STRUCTURE OF THE NERVOUS SYSTEM 



725 



their opaque,, white aspect. When perfectly fresh they appear to be homogeneous; 

 but soon after removal from the body each fiber presents, when examined by trans- 

 mitted light, a double outline or contour, as if consisting of two parts (Fig. 630). 

 The central portion is named the axis-cylinder; around this is a sheath of fatty 

 material, staining black with osmic acid, named the white substance of Schwann 

 or medullary sheath, which gives to the fiber its double contour, and the whole 

 is enclosed in a delicate membrane, the neurolemma, primitive sheath, or nucleated 

 sheath of Schwann (Fig. 633) 



-d 



FIG. 629. Nerve cells of kitten, showing neurofibrils. (Cajal.) a. Axon. 6. Cyton. c. Nucleus, d. Neurofibrils. 



The axis-cylinder is the essential part of the nerve fiber, and is always present; 

 the medullary sheath and the neurolemma are occasionally absent, expecially at 

 the origin and termination of the nerve fiber. The axis-cylinder undergoes no 

 interruption from its origin in the nerve center to its peripheral termination, and 

 must be regarded as a direct prolongation of a nerve cell. It constitutes about 

 one-half or one-third of the nerve fiber, being greater in proportion in the fibers 

 of the central organs than in those of the nerves. It is quite transparent, and is 

 therefore indistinguishable in a perfectly fresh and natural state of the nerve. 

 It is made up of exceedingly fine fibrils, which stain darkly with gold chloride 

 (Fig. 632), and at its termination may be seen to break up into these fibrillae. The 

 fibrillse have been termed the primitive nbrillae of Schultze. The axis-cylinder is 

 said by some to be enveloped in a special reticular sheath, which separates it from 

 the medullary sheath, and is composed of a substance called neurokeratin. The 

 more common opinion is that this network or reticulum is contained in the white 



