Donaldson and Hoke, Medullary Sheath. 15 



average of all the measurements (average observed) ; (3) with 

 the smallest sheath observed in the series, and (4) with the lar- 

 gest sheath observed in the series (extreme variations). 



B}' the study of Fig. i, some notion of the range in the size 

 of the sheath is obtained, but at the same time it must be re- 

 membered that these extreme cases as seen in (3) and (4) are 

 exceptional, and that most of the deviations in the sheath are 

 of decidedly less extent. 



ConcliistoJis. 



The measurements in Table VI have been conipared with 

 one another in so far as they permitted the comparison of (i) 

 nerves having different diameters ; (2) those afferent or efferent 

 in function ; (3) taken from various parts of the body ; (4) in 

 males and females; (5) at different seasons; (6) at different 

 ages ; and (7) in animals active and those slow in their move- 

 ments. 



From such comparisons there is no indication that any of 

 these conditions modify the relative development of the medul- 

 lary sheath. 



To make these statements with regard to area the basis 

 for a statement with regard to the relative volumes of axis and 

 sheath, is justified for the reason that the medullary sheath in 

 the peripheral system is very nearly coextensive with the en- 

 closed axis cylinder, and the short stretches at either end of the 

 fiber would serve to give only a slight increase in the volume of 

 the axis. Since in cross sections the sheath is on the average a 

 trifle in excess of the axis, this difference would tend to make 

 the similarity in volume even closer. 



If the foregoing statements are well founded it should fol- 

 low that the figures which represent cross sections of nerve 

 fibers treated by the osmic acid method and which have been 

 published in special articles or in text-books should accord with 

 the results here given. 



We have examined such figures as they appear in Ranvier 



('78), KUPFFER ('83), GasKELL ('85), CaJAL ('99), HUBER ('OO), 



and find that when the conditions are observed of selecting only 



