STRUCTURE OF NERVE-FIBRES. 



529 



Fig. 374. 



axis-cylinder outwards, which are stained with logwood and carmine. The rods are probably 

 not distinct from each other, but are perhaps part of the neuro-keratin network already 

 described.] 



Action of Nitrate of Silver. When a small nerve, e.g., the intercostal nerve of a mouse, is 

 acted on by silver nitrate, it is seen to be covered by an endothelial sheath composed of flattened 

 endothelial cells (fig. 374), while the nerve-fibres themselves exhibit crosses along their course. 

 These crosses are due to the penetration of the silver solution 

 at the nodes, where it stains the cement-substance and also 

 part of the axis-cylinder, so that the latter sometimes exhibits 

 transverse markings called Fromann's lines (fig. 368, 8).] 



[New Methods. Much progress has recently been made 

 in tracing the course of medullated nerve-fibres by the action 

 of new staining reagents; thus acid fuchsin stains the myelin 

 deeply, leaving the other parts unstained, at least it can be 

 so manipulated as to yield this result. Weigert's Method 

 and its modifications have yielded most important results, 

 and proved that medullated nerve-fibres exist in many parts 

 of the central nervous system where they cannot be seen in 

 the ordinary way. The nerve-tissue is hardened in a solu- 

 tion of a chromium salt, and placed in a half-saturated 

 solution of cupric acetate ; it is then stained with logwood, 

 and afterwards the elements are differentiated by steeping 

 the sections in a solution of ferricyanide of potash and 

 borax. The myelin is coloured a logwood tint.] 



In the spinal nerves, those fibres are thickest which have 

 the longest course before they reach their end-organ 

 {Schwalbe), while those ganglion-cells are largest which 

 send out the longest nerve-fibres (Pierret). [Gaskell finds 

 that the longest nerves are nbt necessarily the thickest, for 

 the visceral nerves in the vagus are small nerves, and yet 

 run a very long course.] , . 



Division of Nerves. -Nerve-fibres run in the nerve-trunks Intercostal nerve of a mouse (single 

 without dividing; but when they approach their termination ^cicuhis of nerve-fibres) stained 

 they often divide dichotomously [at a node], giving rise to with silver nxtrate. Endothelial 

 two similar fibres, but there may be several branches at a sheath stained > and ! me nodes 

 node (fig. 376, t). [The divisions are numerous in motor of Ranvier indicated by crosses, 

 nerves to striped muscles.] In the electrical nerves of the malapterurus and gymnotus, there 

 is a great accumula- 

 tion of Schwann's 

 sheaths round a nerve, 

 so that a nerve-fibre 

 is as thick as a sew- 

 ing-needle. Such a 

 fibre, when it divides, 

 breaks up into a 

 bundle of smaller 

 fibres. 



[Nerve-Sheaths. 

 A nerve-trunk con- 

 sists of bundles of 

 nerve - fibres. The 

 bundles are held to- 

 gether by a common 

 connective - tissue 

 sheath (fig. 375, ep), 

 the epineurium which 

 contains the largei 

 blood-vessels, lymph- 

 atics, and sometimes 

 fat and plasma cells. 

 Each bundle is sur- 

 rounded with its own 

 sheath or perineu- 

 rium (pe), which con- 

 sists of lamellated 



connective-tissue disposed circularly, and between the lamellae are lymph spaces lined by 

 flattened endothelial plates. These lymph spaces may be injected from and communicate with 



2 L 



Trans, section of a nerve (median), ep, epineurium 

 ed, endoneurium. 



pe, perineurium 



