THE SPINAL CORD AS A CONDUCTING PATH 



615 



localized in these fasciculi at different levels of the cord. But, the 

 origin and distribution of the latter are still rather obscure so that the 

 following physiological classification must necessarily be subject to 

 frequent revision. 



1. Descending Tracts, (a) Pyramidal tracts. — We have previously seen that 

 the fibers composing the direct (anterior) and crossed (lateral) pyramidal tracts, 

 originate in the large cells of Betz of the motor 

 areas of the cerebrum. Hence, an injury to 

 these regions or a transverse division of these 

 paths at a lower level must result in a down- 

 ward degeneration of these tracts. It should / 

 be remembered, however, that by far the largest/ '' 

 number of these fibers cross to the opposite side^' 

 so that, say, the left cerebral hemisphere eventu- 

 ally obtains control over the musculature of the 

 right side of the body, and vice versa. Only a 

 few fibers remain on the same side, where they 

 eventually enter the lateral column. The afore- 

 said crossing is effected principallj^ in the pyra- 

 midal decussation in the lower region of the 

 medulla, but in part also in the spinal cord itself. 

 Thus, it appears that the crossed pyramidal 

 tract is made up of fibers which have gained 

 the opposite side in the medulla, while the an- 

 terior pyramidal tract comprises in addition a 

 certain number of fibers which have failed to 

 cross in the medulla but which seek the opposite 

 side gradually by way of the anterior commis- 

 sure. As this crossing is completed in the mid- 

 dorsal region, these anterior tracts disappear 

 at this level. In fact, it is said that they are 

 entirely wanting in about 15 percent, of human 

 spinal cords, because in these cases the decussa- 

 tion is had solely in the medulla, the fibers being 

 distributed from here exclusively to the crossed 

 pyramidal tract. ^ This condition also prevails 

 in the cat, while in the mole the fibers remain 

 uncrossed and descend anteriorly. In the frog 

 this system is absent. 



It may be concluded, therefore, that the 

 pyramidal tracts are efferent in their nature and 

 form the motor path for those impulses which 

 originate in the motor cells of the cerebrum and 

 are finally transferred to the large motor neurons 

 in the anterior horn of the spinal gray matter, 

 whence they are distributed to the skeletal mus- 

 culature. From this discussion it may be in- 

 ferred that they are the chief constituents of 

 the efferent side of the cerebral projection sys- 

 tem. Hence, any injury to this path must re- 

 sult in a loss of voluntary control over the action of the corresponding skeletal 

 muscles, but naturally, the ordinary reflex movements of the cord are not inter- 

 fered with unless the lesion is situated at a low level. High lesions of the pyra- 

 midal system, as has been stated above, really tend to exaggerate the activity 



^Simpson, Quart. Jour, of Exp. Physiol., viii, 1914, 79; also: Lenhossek, Bau 

 des Nervensystemes, 1895. 



Fig. 305. — Schema Represent- 

 ing THE Course of the Fibers of 

 THE Pyramidal System. 



1, Fibers to the nuclei of the 

 cranial nerve; 2, uncrossed fibers 

 to the lateral pyramidal fasciculus ; 

 3, fibers to the anterior pyra- 

 midal fasciculus crossing in the 

 cord ; 4 and 5, fibers that cross in 

 the pyramidal decussation to 

 make the lateral pyramidal tract 

 of the opposite side. {Howell.) 



