PYRAMID TRACT IN SQUIRREL AND CHIPMUNK 149 



than formerly. The bundles interlace very distinctly with those 

 of the normal pyramid. The pyramid is much reduced in size 

 and the area of degeneration dorsal to the gray matter is cor- 

 respondingly enlarged. The latter is somewhat triangular in out- 

 line and the innermost fibers lie nearer to the middle line but not 

 within the limits of the funiculus gracilis. It will be observed 

 that most of the decussation has taken place above the level of 

 the lower end of the inferior olivary nucleus, and that it is very 

 abrupt, the fibers running ahnost at right angles to the direction 

 of the pyramid in their passage across the gray matter. 



Transverse section through medulla oblongata about the level of 

 the lower extremity of the pyramidal decussation (fig. 10.) What 

 remains of the pyramid is here seen as a narrow zone of degen- 

 eration bordering the ventral longitudinal fissure, but most of the 

 fibers have crossed and are to be found in the posterior column 

 where they form a large tract, the sharp inner angle of which 

 reaches almost to the middle line. 



Transverse section near junction of spinal cord and medulla 

 oblongata (fig. 11). The decussation is now over and the crossed 

 pyramid tract occupies a comparatively large area in the poste- 

 rior or dorsal column, in contact with the posterior commissure and 

 extending from the mesial septum laterally into the funiculus 

 cuneatus. No crossed fibers at any level have been observed 

 turning outward through the gray matter to reach the lateral 

 column, nor have any been found to pass into the dorsal or lateral 

 column of the same side. In this anunal the decussation ap- 

 pears to be complete and the crossed pyi-amid tract is confined to 

 the dorsal column. 



Transverse section through first cervical segment of spinal cord 

 (fig. 12). The pyramid tract in the cord occupies the same rela- 

 tive position as in the last section. The lateral diameter of 

 the area is somewhat diminished and the antero-posterior diam- 

 eter increased while the inner extremity is pressed up against the 

 posterior median septum. 



Transverse section through second- cervical segment (fig. 13). The 

 appearance of the tract is now greatly changed. Its outline 

 IS that of an isosceles triangle with the base lying against the 



