22 



Anatomy of the Rabbit. 



A nerve fibre consists of a central core, the axial cord, enclosed, except 

 in the case of those of the olfactory nerve, by certain membranes. Two 

 kinds of fibres are distinguished — medullated fibres, and non-medullated 

 fibres. The former are characteristic of the peripheral nerves. In 

 these (Fig. 17) the axial cord is surrounded by a comparatively thick 

 membrane of fatty material, the medullary or 

 myelin sheath. The latter is continuous except 

 at certain points, the nodes of Ranvier, where the 

 axial cord appears free except for an external 

 investment of the whole fibre, the neurilemma. 



In the non-medullated nerve fibres the myelin 

 sheath is lacking. This type of fibre is chiefly 

 distributed in connection with the sympathetic 

 system. 



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 a nerve is due to the 

 fatty materials of the myelin sheaths, but nerves 

 are commonly found imbedded in a fatty con- 

 nective tissue which is associated with the 

 epineurium and is also of white coloration. 



Nerve fibres, and also nerves, are distinguished 



functionally as afferent and efferent. They are 



organs of conduction, which carry impulses either from the peripheral 



parts of the body to the central nervous system, or in the opposite 



direction. Sensory nerves are 



afferent, while motor nerves SI 7>P- 



are. efferent. Nerves, however, 

 usually contain both afferent 

 and efferent fibres and are 

 hence described as mixed. In 

 the distribution of afferent 

 and efferent fibres there is a 

 marked difference between 

 the external or somatic por- 

 tions of the body and the 

 internal or visceral portions. 

 Consequently, both somatic 

 and visceral kinds of afferent 

 and efferent fibres are con- 

 veniently distinguished. 



On account of certain diff- 

 erences in coloration, the 

 cellular and white fibrous* 

 constituents of the central 

 nervous system produce characteristic patterns where either one is 



Fir. 17. Portions of two 

 meoullated nerve - fibres 

 from the sciatic of the 

 rabbit; a.c, axial cord; 

 m.s., v myelin . sheath, 

 stained black with osmic 

 acid; n., neurilemma; 

 n.r., node of Ranvier. 



Jim.Q 



Fig. 18. Section of the spinal cord of the rabbit: 

 c , central canal ; f.m. a., anterior median fissure; s.m.p., 

 posterior median sulcus; f.a., f.L, and f.p., anterior, 

 lateral, and posterior funiculi of the cord; e.g. a. and 

 c.ff.p., anterior and posterior grey columns (horns of 

 grey matter); r.a., and r.p., anterior and posterior nerve 

 roots; s.a., white substance. 



