THE NERVOUS SYSTEM. 
175 
The spinal cord is a centre for originating involuntary 
actions, and is also a conductor—propagating through its 
central gray matter the impressions received by the nerves 
to the brain, and taking back through its fibrous part the 
impulses of the brain. c 
In Man, thirty-one pairs 
of nerves arise from the 
cord to supply the whole 
body, except the head. 
Each nerve has an ante¬ 
rior and a posterior root. 
The fibres of the former 
go to the muscles, and 
hence carry the impulses 
which cause muscular 
contraction (hence call¬ 
ed motor fibres ); those 
of the posterior root con¬ 
vey sensations from the 
exterior to the central 
organs {sensory). The 
fibres leading from the 
brain to the cord cross 
with its ganglion; a', anterior branch; p\ pos- 
if the right cerebral terior branch; s, sympathetic; e, its double 
, . . . ,. , junction by white and gray filaments. 
hemisphere be diseased, 
the left side of the body loses the power of voluntary 
motion. 
The sympathetic nervous system is a double chain of 
ganglia, lying along the sides of the vertebral column in 
the ventral cavity. From these ganglia nerves are given 
off, which, instead of going to the skin and muscles, like the 
spinal nerves, form net-works about those internal organs 
over which the will has no control, as the heart, stomach, 
