616 STRUCTURE OF THE SPINAL CORD [CH. XLII. 



The two pyramidal tracts come down from the brain ; in the 

 medulla oblongata, the greater number of the pyramidal fibres cross 

 over to the other side of the cord which they descend ; hence the 

 term crossed pyramidal tract ; a smaller collection of the pyramidal 

 fibres goes straight on, on the same side of the cord, and these cross 

 at different levels in the anterior commissure of the cord lower down ; 

 hence the disappearance of the direct pyramidal tract in the lower 

 part of the cord. The fact that the crossed pyramidal tract of one 

 side is the fellow of the direct pyramidal tract of the other side, is 

 indicated in the diagram by the direction of shading (see fig. 450). 



Comma tract Septet-marginal 



Oval bundle 



Crossed 

 pyramidal' 



Prepyramidal 

 ry tract * 



V , / [ . \ \ 



'Bundle of 



Antero-lateral^r-^ i! V ^f Helweg 

 descending 



Direct pyramidal 

 tract 



FIG. 450. Tracts of descending degeneration. For the sake of clearness each is shown on only one 



side. (After Schafer). 



Mingled with the fibres of the crossed pyramidal tract are a few 

 fibres of the pyramid which have not crossed in the medulla 

 oblongata, and are therefore derived from the same side of the 

 cerebrum (uncrossed lateral pyramidal fibres). 



The pyramidal fibres are not found at all in vertebrates below the mammals. 

 In the lower mammals they are very small, and in some rodents (rat, mouse, 

 guinea-pig) they are placed in the posterior columns. The direct pyramidal tract is 

 found only in man and the higher apes. 



The paralysis that results from the section of the pyramidal 

 tracts passes off very soon in many animals, whereas that which 

 results from section of the anterior column and the adjacent part of 

 the lateral column is more permanent. It is probable that the two 

 tracts next to be described may be the second path for volitional 

 impulses, and perhaps derive their importance from the fact that the 

 impulses which travel down them are necessary in the maintenance 

 of the tone of the anterior horn cells. 



(3.) Antero -lateral descending tract, or tract of Loewenthal, lies by 

 the side of the anterior median fissure, and extends along the margin 



