554 FUNCTIONS OF THE CEREBRO -SPINAL NERVOUS SYSTEM. 



and nerves ramify. From the internal surface of this, processes are given 

 off which divide the nerve into smaller bundles or fnn.iculi. And these 

 funiculi, according to Ranvier, are invested by an epithelium exhibited at 

 Fig. 185, consisting of large flat cells. These are considered by some ob- 

 servers to represent the lining of a lymphatic space. 1 On further dissec- 

 tion, the individual fibres may be separated from one another, and in the per- 

 fectly fresh state appear as clear, transparent, and highly retractile threads, 

 in which no trace of structure can be discovered. After short exposure to 

 the action of air, water, and other reagents, a kind of coagulation occurs, 

 and it then appears that each fibre is composed of a very delicate sheath, 



FIG. 185. 



ffi- 



FIG. 185. Funiculus of a mouse after impregnation with nitrate of silver. Large flat epithelial 

 cells are seen covering its surface. The explanation of the small crosses is seen by reference to the 

 next figure. 



FIG. 186. Nerve-fibre from the sciatic nerve of the rabbit after the action of nitrate of silver: a, 

 ring formed by thickened membrane of Schwann ; TO, white substance of Schwann rendered trans- 

 parent by glycerin ; Cy, cylinder-axis, which just above and below the level of the annular constriction 

 presents the stria; of Frommann. 



the sheath of Schwann, or neurilemma (1, Fig. 187), within which is an ole- 

 aginous material termed the white or medullary substance of Schwann (2), 

 whilst the core or centre is formed by an albuminous round or flattened 

 thread, the axis-cylinder, or primitive band (3). Ranvier 2 de- 

 FIG. isy. scribes certain annular constrictions (Fig. 186) on all medullated 

 nerve-fibres at intervals of about Tvjth of an inch, which may be 

 brought into view with various reagents, as the picro-carminate 

 of ammonia and osmic acid, and which he thinks are caused by 

 a thickening of the membrane of Schwann. The constrictions 

 appear to divide the white substance of Schwann into segments, 

 each containing a nucleus. The cylinder-axis runs continuously 

 through the constriction without presenting any trace of seg- 

 ii:i-iamof nicntation, except that when the libre is treated with nitrate of 

 Kfnid.uri! of silver, it presents a laminated aspect, just above and below the 

 Nerve-fibre, constriction. No such constrictions are presented by the fibres 



1 Tamamschcf, Centralblatt fiir die Mecl. Wiss., 1872, p. 593, who also gives a com- 

 parative; table (if llie action of various reagents on the cylinder-axis and on albumen. 

 Sec ;ils. i A. v. Tu'rok, Wiirzburg Physikal-Med. Ve.rhand., Bd. iii, 41. 



2 Brown-86quard's Archives de Physiologic, 1872, p. 1 _'!), No. 2. Sachs, Central- 

 blatt, 1873, p. 578. An account of llanvier's discovery will be found in the Lancet, 

 1872, vol. i, p. 477. 



