THK 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 

 t6 the brain, and taking back through its fibrous part the 

 impulses of the brain. 

 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 jibres) ; those 

 of the posterior root con- 

 vey sensations from the 

 exterior to the cenh'al 

 organs {sensory). The 

 fibres leading from the 

 brain to the cord cross 

 one another in the me- 

 dulla oblongata, so that 

 if the right cerebral 

 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. 



Fig. 146— Eelntioii of the Sympnthelic and Spinal 

 Nerves: c, Assure of epintil cord; a, imlerior 

 root of a dorsal spinal nerve : p, posterior root, 

 with its gauQ:Iion ; a', anterior 1>rancli; /■', pos- 

 terior In-iiricii; s, sympjitheiic ; e, its double 

 junction l)y wliite luul ^rjiy tllainents. 



