308 MANUAL OF HISTOLOGY. 



in. the demonstration of the structure in question, is collodium, recom- 

 mended by Pfliiger. Under its action the axis cylinder makes its appear- 

 ance almost instantaneously throughout the whole length of every fibre, fre- 

 quently bent over strongly to one side (c). Tinction with carmine may 

 also be employed, and aniline (Frey) or chloroform ( Waldeyer). 



Very instructive objects, as regards the nature of the structure just 

 described, may be prepared from transverse sections of nervous trunks 

 previously artificially hardened (Reissner}. In these we recognise the 

 envelope of each tube, its axis cylinder as a small central formation, 

 and between the two the medullary sheath. In the latter may be seen 

 an irregular concentric marking, first observed by Lister and Turner^ 

 which is probably the optical expression of lamination in the medullary 

 substance. Transverse sections also, through the white substance of the 

 spinal cord, present the same views of the axis cylinder, and medullary 

 matter. 

 REMARK. Quart. Journ. of Microsc. Science, 1860, p. 29, pi. 2. 



176. 



Turning now to ike fine dar/c-edged nerve fibres (fig. 296, c, d, e), we find 

 it possible here also to demonstrate in many cases the presence of the 

 primitive sheath, although with greater difficulty. We recognise at the 

 same time the axis cylinder, especially in the fibres of the brain and 

 spinal cord (fig. 298, /, g, h), where the primitive sheath is no longer present. 



It is a striking fact that, in these fine nervous tubes, we do not remark 

 the same inclination to lumpy or granular coagulation as is seen to such 

 an extent in the broader ones; we find them rather preserving their 

 transparency, whether their contour appear double, as in larger speci- 

 mens (fig. 298, /), or single, as in the more minute (fig. 296, c, d, e). 



In a degree proportionate to their thinness, these fine nerve tubes are 

 remarkable for being subject to a displacement, and the formation into 

 globules of their medullary substance under the action of water, or from 

 pressure, twisting, &c., in consequence of which they often present a 

 knotted appearance (fig. 296, c, d, e, and 298, h). These swellings are 

 known as " varicosities," and are nothing, we repeat it, but artificial 

 productions, which do not exist in the living body. 



Next in succession to these dark-edged medullated fibres, we come now 

 to a second species, namely, to the pale or non-medullated. 



This is the primary form of all the fibrous neural elements in the 

 embryos of man and the vertebrates. 



In the family of the petromyzon, a lowly organised fish, this non- 

 medullated pale appearance of the fibre, presenting simply an axis 

 cylinder, persists throughout life (fig. 298, d). But even in the bodies oi 

 the higher vertebrates, and human beings also, the nerve tubes may still 

 preserve this original embryonic condition in various positions ; thus, in 

 the nervus olfactorius, as soon as it enters the nose. 



While there can be but little doubt as regards the nature of the 

 fibre-elements in the olfactory nerves, it is quite a different matter in 

 the course and distribution of the sympathetic. Here we encounter in 

 the human body and among the higher vertebrates, together with medul- 

 lated tubes, the so-called fibres of Remak (ganglionic nerve fibres), which 

 may even preponderate. These are transparent sometimes ; flat bands of 

 about 0-0038-0-0068 mm. in breadth and 0*0018 mm. in thickness (fig. 

 299, 300, &). Their appearance is usually homogeneous, while at intervals 



