5 i8 TEXT-BOOK OF PHYSIOLOGY. 



smaller as its fibers cross the anterior commissure to enter the gray 

 matter of the opposite side. Thus all the fibers of the pyramidal tract 

 from each cerebral hemisphere eventually are brought into relation with 

 the cells of the gray matter of the opposite side of the cord. 

 That the pyramidal tracts are the conductors of volitional impulses 

 throughout the length of the cord to its various segments has been made 

 evident by the results of section, electric stimulation, and disease. Division 

 of the anterior and lateral columns of one side of the cord in any part of its 

 extent is invariably followed by a loss of motion or paralysis of the muscles 

 below the section, while electric stimulation of the peripheral end of the 

 isolated crossed pyramidal tract is followed by marked characteristic move- 

 ments of the muscles. Similar results follow division of the pyramidal tract 

 in any part of its course from the cerebral cortex downward. Electric 

 stimulation of the cortical cells which give origin to the pyramidal tract is 

 also followed by contraction of the muscles of the opposite side, while their 

 destruction is attended by paralysis of the same muscles. As the nutrition of 

 the fibers is governed by the cells, it follows that when the axon is separated 

 from its cell-body it degenerates. It has been found that a lesion of the 

 pyramidal tract in any part of its course is followed by descending degenera- 

 tion, which is taken in evidence that it conducts nerve impulses from above 

 downward. Thus experimental investigation and pathologic observation are 

 in accord in the view that physiologically these nerve-fibers are the pathways 

 for the transmission of motor or volitional impulses from the encephalon 

 to the spinal cord. 



