30 



ANATOMY OF THE RABBIT 



-a 



.— nr 



n> 



bodies of other neurons, 



by terminals of various 



Unmyelinated fibres lack 



tance before it ends. A nerve fibre consists of an axon with, in 

 some cases, certain enclosing sheaths. Two kinds of nerve fibres 

 are distinguished — myelinated and unmyelinated fibres. In the 

 former, the axon is surrounded by a layer of fatty material, the 

 medullary or myelin sheath. In peripheral 

 nerves, a second sheath, the neurilemma, which 

 is composed of independent cells, encloses the 

 myelin, and as the latter is interrupted at certain 

 points, the nodes of Ranvier, the neurilemma 

 there comes into contact with the axon itself. 

 Near the peripheral end of the axon, first the 

 myelin and then the neurilemma disappears, 

 so that the nerve-ending is devoid of either 

 covering. The naked portion usually breaks up 

 into numerous small branches which end on 

 the dendrites or cell 

 on muscle fibres, etc. 

 characteristic forms, 

 the myelin sheath. 



Both types of fibres are present, in varying 

 proportions, in many peripheral nerves as well as 

 in the central organs. 



A nerve is an association of nerve fibres, the 

 latter being disposed in a parallel fashion and 

 united together into bundles of larger or smaller 

 size by connective tissue, which also forms a 

 general peripheral investment, the epineurium. 

 The dead-white coloration of most nerves is due 

 to the fatty materials of the myelin sheaths, but nerves are com- 

 monly found imbedded in a fatty connective tissue which is as- 

 sociated with the epineurium and is also of white coloration. 



Nerve fibres, and hence nerves, are organs of conduction and as 

 such are designated afferent if they conduct impulses towards the 

 central nervous system or efferent if they conduct away from it. 

 Sensory nerves are afferent, while motor nerves are efferent. 

 Nerves, however, usually contain both afferent and efTerent fibres 

 and when the proportions of both are considerable, they are de- 

 scribed as mixed. In the central connections as well as in the 



Fig. 17. Parts of two 

 myelinated nerve fibres 

 from a teased prepara- 

 tion of the sciatic nerve 

 of a rabbit in which 

 the myeHn sheaths have 

 been stained dark with 

 osmic acid. a. axon; 

 m. myehn sheath; n, 

 neurilemma; nc. neuri- 

 lemma cell; nr, node of 

 Ranvier. 



