340 Sheath Cells and Axoiie Sheaths in the Central Xervous System 



has proceeded till there are much fewer fibers of types b and c, Fig. 5. 

 At birth there may be found in the white substance, but very rarely 

 indeed, fibers totally void of myelin such as one of those shown in group a. 

 Sheath cells of the form of the seal-ring cells of the 31-centimeter pig 

 are also more difficult to find in the later stages. This is apparently due 

 to their being relatively less numerous and to the fact that when found 

 they show relatively less protoplasm about their nuclei and about the 

 fibers they clasp. 



In the adult especially are unquestionable examples of these cells diffi- 

 cult to find. Usually the protoplasm has apparently been used up or so 



Fig. 6. Types of fibers selected from teased preparations ot spinal cord 

 of adult hog. Fixation, etc., same as in Fig. 4. e and ^7 = types of small, 

 thinly medullated fibers from white substance; fr = framework of sheath 

 from which myelin has been washed in teasing; sc = sheath cells (sheath 

 nuclei); p = peripheral sheath, x 5-50. 



dispersed that little more than the nucleus remains and the only sugges- 

 tion that such nuclei do not belong to the general class of the neuroglia 

 nuclei present throughout the inter-axone spaces, is their flattened shape 

 and their position upon the nerve fibers, and, when observed in the teased 

 preparations, the fact that they are not washed off in the process of teas- 

 ing. It is possible, of course, that during growth neuroglia nuclei proper 

 may be flattened against the medullary sheaths. 



In Fig. 6 are presented some of the types of fibers to be found in the 



