598 THE HUMAN BODY. 



middle line at what is known as the decussation of the pyr- 

 amids and enter the spinal cord on the opposite side. The 

 area of descending degeneration, dpi, lying close to the an- 

 terior fissure is the direct pyramidal tract. Its fibres arise in 

 the same cerebral region as those of pyramidal tract, and have a 

 similar course and ending, except that they do not cross to the 

 other side in the medulla oblongata, but gradually pass over 

 in the spinal cord itself, to end in the gray matter connected 

 with the origin of the anterior spinal roots: the direct pyram- 

 idal tract does not extend so far down the cord as the crossed 

 tract, pt. Another tract of descending degeneration is dc.al, 

 the descending antero-lateral : it represents rather an area over 

 which are to be found some degenerated fibres scattered among 

 many undegenerated, than a distinct group of fibres. The 

 same may be said of ct, the comma tract: it only extends 

 a short way down in the external posterior column of the cord 

 after a hemisection has been made on the same side. Its fibres 

 are posterior root-fibres running back in the white matter a 

 little distance before entering the gray core. 



A conspicuous tract of ascending degeneration is the cere- 

 bellar tract cb.t. It lies on the outer side of the pyramidal 

 tract and can be traced along the dorsal side of the medulla 

 oblongata to the cerebellum. It commences in the lumbar 

 region of the cord, and seems to contain two sets of fibres; 

 some originating in the gray matter and passing on to re-enter 

 it at a higher level of the cord ; and others continued to the 

 cerebellum. The nerve-fibres of this tract are very large. 

 Another important ascending tract, s, e, f, c, lies in the median 

 portion of the posterior white column and is named the median 

 posterior tract. It commences in the lower portion of the 

 cord and gradually increases in size upwards. Its degenera- 

 tion follows not only section of the posterior column, but 

 section of the dorsal roots of the spinal nerves : sections of 

 these roots in the sacral, lumbar, thoracic, and cervical nerves 

 cause degenerations in the areas marked respectively s, e,t, c\ 

 hemisection of the cord is followed above the section by de- 

 generations in this tract corresponding to all tiie spinal nerves 

 which join the cord below the section. The posterior median 

 tract is lost as a distinct tract in the medulla oblongata: its 

 fibres are nearly all small. The ascending antero-lateral tract, 

 ac.al, contains many fibres which undergo degeneration after 

 section of the cord on the same side, mixed with many fibres 



