THE REGULATING MECHANISM 189 



impulses outwards from neurones in the 

 grey matter. These fibres mostly supply the 

 muscles of the trunk and limbs on the same 

 side. They also pass to blood vessels, and 

 probably to glands, through the ganglia of 

 the sympathetic The posterior roots con- 

 sist of fibres that convey sensory impulses 

 into the cord. On this root there is a gan- 

 glion containing neurones. Sensory fibres, 

 coming from the skin, muscles, and other 

 organs, are related to the neurones in the 

 ganglia, and from these neurones new fibres 

 spring, which carry impulses into the cord. 

 The neurones in the ganglia on the posterior 

 roots are the first receiving stations of 

 sensory impulses. Many of such impulses are 

 then conveyed upwards to the brain, and may 

 give rise to sensations of various kinds. Each 

 segment of the cord, however, is connected with 

 a number of segments both above and below it. 

 Many of the sensory fibres of the posterior roots 

 come into relation w r ith neurones in the cord 

 in one or more segments. From these neur- 

 ones axons arise, which find their way into 

 the anterior roots and thence to the muscles. 



