74 Journal of Comparative Neurology. 



and the ramus communicans. This explanation of the changes 

 as due to growth will be discussed later. 



VI. The Role Played by the Smaller Fibers. 



It was suggested that it would be well worth while to de- 

 termine whether the decrease in the number of fibers in the 

 roots and trunk occurs through variations in the number of the 

 large or the small fibers. It is known from previous descrip- 

 tions that the two roots differ from each other in the general 

 appearance of their transverse sections. The ventral root shows 

 much more uniformity in the diameter of its fibers than does 

 the dorsal. The ventral roots of the spinal nerves of the frog, 

 while not possessing the very largest fibers, yet, with the excep- 

 tion of the ventral root of the Ilird nerve, have the great ma- 

 jority of their fibers uniformly large. This excess of large 

 fibers is interspersed with fibers distinctly small and with a very 

 few intermediate in size. Plates VIII, X, and XII represent 

 sections of the ventral root of the Vlllth nerve which is quite 

 typical. 



The dorsal root, on the other hand, shows no uniformity as 

 to the diameter of its fibers. It is rather a mixture of fibers of 

 all sizes from the very small up to a few larger than any found 

 in the ventral root. 



In order, therefore, to find if possible, in fibers of what 

 general diameter the above changes in number occur, a second 

 count was made of ten of the sections of the roots. Sections 

 of the smaller roots were chosen since the labor of counting 

 would be less and the results obtained would be fully as signifi- 

 cant as in case of larger ones. 



As will be explained in that part of this paper which deals 

 with the methods employed, each count was made from a pho- 

 tograph of the section and controlled by having the section at 

 at the same time under the microscope and highly magnified. 

 Thus the use of the photograph guarded against any fibers be- 

 ing overlooked or recounted, and all doubtful cases could be 

 settled at once and accurately by an appeal to the microscope. 

 In the investigation with regard to the small fibers, all those 



