TRACT OF LISSAUER 121 



SUMMARY 



1. The tract of Lissauer is present in man and in the monkey, 

 cat, rabbit, squirrel, rat and guinea-pig, and possesses the same 

 structure in all. 



2. It is composed of small, somewhat widely separated, med- 

 ullated fibers, and great numbers of fine non-medullated axons. 



3. While medullated fibers can be traced from the dorsal root 

 into Lissauer's tract in some animals (cat and monkey), these 

 fibers are not numerous. Some of these run horizontally through 

 the tract to enter the substantia gelatinosa. It is probable that 

 a few of them turn vertically in the tract to lose themselves among 

 the other vertical medullated fibers of the tract. 



4. It is clear that the number of medullated fibers entering the 

 tract from the dorsal root is not sufficient to account for all the 

 vertical medullated fibers found there, and that many, probably 

 a majority of these vertical medullated fibers are of endogenous 

 origin. This conclusion is in keeping with the results obtained 

 from the study of the degeneration within the cord produced by 

 lesions of the dorsal roots. 



5. Great numbers of non-medullated fibers can be traced into 

 the tract from the dorsal roots in man and in all the animals 

 studied. These root fibers form a large part, perhaps the major 

 part, of the non-medullated fibers of the tract. 



6. In all cases there is manifested a tendency for the tract to 

 spread out into the lateral funiculus at some level in the cord. 

 This is particularly evident in the monkey and in the rat. Since 

 there are no oblique fibers running ventro-laterally from the 

 dorsal part of the tract into this lateral expansion of the tract, 

 it seems quite certain that the fibers located here are not de- 

 rived from the dorsal root. This leads to the conclusion that 

 this lateral part of the tract is of endogenous origin. It would 

 seem probable, therefore, that a part of the non-medullated fibers 

 in the tract of the cat and other animals in which this lateral 

 expansion is not so well developed are also of endogenous origin. 

 Indeed, it would seem that the variations in the shape of the tract 

 in different animals were chiefly due to variations in the position 

 of the endogenous non-medullated fibers. 



