X.] STRUCTURE OF NERVOUS TISSUES. 107 



occur usually at intervals of about twice the 

 diameter of the field, varying considerably, 

 however, in different nerves. The axis cy- 

 linder may be seen continuing across the 

 nodes. 



c. The nuclei of the primitive sheath ; one to 

 each internodal nerve portion situated about 

 halfway between two nodes, and appearing 

 as a small transparent elongated body pro- 

 jecting usually into the medulla. 



The nuclei of the primitive sheath may be 

 stained by placing a piece of the nerve, after 

 treatment with osmic acid, in Frey's carmine 

 for two or more days. In such a preparation, 

 the cells of the fine connective tissue around 

 the nerve fibres will also be well seen. 



4. Stain with carmine a short piece of a rather 

 thick nerve which has been preserved in ammo- 

 nium bichromate 2 p.c. (cp. Less. I. F. 4 and 

 footnote p. 69). Imbed, and cut thin transverse 

 sections, treat these with creosote and turpentine 

 (cp. p. 75 footnote) and mount in Canada 

 balsam. Observe 



a. The cut ends of the nerve-fibres varying in 

 diameter, and in each 



b. The section of the stained axis cylinder sur- 

 rounded by 



c. A transparent ring indicating the former 

 position of the medullary sheath, which has 

 been dissolved or made transparent in the 



