NER VE- TISS UE. I Of 



from the animal, and swells and assumes a multitude 

 of bizarre forms on addition of water ; it is soluble 

 in alcohol, chloroform, and ether ; and, like fat, is 

 hardened and turned black under the action of osmic 

 acid. The medullary sheath does not form a con- 

 tinuous tube, but at tolerably regular intervals is 

 separated into segments. 



3. The neurilemma or sheath of Schwann is an 

 extremely thin, structureless, membranous tube, 

 which tightly encloses the medullary sheath, and, 

 like the latter, is broken up into segments. At the 

 ends of the segments is a constriction around the 

 fibre, at the expense of the medullary sheath, and 

 the ends of the neurilemma segments are joined 

 together by a thin layer of cement substance, which 

 extends inward to the axis cylinder. There is rea- 

 son to believe that the neurilemma extends inward 

 and between the medullary sheath and the axis 

 cylinder, entirely enclosing the segments of the me- 

 dullary sheath. 



Within each neurilemma segment, called interannu- 

 lar segment, and about midway between the constric- 

 tions, lies a flattened, elongated, elliptical nucleus. 

 We may regard the neurilemma segments, with 

 their nuclei, as cylindrical cells cemented together, 

 end to end, enclosing the segments of the medullary 

 sheath and surrounding the axis cylinder, the latter 

 passing uninterruptedly through the axis of the seg- 

 ments. In addition to these structural features, we 



