144 'Journal of Comparative Neurology and Psychology. 



of skin and weight of muscle, the relative sensory supply is supe- 

 rior only in the head and trunk, but not in the skin and muscles 

 of the limbs. 



Such anatomical differences as these just described, suggest 

 corresponding physiological differences between the two species. 

 In pursuance of this suggestion, I sent out a letter of inquiry to 

 my physiological colleagues in May of 1907. I take this oppor- 

 tunity of thanking my numerous correspondents for their courteous 

 replies, but at the same time must report with regret that there 

 do not appear to be any data bearing on the possible physiological 

 differences, concerning which inquiry was made. The number 

 of medullated nerve fibers in the spinal nerve roots of R. tempo- 

 raria, has still to be determined. 



A comparison of the length of the internodal segments in the 

 fibers of the sciatic nerve of R. pipiens and R. esculenta. — As the 

 heading indicates, the comparison will here be limited to fibers 

 from one nerve. Boycott ('04) has determined the length of the 

 internodes in fibers taken from the sciatic nerve just at the point 

 where it divides into the nervus tibialis and the nervus peroneus. 

 The length of the internodes at this locality depends on two fac- 

 tors, first the size (length) of the frog, and second the diameter 

 of the fiber; the internodes becoming longer, the larger the frog 

 and the greater the diameter of the fiber examined. 



By a study of specimens of R. temporaria of different lengths. 

 Boycott was able to show that the average length of the internodes 

 of fibers of all diameters taken from the sciatic, increased in the 

 same proportion as the length of the sciatic, the curves represent- 

 ing the two series of measurements running parallel. Accepting 

 this result, it is possible to calculate the average length of the inter- 

 nodes at this point for frogs of different sizes. 



Below is given a table containing the data on the six largest 

 specimens of R. temporaria examined by Boycott ('04, p. 375). 

 These are arranged in the order of increasing body length, the 

 measurement being made from the tip of the nose to the end of the 

 urostyle. 



The body weight here given was taken as usual. The length 

 of the sciatic in millimeters is defined by Boycott {loc. cit., p. 

 371), as follows: "The upper end has been taken throughout as 

 the point of emergence from the vertebrae of the upper of the two 

 larger branches of the plexus. 



