2 Journal of Comparative Neurology and Psychology. 



(Donaldson, 1895, p. 154). The fact that it was maintained 

 in the growing fiber was also noted (Donaldson, 1901, p. 180; 

 1901, A, p. 326); and later Dunn ('go, '02) verified the relation 

 by the study of the meduUated fibers in the sciatic nerve of the 

 frog. It seemed desirable, however, to extend the observations 

 on this point, and in 1901, Mr. Hoke took up the question and 

 determined the relative area of the axis and sheath in cross sec- 

 tions of fibers from the nerves of 27 species of vertebrates rep- 

 resenting the five great classes.^ 



Technique. — The animals were killed with chloroform. 

 The nerve, usually from the brachial plexus, laid bare and par- 

 tially fixed in sitii with osmic acid (i ^ sol.). After half an 

 hour the nerve was removed on a piece of cardboard to prevent 

 shrinking, and replaced in a i ^ solution of osmic acid for 24 

 hours. Then imbedded in paraffin by the usual method. The 

 sections were cut 3.5 a thick and mounted in colophonium. 



The measurements of the large fibers were made under the 

 magnification of 340 diameters, or, in some cases, 265 diame- 

 ters. The very small fibers were measured with the 1-12 oil 

 immersion. 



Tlic Effect of Osmic Acid Treatment upon the Size of 

 MeduUated Peripheral Nerves. — This reaction was studied be- 

 cause of its obvious bearing upon the observations here pre- 

 sented. 



Boll ('76) states that i % osmic acid causes a swelling of 

 the sheath to almost double its normal size. This, however, 

 was the result of putting a fiber which had been teased out 

 ivJiile fresh, in a drop of the acid and examining it after a short 

 time. Prolonged immersion in osmic, he says, in the same 

 place, is followed by a shrinkage of the fiber. These statements 

 lose value by reason of the fact that teasing out the fresh fibers 

 stretches them and thus alters their reaction to the reagent. 



BovEKi ('85) concludes that the normal form and structure 

 of the medullary sheath is very little modified by the osmic acid 



' Mr. Hoke's account of his work was accepted as a thesis for the degree of 

 Master of Science in the department of Neurology of the University of Chicago 

 in 1902. The data in that thesis form the basis of the present paper. 



