306 Journal of Comparative Neurology. 



thigh^branches cannot be found in the sciatic nerve at a level 

 below the thigh branches, but do appear among the fibers pass- 

 ing by the branches to the thigh. 



These two facts pointed so strongly to the necessity for 

 a revision of our beliefs regarding the destination of the largest 

 nerve fibers and the interpretation of the variations in the size 

 of nerve fibers that a more complete study of the number and 

 diameter of the nerve fibers at stated levels, increasing in dis- 

 tance from the spinal cord, seemed very desirable. A study of 

 the fibers in the branches to the thigh and to the shank was also 

 fortunately completed upon the same frog, thus giving data for 

 comparison which were all obtained from a single specimen. 



So many very small branches required to be sectioned in 

 this examination that a frog of the maximum size was selected. 



The choice in this second series of observations fell upon a 

 frog, designated as Frog II B, female, corrected weight 61.5 

 grams, length 234 mm. Both the enumeration of the fibers and 

 the calculation of the areas were carried through for the right 

 and left hind extremities. 



I. Levels at which observations were made. 



The exact points at which observations were made are indi- 

 cated in Figure 2 by the breaks in the continuity of the nerve 

 trunks and branches. Figure 2 is a reproduction of the main 

 branches shown in Figure i. The designations are those used 

 for Tables I and II and Figure i, with the addition of S^ S., , and 

 T + P, to indicate the levels on the main trunks at which sec- 

 tions were made. 



The levels on the main trunks chosen for comparison were 

 four; the first at the point just above the branches to the thigh, 

 indicated by C and S^ ; the second at a point just below the 

 thigh branches at S^ . The data furnished by the section at 

 S , just above the point of division of the sciatic nerve, acted 

 as a control upon the results obtained at S, . The third level 

 was at a point just above the branches to the shank, cutting 

 the tibial and peroneal nerves at T and P. The section at T 

 4- P, the immediate point of division of the sciatic nerve, fur- 



