Correlation Centres and Tracts 53 



Containing both ascending and descending correlation 

 fibres, is the medial longitudinal bundle {fascicuhis longitudi- 

 nalis medialis, dorsalis, seu posterior), a rather distinct bundle 

 lying close to the central gray matter immediately on each 

 side of the raphe, and containing many different kinds of 

 fibres (Pis. MI.-X\\). 



The most conspicuous descending tract is the pyramidal 

 tract, which runs directly posteriorly in a ventral position, 

 carrying motor impulses to the somatic muscles. As it enters 

 the region of the pons from the midbrain, this tract is accom- 

 panied by numerous cortico-pontine fibres which end in the 

 nucleus pontis (infra), thus decreasing considerably the size 

 of the bundle as a whole. It is also accompanied by cortico- 

 bulbar fibres, which pass to the motor nuclei of the medulla 

 oblongata. The remainder of the tract^ (cortico-spinal or 

 pyramidal tract), when it reaches the lower end of the ob- 

 longata, decussates in the form of numerous rather large 

 bundles — the pyramidal decussation (PI. VL) — and runs 

 dorsally to take up a position in the ventral part of the 

 dorsal funiculus of the spinal cord, where it has already been 

 observed. 



In man and some other mammals, the decussation is 

 incomplete, a small proportion of the fibres continuing straight 

 down into the ventral funiculus of the same side and crossing 

 over singly just before ending in the ventral gray matter. 

 In the rat, however, the decussation appears to be complete 

 or practically so. 



Related functionally with the pyramidal tract is the rubro- 

 spinal tract {fasciculus rubro-spinalis), which conveys impulses 



^Bregmann finds that the area of cross section of the cortico-spinal 

 tract just before it decussates is 5.3% of the area of the section of the 

 spinal cord just behind the decussation in the rat. The corresponding 

 ratio for man is 30.0%. These figures do not represent the relative 

 number of fibres, however, since they do not consider the relative sizes 

 of the individual axons, a factor which is complicated by the great difference 

 in myelination. 



