326 



Journal of Comparative Neurology. 



that the dimunition in the diameter of the fiber occurs in a large 

 nerve trunk and at a considerable distance from any peripheral 

 branches. 



5. Area of the axis cyhnder substance proportional to the 

 area of the section. 



In the discussion of the innervation of the various por- 

 tions of the hind extremity of Rana virescens we have neces- 

 sarily included both the medullary sheath and the axis cylinder 

 in the determination of the average areas of the nerve fibers. 



In attempting to eliminate as completely as possible all 

 sources of error, there arose the question of a possible change 

 from the normal proportion of the medullary sheath to the axis 

 cylinder in these sections. Measurements were therefore made 

 at the three chief levels to ascertain the existing proportion of 

 the average area of the ten largest fibers at each level to the 

 average area of their axis cylinders. 



The results are embodied in Table XVIII. 



TABLE XVIII. 



Showing the average areas for the ten largest fibers and for their axis 

 cylinders. 



Reference to this tabulation shows that at each of the 

 three levels the average area of the axis cylinders of the ten 

 fibers measured is approximately one-half that of the average 

 areas of the entire fibers. Hence the 1:1 relation (Donaldson, 

 1895) of the medullary sheath to the axis cylinder prevails 

 here also. 



The results at each level are so nearly uniform that we are 

 justified in the conclusion that, in all the measurements, not 

 only the areas of the entire fibers bear certain relations to one 

 another, but that their most actively functional portions, the 



