38 ANATOMY OF THE RABBIT. 
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 conducuon, 
which carry impulses 
either from the peri- 
pheral parts of the body 
to the central nervous 
system, or in the oppo- 
a site direction. Sensory 
fn.a nerves are afferent, while 
Fic. 19. Section of the spinal cord of the rabbit: motor nerves aro efier- 
c., central canal; f.m.a., anterior median fissure; s.m.p., ; 
posterior median sulcus; f.a., f.l., and f.p., anterior, ent. Nerves, however, 
lateral, and posterior funiculi of the cord; c.g.a. and - 
c.g.p., anterior and posterior grey columns (horns of usually contain both 
grey matter); r.a., and r.p., anterior and posterior nerve a 
roots; s.a., white substance. 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- 

