158 SUTHERLAND SIMPSON 



Transverse sections through tiveJfth thoracic and first lumbar seg- 

 ments (figs. 34 and 35). Tn the twelfth thoracic segment the 

 tract stil] meets its fellow of the opposite side at the posterior 

 median septum but in the first lumbar segment this is no longer 

 the case; it has now left the septum and is separated from it 

 by a zone of sound fibers, the degenerated area lying against the 

 mesial border of the posterior horn. 



Transverse sections through third lumbar and first sacral segments 

 (figs. 35 and 36). At the level of the third lumbar segment the 

 degenerated fibers are greatly reduced in number and form a 

 narrow zon^ along the ventral part of the mesial border of the 

 posterior horn. In the first sacral segment verj'' few fibers re- 

 main. 



Transverse section through fourth sacral segment (fig. 37). At 

 this level six or eight degenerated fibers can still be seen in the 

 posterior column. 



Comparing the figures at corresponding levels in the medulla 

 oblongata and spinal cord, it will be seen that the distribution 

 of the pyramid tract in the squirrel and chipmunk agrees closely. 

 In both species it is confined to the dorsal column in the spinal 

 cord and there is no trace of a direct tract. 



SUMMARY 



The cortical motor areas in the left cerebral hemisphere were 

 extirpated in three red squirrels (Sciurus hudsonius loquax) and 

 three chipmunks (Tamias striatus lysteri) and the resulting 

 degeneration followed by the Marchi method. 



The pyramid tract occupies the usual position in the crusta, 

 pons and medulla oblongata until the decussation is reached. 

 Here, in the lower part of the bulb, the fibers cross the middle 

 line abruptly in bundles which interlace with those of the sound 

 side and pass through the gray matter into the funiculus cuneatus 

 where they turn caudalwards to enter the spinal cord and form 

 the crossed pyramid tract. The decussation is complete, no 

 fibers remaining on the same side when the spinal cord is reached. 



