122 S. WALTER R ANSON 



7. The narrow band-like tract in the rats and guinea-pigs 

 covers a relatively small area. We can best explain the small 

 size of the tract in these animals by assuming that most of the en- 

 dogenous fibers are located in the lateral expansion of the tract 

 in the lateral funiculus, and that the tract of Lissauer proper 

 is in these animals chiefly composed of exogenous fibers. We 

 need, however, more information on this subject. 



8. The tract of Lissauer and the substantia gelatinosa Rolandi 

 are intimately related to each other. But there seems to be no 

 satisfactory explanation at present for the fact that in the rat 

 and guinea-pig, where the substantia gelatinosa is excessively 

 developed, the tract of Lissauer (exclusive of the lateral expan- 

 sion) is least well developed. Sano's idea, that the enlarged 

 substantia gelatinosa in a purely mechanical way pushes the 

 tract of Lissauer lateralward, does not seem to be satisfactory. 

 If this were the case, we would expect to find oblique fibers 

 marking the course from the dorsal roots to the lateral expansion 

 of Lissauer's tract. 



9. Two layers can be distinguished in the substantia gelati- 

 nosa Rolandi and a third at the border between it and the ven- 

 trally-lying gray substance. 



10. The most superficial layer of the substantia gelatinosa is 

 the stratum zonale. It contains many non-medullated axons 

 and fewer medullated fibers. It is most abundant imme- 

 diately beneath the tract of Lissauer, and between the two 

 there is a free interchange of fibers. From these facts one 

 may assume that this layer serves as a means by which the 

 fibers of Lissauer's tract reach the substantia gelatinosa. 



11. The second layer, the substantia gelatinosa proper, con- 

 tains a plexus somewhat less dense than that in the preceding 

 layer. The fibers are almost all non-medullated. 



12. The intermediate layer at the boundary between the sub- 

 stantia gelatinosa and the ventrally-lying gray matter is a dense 

 plexus of fibers mostly non-medullated. A majority of these 

 fibers have a vertical course. Rosenzweig has spoken of them 

 as a special longitudinal conduction path formed of non-med- 

 ullated fibers. 



