544 S. WALTER HANSON 



ganglia were not removed in these experiments so as to exclude 

 the possibility of regeneration. Only by excluding this possi- 

 bility can the question of efferent dorsal root fibers be settled. 



CONCLUSIONS 



The results of these experiments confirm those of earlier his- 

 tological studies and show that both fine medullated and non- 

 medullated fibers from the dorsal root enter the tract of Lissauer. 



The tract of Lissauer in the Imnbo-sacral region of the cat's 

 cord consists of two parts, approximately a medial and a lateral 

 half. The fibers of the lateral half, both medullated and non- 

 medullated, are of endogenous origin. The fibers of the medial 

 half, both medullated and non-medullated, are in part endoge- 

 nous and in part exogenous. While it may be that in other animals 

 there would not be this sharp division of the tract of Lissauer 

 into medial and lateral halves, we may safely generalize our 

 conclusions so far as to say that the tract of Lissauer is a mixed 

 tract consisting of medullated and non-medullated fibers of both 

 endogenous and exogenous origin. Variations in the shape of the 

 tract in different animals are probably associated with variations 

 in the development and position of the endogenous fibers of the 

 tract. 



Our results do not permit us to trace the non-medullated fibers 

 to their tennination in the cord nor to decide the question of the 

 existence of efferent dorsal root fibers. 



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