330 Sheath Cells and A.xoiie Sheaths in the Central Xervous System 



made upon the spinal coi'd alone, and usually u\)nn pieces taken from the 

 cervical region. Preparations from the adult liog were compared with 

 similar preparations from the adult human. After some study of younger 

 stages, it appeared that only pigs of about IG centimeters in length and 

 above were necessarily concerned in the study of the structures involved. 

 Prior to this all the fibers of the spinal cord are non-medullated or in the 

 very early stages of medullation and none of the nuclei and their sur- 

 rounding protoplasm show evidences of the differentiation in mind. The 

 study, therefore, has chiefly involved fcetal pigs of 16, 19, 21 and 28 

 centimeters, suckling pigs of about two weeks, and the adult. 



Both transverse and longitudinal sections were used, supplemented 

 by teased prejsarations. Some of the sections from each specimen were 

 prepared by the Benda neuroglia method as employed by Huber. Others 

 were made from pieces fixed either in Zenker's fluid or Van Gehuchten's 

 mixture and stained lightly with hematoxylin and counterstained -with 

 Congo red. The latter method was employed in that, with other tissues, 

 Congo red is efficient for bringing out cell outlines. Also sections from 

 certain of the stages after medullation has begun and sections from the 

 adult were stained by Mallory's method for wdiite fibrous tissue. Other 

 sections of the adult spinal cord were subjected to the action of pancreatin 

 by the method of " digestion on the slide " described by Flint. 



The pieces of spinal cord of different ages from which the teased prepa- 

 rations were made were first split with a sharp razor into thin longitudinal 

 strips about one centimeter long and these were fixed in a mixture of 

 equal parts of saturated aqueous corrosive sublimate and 1 per cent osmic 

 acid. In order to fix the strips straight and extended, they were at first 

 merely immersed in the fluid and then placed in adherence to the walls 

 of the vials containing the fluid with the lower end of each strip alone in 

 the fluid. Corking the vials and allowing the fluid to act for 10 or 15 

 minutes was found sufficient to stiffen the strips so that they would remain 

 straight and extended when shaken down into the fluid. After doing 

 this they w-ere allowed to remain in the fluid for 12 to 21: hours. The 

 strips were then washed for several hours in water frequently changed. 



Especially in the younger stages the closely bundled nerve fibers of 

 the spinal cord are so friable that it was found impossible to satisfactorily 

 dissociate them with even the finest teasing needles. Even after dehy- 

 dration and while in clearing oil (in wdiich condition nerve fibers usually 

 tease more easily on the slide than when in water and may be mounted 

 in balsam immediately without disturbing their separated positions) 

 but few pieces of isolated fibers could be obtained of sufficient length for 

 satisfactory study. Teasing in glycerine witli needles gave no better 



