PYRAMID TRACT IN SQUIRREL AND CHIPMUNK 153 



Chipmunk 



In the chipmunk the degeneration is similar to that found 

 in the squirrel in the mid-brain, pons and medulla oblongata 

 until the decussation is reached, so that it will be unnecessary to 

 describe sections above this level. It is confined to the side of 

 the lesion. Of. the three animals used, one is selected for descrip- 

 tion but the results obtained in all are in close agreement. The 

 lesion in the left cerebral hemisphere as it appeared in this animal 

 when the brain was removed is shown in figure 20. 



Transverse section through medulla oblongata near upper limit 

 of decussation of pyramids (fig. 21). The pyramid, at this level, 

 lies ventro-mesial to the inferior olivary nucleus. Its transverse 

 section is oval in outline, pointed at the inner and outer extrem- 

 ities. From the inner angle a few fibers, cut longitudinally, 

 can be seen passing toward the raphe but none as yet reach it; 

 this represents the beginning of the decussation. The degen- 

 eration appears to be confined entirely to the left side, that is, 

 to the side of the cortical lesion. 



Transverse section of medidla oblongata through pijranddal 

 decussation (fig. 22). The pyramid is still oval-shaped but the 

 direction of its long axis is more dorso-ventral than in the last 

 section. Degenerated fibers are seen crossing the raphe in bundles 

 which interlace with those of the sound pyramid. They curve dor- 

 salward through the gray matter and pass into the funiculus 

 cuneatus in which they turn toward the spinal cord. 



Transverse section of medulla oblongata through middle of pyra- 

 midal decussation (fig. 23). This section passes through the middle 

 of the decussation; the inferior olivary nucleus is still seen. 

 The degenerated fibers are crossing in large bundles and the 

 pyramid is much reduced in size. A large crossed tract is now 

 present in the funiculus cuneatus and it has moved nearer to the 

 middle line. No fibers, at this or at any other level, turn outward 

 in the gray matter toward the lateral column and none join the 

 bundles of the sound pyramid and pass to the dorsal column 

 of the same side, as they can be seen to do in the porcupine. 



