Donaldson and Hoke, Medullajy Sheath. 5 



The results of these observations seem to justify the con- 

 clusion that the osmic acid treatment, followed by the prepara- 

 tion for examination, produces but little change from the nor- 

 mal diameter (or area) of peripheral nerve fibers. 



Conditions Dcternwnng the Choice of the FibcTS to be Meas- 

 ured. — There were used for measurement only those fibers 

 which had been cut at right angles to their long axis and which 

 stood vertically. It is easy to see that any departure of a fiber 

 from the vertical would make the measurements for the thick- 

 ness of the sheath too large, and those for the axis correspond- 

 ingly too small. From measurement M'ere excluded those 

 fibers in which the medullary sheath appeared double, as occa- 

 sionally occurs when the fiber has been cut through an enlarged 

 cleft of Lantermann. 



The rare instances in which the section passes through an 

 internodal nucleus, or just above or below a node — where the 

 relative area of the medullaiy sheath is very greatly increased — 

 were easily avoided. A fiber in which the sheath was wrinkled, 

 or which departed much from the circular form, was not meas- 

 ured. Where the section of the nerve fiber was suitable in 

 other respects — and at the same time was slightly oval — two 

 diameters were taken and the mean taken as the value to be 

 used. Among the small fibers those that were stained gray 

 and not black were classed as immature, and were not meas- 

 ured. It is among the small fibers that the greatest normal 

 range in the relative development of the medullary sheath ap- 

 pears, and it is here too, that the greatest difficulties in making 

 exact measurements are met, any departure from the vertical 

 being especially disturbing. 



The very small medullated fibers which appear in the rami 

 communicantes were not studied in this investigation. 



Method of Measurement. — On sections of fibers thus pre- 

 pared and thus selected, the diameter of the entire fiber was 

 first measured and then the diameter of the axis. In order 

 to give an idea of this procedure we may take as an example 

 the first group of ten measurements, the final results of which ap- 

 pear in Table VI after Specimen I. In the first column of Table 



