XVJL] 



NERVE-FIBRES. 



203 



Ranvier is an interannular segment, or 



internode, and about the centre of this, 

 under the neurilemma, is a flattened oval 

 nucleus nerve-corpuscle surrounded by 

 a small quantity of protoplasm, and lying 

 in a slight depression of the myelin. 

 Nodes of Ranvier are absent from the 

 nerve-fibres of the brain and spinal cord. 

 Osmic acid blackens the myelin ; silver 

 nitrate produces the so-called Ranvier's 

 crosses. The manner of their production 

 is given in the text. Nerve-fibres do not, 

 as a rule, branch except towards their 

 terminations. 



The fibres vary greatly in diameter, 

 some being only half as broad as a red 

 blood-corpuscle (4 p, ^-^tr inch), others as 

 broad (8 ^ -g-^Vo" inch)? and others broader 

 still; so that they vary in diameter from -g 

 2 /A to 20 /JL. 1 



II. Non-Medullated Nerve - Fibres. f 

 They occur specially in the sympathetic S 

 system, but are also present in the cerebro- | 

 spinal nerves. Each fibre consists of a | 

 bundle of fibrils enclosed in a transparent c 

 structureless sheath. Some observers doubt 

 the existence of this sheath. The fibres 

 are somewhat flattened ; they branch and 

 anastomose, and in their course are oval 

 nuclei (fig. 182). As they have no myelin, 

 they are not blackened by osmic acid, so 

 that this reagent serves to distinguish the 

 two kinds of fibres. 



Nerve-trunks consist of bundles or funi- 

 culi of nerve-fibres, each bundle containing 

 a greater or less number of fibres. Several 

 bundles are held together by a common 

 connective-tissue sheath the epineurium. 

 The sheath around each funiculus is com- 

 posed of lamellated connective tissue, 

 covered on both surfaces by endothelial 

 cells, and is called the perineurium or 

 lamellated sheath. Lymph spaces exist 

 between the lamellae. Delicate fibrils of 

 connective tissue lie between the nerve- * IG ' 



.Node of 

 Uanvier. 



Primitive 

 sheath. 



.Nerve 

 corpuscle. 



. Axis-cylinder. 



White substance 

 of Schwann. 



Node of 

 "llunvier. 



