THE SPINAL CORD AND REFLEX ACTIONS. 597 



some regions of the white substance of the cord undergo no 

 degeneration as the result of injuries above or below them. 



The details as to the result of sections or injuries at various 

 levels differ considerably, but their broad features are indi- 

 cated in Fig. 175, in which tracts of descending degeneration 



FIG. 175. Diagram of a cross-section of the spinal cord near the upper part of 

 the cervical enlargement to indicate the main tracts of ascending and descending 

 degenerations. The gray matter is in solid black; tracts of descending degenera- 

 tion are shaded in vertical and of ascending in horizontal lines; pt, pyramidal or 

 crossed pyramidal tract; dpt, direct pyramidal tract; dc-al, descending antero- 

 lateral tract; ct, comma tract; cbt. cerebellar tract; ac-al, ascending antero-lateral 

 tract; s, e, t, c, posterior median tract: It, tract of Lissauer-, epc, external posterior 

 column; t'.aJinternal antero-lateral column. 



are shaded in vertical lines, and of ascending degeneration in 

 black. It represents a cross-section of the cord at about the 

 level of the fifth cervical nerve. The descending area of de- 

 generation, pt, is the pyramidal tract or crossed pyramidal 

 tract; its fibres degenerate posterior to any hemisection of the 

 cord on the same side, and to section of the anterior pyramid 

 of the medulla oblongata, or of the cms cerebri on the 

 opposite side, or as a result of disease or lesions of certain 

 parts of the convolutions of the cerebral hemisphere of the 

 opposite side. This tract is large in the upper part of the 

 cord and becomes smaller the further down it is examined, 

 because fibres are all the way separating from it to end in the 

 gray matter of the cord, where they join the mechanisms from 

 which the motor fibres of the anterior spinal roots arise. The 

 fibres of the pyramidal tract originate and have their nutri- 

 tive centres in what is known as the motor area of the 

 cerebral cortex : from there they converge and are collected 

 into the ventral portion of the cms cerebri and pass through 

 it and the pons Varolii to the ventral median portion (an- 

 terior pyramid) of the medulla oblongata, and there cross the 



