80 TISSUES OF THE NERVOUS SYSTEM. 



in common alcohol for a few minutes, and to stain the prepa- 

 ration with carmine. It must then be put in absolute alcohol 

 for twenty to thirty minutes, after previously teasing it out 

 somewhat. If it is allowed to remain twelve hours or more in 

 oil of turpentine, and then covered in dammar varnish, it will 

 be found that all the nerve fibres are more or less completely 

 deprived of their medullary sheaths. The axis-cylinder ap- 

 pears in general to consist of granulous substance, but here 

 and there distinct longitudinal streaking can be recognized. 

 The axis-cylinder can also be freed of its medullary sheath if 

 chloroform or collodion be added to a teased preparation of 

 fresh nerve, which is as nearly dry as possible without being 

 thoroughly desiccated. Occasionally the primitive fibrils of 

 the non-medullated nerve fibres are beset with small varicosi- 

 tics at nearly regular intervals, which, when treated with cer- 

 tain reagents (perosmic acid, chloride of gold), become very 

 distinct. 



Medullary Sheath. In a teased preparation of a fresh 

 sciatic nerve of the frog, in half per cent, salt solution, the 

 individual nerve fibres are seen to be invested by a sheath of 

 transparent, highly refractive material, which, when it presents 

 its surface, appears hyaline, but, as seen at the edge of the 

 nerve, exhibits a double outline. Thus the medullary sheath 

 confers on the nerve fibre a dark edge or double contour; so 

 that these appearances in a nerve are characteristic of its 

 presence. Soon after the preparation has been made, it is ob- 

 served that the sheaths of many fibres become beset with drop- 

 like bodies of irregular form, which are either bright and 

 shining, or granulous and turbid. They are produced by a 

 coagulation of the medulla. In preparations made in iodized 

 serum, the fibres remain, however, for several hours quite 

 smooth, without undergoing this change. In the nervous 

 centres, the medullated fibres which possess no Schwann's 

 sheath often present a necklace-like appearance, due to this 

 coagulation of the medullary sheath (the so-called varicose 

 fibres). The medullary sheath exhibits a remarkable arrange- 

 ment at those points of the course of the nerve at which it 

 divides into two or more branches. At such points the sheath 

 becomes considerabty attenuated, as well as contracted. To 

 show this, the membrana nictitans of a frog is carefully ex- 

 cised, spread out in a drop of humor aqueus, and covered 

 care being taken to introduce strips of paper under the cover- 

 glass so as to prevent pressure. The thoracic cutaneous 

 muscle of the frog may be prepared in the same wa} r . Where 

 a medullated nerve fibre passes into a non-medullated, as in 

 the objects above mentioned, the sheath is usualty thinned out 

 towards the point where it is about to cease, in which case 

 the thin portion ma} 7 either extend up to the line at which it 



