Donaldson and Hoke, Medullary Sheath. 13 



The deviations observed. — Table VI shows that the aver- 

 age deviation for all the 1540 fibers is +0.45 %. 



On determining the deviations separately for the Fish, 

 Amphibia, Reptiles, Birds and Mammals we obtain the follow- 

 ing results : 



, Number of Fibers Deviation 



- 0.46% 

 + i-70fo 



- i.yofc 



+ 1-40/0 



This tabulation serves to show that in no class do the de- 

 viations rise very high, and that with the exception of the 

 mammals, which have been commented on already, the aver- 

 age deviation tends to diminish as the number of cases increases. 



On further looking over the individual deviations, as shown 

 in the original records, it appears that in 580 cases (or 37% of 

 1540), they are more than 6% plus or minus — 266 cases being 

 minus and 314 plus. Thus the excessive deviations are rather 

 evenly divided on either side of the average. 



It follows from this that 63 % of the cases deviate only 6% 

 or less, or are within the anticipated error of observation. 



From the foregoing it is concluded that despite the indi- 

 vidual variations which may be often quite large, there is no 

 tendency for the sheath to depart materially from the ideal 

 standard, when this relation is determined by averaging the 

 measurements obtained from groups of twenty or more fibers. 



Figure illustrating variatiofis in the thickness of the medul- 

 lary sheath. — In order to give a meaning to the numbers that 

 appear in the foregoing Table VI, the measurements applying 

 to Specimens I, XI, XXII, XXIV, XXVIII, and XLV have 

 been selected for illustration (see Fig. i). This selection gives 

 one specimen from each of the great classes below the mam- 

 mals. In the case of each specimen, the diameter of the aver- 

 age fiber is taken as a standard, and this is shown, enlarged one 

 thousand diameters, (i) with a sheath having just half the area 

 of the entire fiber (Ideal); (2) with the sheath determined from the 



