Dunn, Medullated Nerve Fibers. 713 



to the foot on the operated left side of frog E, there are 1535 medul- 

 lated nerve fibers. On the right side we find 1998 combined afferent 

 and efferent nerve fibers. Hov^r these numbers are distributed to 

 the muscles and skin of the foot we do not know, and an estimation 

 based upon the distribution in the thigh and shank does not seem 

 profitable. At some future time it may be possible to count the 

 number to the skin and to the muscles of the foot. When that is 

 done we may be able to assign the afferent and efferent fibers to their 

 destinations and consider their relations. It is, however, of interest 

 to note that of the total number of afferent fibers to the leg of the 

 frog 45 per cent pass to innervate the thigh, 21 per cent to the shank, 

 and 34 per cent to the foot. 



The Size of the Afferent Medullated Nerve Fibers, and the 

 Level of their Distribution. 



Turning our attention to the size of the medullated nerve fibers, 

 we are able to make a comparison of the size of the largest cutaneous 

 afferent nerve fibers and the largest muscular afferent nerve fibers. 

 This is done for both the thigh and the shank by selecting and meas- 

 uring the largest nerve fibers from primary branches containing 

 approximately the same number of nerve fibers. 



The average areas in square micra are given in Table XYI. It 

 appears that very large afferent nerve fibers pass to the skin and to 

 the muscles, and that in both the thigh and the shank the muscular 

 branches contain the largest afferent fibers. The ten largest fibers 

 in muscular branches to the thigh average 183 square micra, the ten 

 largest in cutaneous branches to the thigh average 174 square micra. 

 For the shank the ten largest fibers in branches to the muscles average 

 139 square micra, the ten largest nerve fibers in branches to the 

 skin average 124 square micra. 



Measurements were made of the largest fibers alone because of 

 technical difficulties. A study of the branches in which purely 

 muscular or purely cutaneous afferent fibers were found, such a 

 study as could be made without measurements, showed that both the 

 muscles and the skin received fibers of various sizes, but that the 

 supply of small fibers to the skin was much more abundant than that 



