336 



OF THE PRIMARY TISSUES OF THE HUMAN BODY. 



continuous sheath, which isolates the contained matter from the surrounding 

 tissue, along the whole course of the nerve-trunk, from its central to its peri- 

 pheral extremity. When the nerve-fibres are examined in a very fresh state, 

 their contents appear pellucid and homogeneous, and of a fluid consistence ; so 

 that each tube or fibre looks like a cylinder of clear glass, with simple, well- 

 defined, dark edges. But a kind of coagulation soon takes place in the contained 

 substance, making it easily distinguishable from the tube itself; for the latter is 

 then marked by a double line, as shown in Fig. 106, A. The substance which 

 is in immediate contact with the inner wall of the nerve-tube, is more opaque 

 than that which occupies its centre, and of a different refractive power ; and 

 thus it forms a hollow cylinder, which surrounds the latter, and which is knows 

 under the name of the " white substance of Schwann." The centre or axis of 

 the tube is occupied by a substance that preserves its transparency; and this is 

 the " axis-cylinder'' of Rosenthal and Purkinje, the " primitive band" of Remak. 

 Much discussion has taken place respecting the condition under which this central 

 substance exists in the living nerve-fibre ; some microscopists considering that it 

 is then soft and semi-fluid, and that its subsequent firmness is derived from a 

 kind of coagulation; whilst others maintain that it always possesses a considera- 

 ble degree of firmness and tenacity, and that it must be considered, not only as 

 having a proper independent existence, but as being the essential part of the 

 tubular fibre. This last view has recently been strenuously contended for by 

 Prof. Kolliker, 1 who maintains that we do not distinguish it in the living nerve- 

 tube, merely because it then possesses the same refractive power as the surround- 

 ing substance, and who urges the readiness with which it may be brought into 

 view by various reagents (such as concentrated acetic acid, alcohol, ether, iodine, 

 &c.), in nerves taken directly from the living animal, as a proof of the distinct- 

 ness of its character. The effects of reagents further lead to the suspicion, that 

 the " axis-band" (this term being preferable to axis-cylinder, since the substance 

 seems more commonly to possess a flattened than a cylindrical form), is of an 



Fig. 106. 



Stricture of Tubular Nerve-fibres, magnified 350 diameters : A, cylindrical tubuli from nerve ; B, varicose 

 tubuli from brain; c, nerve-tubes, of which one exhibits the remains of nuclei in its walls. 



albuminous nature ; and that the "white substance of Schwann" is the oleaginous 

 constituent of the nerve-fibre. The best evidence of the superior consistence of the 

 axis-band during life, appears to the Author to be derived from the distinct con- 

 tinuity which it may be not unfrequently seen to present when prolonged beyond 

 its envelops ( 343); and from its occasional fission into finer fibrilke, the 



1 " Mikroskopisch* Anatomic," band ii. p. 393, et scq. 



