^6 Journal of Comparative Neurology. 



entire numbers previously found, yet it is seen that by far the 

 greater portion of a difference does occur among the small 

 fibers ; i. e. , those 5 micra or less in diameter. The lack of 

 complete accordance is perhaps due to fibers whose diameters 

 were so near the border line between large and small that a 

 slight distortion, or the lack of it, resulted in their being 

 classed in the one category at one level and in the other at 

 another. 



These counts for small fibers were of dorsal and ventral 

 roots exclusively. If the variations in the number of fibers in 

 the roots can be shown to take place in the smaller fibers, it is 

 reasonable to assume that the variations found in the trunk, oc- 

 cur among the small fibers also. 



Birge ('82) made counts of the fibers at a single level of 

 the two roots and of the trunk of frogs of different weights or 

 ages. In regard to the fibers of the ventral roots, he found 

 upon summing up the entire number of motor fibers deter- 

 mined for one side of each animal, that there existed such pro- 

 portions between the numbers of these fibers and the weights 

 of the frogs, that, given the one he could compute the other 

 approximately. Assuming the number of ventral root fibers 

 to be the same for the two sides of the animal, he found that 

 an increase in the weight of the frog was accompanied by an 

 increase of about 5 5 ventral root fibers for each gram of body 

 weight. 



This author also determined that the average diameter of 

 the fibers in the roots of the younger specimens was less than 

 that in the older and expressed the opinion that, during growth, 

 small fibers thicken into larger ones. 



Korybutt-Daskiewicz ('78) as early as 1878 described the 

 ingrowing of new fibers in the sciatic nerve of the frog. He 

 pictures the first formation of the medullary sheath about the 

 outgrowing neuraxone, either as fine varicosities which blacken 

 with osmic acid, or as first a thin layer, of myelin which gradu- 

 ally thickens. These changes were noted near the end of the 

 growing fiber. The sheath was observed to cease, not gradual- 

 ly, but more as the wood portion of a sharpened lead pencil, 



