Dunn, Nerve Fibers of the Frog. 327 



axis cylinders, stand in the same relations — a fact that increases 

 somewhat the value of this series of observations. 



6. Comparison of the average areas /i^r the fibers with those 

 determined by previous measurements. 



While the general relations of the results of these meas- 

 urements are much the same as those obtained by earlier 

 investigations, so far as the thigh is concerned, yet the resulting 

 figures are considerably lower in value than those heretofore 

 obtained. This may be due to one of several causes. 



First, the proportion of fibers of various sizes may be dif- 

 ferently adjusted in the various frogs. Concerning the numbers 

 and proportions of the fibers of varying sizes in the peripheral 

 nervous system we have no definite information. A fact that 

 seems to negative the probability of changed proportions is the 

 observation that the ten largest fibers at any level are in this 

 series of observations less in diameter than the ten largest fibers 

 at the same level in any previous series of observations. 



Again, some change may have taken place in the individ- 

 ual nerve fibers during the process of preparation of the sec- 

 tions, but if a shrinkage of the fiber in its entirety had taken 

 place the sections would exhibit increased spaces between the 

 fibers. No such separation of fibers appears in these sections. 

 Or, if the change were in the axis cylinder alone, or in the 

 medullary sheath alone, the usual relation of 1:1 existing 

 between the areas of these two portions would be lost. 



Table XIX, showing the average areas of the nerve fibers 

 and of their axis cylinders in Frog B of the first series and 

 Frog B of the second series, shows but slight deviation in either 

 frog from the usual 2: i relation of the fiber to its axis cylinder 

 or of the 1:1 relation between the axis cylinder and the medul- 

 lary sheath. The smaller diameter of these ten fibers in Frog 

 II B, the subject of the present study, is also shown in this 

 table. 



