THE PYRAMID TRACT IN THE RED SQUIRREL 

 (SCIURUS HUDSOXIUS LOQUAX) AND CHIP- 

 MUNK (TAJVIIAS STRIATUS LYSTERI) 



SUTHERLAND SIMPSON 



From the Physiological Laboratory, Medical College, Cornell University 



THIRTY-SEVEN FIGURES 



INTRODUCTION 



The nerv'e fibers which form the pyramid tract take origin 

 from the large pyramidal cells of Betz ('09) in the cerebral motor 

 cortex. Entering the corona radiata they converge and pass 

 caudalward through the internal capsule, crusta, pontine bundles 

 and anterior pyramid until the lower part of the medulla oblon- 

 gata is reached. The tract, in its passage downward, gives off 

 fibers or collaterals to the optic thalamus, substantia nigra, 

 nuclei pontis of the same side, and to the nuclei of the cranial 

 motor nerves mainly of the opposite side. The positions occu- 

 pied by the pyramid tract at these different levels are similar in 

 all the mammalian orders that have been investigated, with 

 slight variations, until the lower part of the bulb is reached, 

 but beyond this, where it passes into the spinal cord, great differ- 

 ences are found to exist. 



In man and the anthropoid apes, at the decussation of the 

 pyramids the fibers divide into three bundles; most of them cross 

 the middle line and enter the dorsal portion of the lateral column 

 of the opposite side, forming the crossed pyramid tract of the 

 cord. A second group of fibers, much less numerous than the 

 first, turn away from the median raphe on the same side, without 

 crossing, and take up a position in the lateral column corre- 

 sponding to that of the crossed pyramid tract of the opposite 

 side; this is termed the direct lateral pyramid tract. A third 

 group continue downward from the pjTamid into the cord, 



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