the Randschleier of 

 the cord in the dorsal zone, close to the boundary of the 

 ventral. It then divides, its fibres running up and down the 

 cord. 



In a transverse section of the medullary canal, these 

 fibres are seen forming the so-called oval bundle, which in 

 the adult corresponds to the columns of Burdach, as well as 

 to the posterior columns of the cord. 



Hence it is apparent that the origins of the motor and 

 sensitive nerve roots of the spinal cord are totally different. 

 The former originate within the central nervous system and 

 leave it by means of the ventral zone. The latter take their 

 origin outside of the central nervous system and enter it 

 through the dorsal zone. 



It is not to be wondered at that the functions of these 

 two sets of fibres should differ so materially from each other ; 

 since they have such totally different origins. 



If we noAv compare the development of the medulla 

 with that of the cord, we shall find that it differs in four 

 salient particulars : 



1st. The widening of the roof. 

 2d. The ventral zones come to lie in one plane. 

 3d. The development of the rhomboidal lip. 

 4th. Formation of the raphe. 



It was shown in the development of the spinal cord 

 that the cavity between the dorsal zones became obliterated, 

 and thus the posterior septum was formed. 



In the medulla the reverse of this process takes place. 



