38 



Anatomy of the Rabbit. 



s.m.p 



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 peri- 

 pheral parts of the body 

 to the central nervous 

 system, or in the oppo- 

 site direction. Sensory 

 nerves are afferent, while 

 motor nerves are effer- 

 ent. Nerves, however, 

 usually contain both 

 afferent and efferent 

 fibres and are hence 

 described as mixed. In the distribution of afferent and efferent 

 fibres chere is a marked difference between the external or somatic 

 portions of the body and the internal or visceral portions. Con- 

 sequently, both somatic and visceral kinds of afferent and efferent 

 fibres are conveniently distinguished. 



On account of certain differences in coloration, the cellular and 

 white fibrous constituents of the central nervous system produce 

 characteristic patterns where either one is concentrated. Thus the 

 cellular material is greyish, and is hence distinguished as the grey 

 substance, while white fibrous material produces when concen- 



Fig. 19. Section of the spinal cord of the rabbit: 

 c, central canal; f.m.a., anteiior 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 

 e.g. p., anterior and posterior grey columns (horns of 

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

 roots; s.a., white substance. 



