THE SPINE. 383 



ANTERIOR REGION. 



1. Direct Pyramidal Tract. This is a small well- 

 defined tract that lies next to the antero-median 

 fissure. As a rule this tract can only be traced down 

 to the middle of the dorsal region. These fibers 

 originate jointly with those of the crossed pyramidal 

 tract; that is, from the large pyramidal cells of the 

 cerebral cortex. The fibers, however, do not cross 

 in the medulla but pass directly down the cord on 

 the same side. The fibers cross in the cord at inter- 

 vals in its course, making use of the anterior com- 

 missure to reach the opposite side, where they ar- 

 borize around the cells of the anterior horn in a 

 manner like those of the crossed pyramidal. It 

 thus follows that the motor cells on one side of the 

 cerebrum control the muscular contraction on the 

 opposite side of the body, either through the crossed 

 or the direct pyramidal tract or through both. 



2. The anterior ground bundle is really a part of 

 the mixed lateral tract already described. It is 

 composed of ascending and descending fibers that 

 have both their origin and their termination in the 

 gray matter of the cord and is therefore an inter- 

 segmental or commissural tract. 



3. Anterior White Commissure. This is composed 

 of medullated nerve fibers passing parallel to the 

 gray commissure between the latter and the bottom 

 of the anterior fissure. It is a decussation of fibers 

 of a mixed variety, many of them being derived 

 from the direct pyramidal tract as already stated. 



