68 ANATOMY OF THE RABBIT. 
ganglion. Impulses passing through the dorsal root are centri- 
petal or afferent in that they pass only in the direction of the 
central nervous system, but they are also in many cases sensory 
in that their effects may be consciously experienced. The most 
characteristic sensory impulses are those which come from the skin. 
Ina similar fashion the impulses of the ventral root are centrifugal 
COMPOSITION OF or efferent, in that they pass only in a 
‘A SPINAL NERVE. direction away from the central nervous 
system, and are in most cases Motor in that 
their effects are commonly observed as muscular contraction. The 
two roots, however, unite immediately outside the spinal cord, and 
subsequently re-divide in such a way that three nerves are formed, 
each containing a proportion of both kinds of fibres. Two of these 
nerves, known as the dorsal and ventral rami and then distributed 
as somatic nerves to the body wall, while a third is distributed as a 
visceral nerve, or ramus communicans, having important se- 
condary connections in the sympathetic nervous system. Unlike 
the somatic nerves, which take a direct course to their terminations, 
the communicating rami of each side, unite in a position ventral 
to the vertebral column to form a longitudinal sympathetic 
trunk consisting of a connected series of ganglia. The latter 
is similarly connected witha prevertebral series of the ganglia, and 
through them with certain peripheral ganglia on the surface of 
the visceral organs. This system forms the sympathetic division 
of the peripheral nervous system. Its ganglia are nerve-cell 
centres, and its fibres, afferent and efferent, are distributed both 
to the visceral organs and to bloodvessels in all parts of the body. 
It is difficult to determine what portion of a muscular con- 
traction, even if considered to be purely voluntary, actually arises 
from an impulse originating in the central nervous system. The 
animal body, however, affords many indications of muscular 
REFLEX ACTION. actions as responses to previous incoming 
stimuli, without conscious experience as a 
necessary factor in the result. Stimulation of the skin, in verte- 
brates in which the spinal cord is divided, and thus separated from 
the brain, is followed by muscular contractions, presumably by 
direct connections of individual dorsal and ventral roots, or exten- 
sion of the stimulus to neighbouring roots. This is known as 
reflex action. Its nature and conditions can be determined by 
