622 STRUCTURE AND ARRANGEMENT OF THE NERVE ELEMENTS. 



and covered here and there with oval nuclei. Dilute acids clear the fibers, 

 without swelling. They are stained brownish red by gold chlorid. They occur 

 in large numbers in the sympathetic nerves. In embryonal life all nerves, as 

 well as the nerves of many invertebrates, are of this variety. In some situations 

 several axis-cylinders are present within one sheath. These are designated Re- 

 mak's fibers. They occur chiefly in the sympathetic system and in the olfactory 

 nerves. 



4. Axis-cylinders or nerve-fibrils surrounded only by a medullary sheath are 

 present in the white and gray substance of the central nervous system, also in 

 the optic and auditory nerves. After death they exhibit a tendency to undergo 

 varicose and nodular thickening in certain areas in consequence of the coagulation 

 of the medullary substance ; hence they are also designated varicose -fibers. Osmic 

 acid acts upon these fibers only imperfectly; otherwise the medulla exhibits the 

 same characteristics as the fibers of the following category. 



5. The medullated fibers, with a sheath of Schwann (Fig. 220, 5, 6), occurring 

 principally in the cerebrospinal nerves, but also in small number in the sympa- 

 thetic nerves, exhibit the most complicated structure. They vary in width from 

 i to 22.6 n. The essentially nervous element of these fibers is the axis-cylinder 

 (Fig. 220, 6, a) , which occupies from one-fourth to one-sixth of the width and is sur- 

 rounded by nerve-marrow like the wick of a candle. Generally it is somewhat 

 flattened, and at times it is somewhat eccentric (Fig. 220, 7). Otherwise the axis- 

 cylinder is composed of fibrils. Its consistence during life is that of semiliquid 

 protoplasm or even more fluid. According to Kupffer a fluid (neuroplasm) is pres- 

 ent between the fibrils. 



Chloroform and collodion render the axis-cylinder visible. It is most readily 

 isolated by means of nitric acid with an excess of potassium chlorid. On treat- 

 ment with silver nitrate, Frommann noted the appearance in places of striation 

 transverse to the axis-cylinder (Fig. 220, 8), the significance of which could not 

 be determined. 



The axis-cylinder is surrounded by the medullary sheath, which in the fresh 

 state is homogeneous and strongly refracting. It is at the same time of fluid 

 consistence, so that it exudes in globular drops (x) from the cut surfaces of the 

 fibers. After death, however, or under the influence of heterogeneous fluids, the 

 medullary substance at first retracts somewhat from the sheath, and as a result 

 the fiber exhibits a double contour. Then the substance by a process of emulsifi- 

 cation breaks up into larger and smaller globules, which tend to lie close together. 

 Peculiar broken-up masses are thus formed in the nerve-fiber, giving it its charac- 

 teristic appearance (Fig. 220, 6). 



The medullary sheath is strongly refracting and positively uniaxially doubly 

 refracting. The optically active body is lecithin. The substance of the medul- 

 lary sheath is especially rich in cerebrin and lecithin, which swell up in warm 

 water and assume similar forms, which have been well named myelin forms. 

 Ether, chloroform, and benzine increase the transparency of the fibers (with disap- 

 pearance of the double refraction) by solution of the fat-like constituents. Osmic 

 acid stains them black. 



Immediately surrounding the medullary sheath is the sheath of Schwann or 

 neurilemma (Fig. 220, 6, c), a delicate, structureless membrane, resembling the 

 sarcolemma. It contains disseminated oblong readily stained nuclei. On addition 

 of acetic acid, and in chromic-acid preparations, this sheath appears in part 

 isolated. 



The sheath of Schwann exhibits, in the case of thick fibers at intervals of 

 considerable length, in that of thin fibers at somewhat shorter intervals, the 

 nodes of Ranvier (Fig. 220, 6, t t, and Fig. 221, f s}. These are annular con- 

 strictions, about which the myelin is wanting. Between each two constrictions 

 is a nucleus, so that such a segment of the fiber is equivalent to a cell from 

 which it may be considered to have originated. At the annular constrictions the 

 nutritive plasma probably enters the fiber for the axis-cylinder, as stains are able 

 to penetrate at this point (8) ; probably also the products of metabolism are re- 

 moved by the same channels. Apparently two segments of the sheath of 

 Schwann are united by cement-substance at the annular constriction. 



The axis-cylinder exhibits at the situation of the annular constriction regu- 

 lar preexisting interruptions, as can best be demonstrated by treatment with 

 silver nitrate. And although the discoverer Engelmann does not believe that in 

 the living fiber a dividing layer of microscopic thickness is interposed in the an- 

 nular constriction between each two adjacent segments of axis-cylinder, yet such 



