BRAIN AND NERVE 93 



the different segments of the grey matter of the cord. Some of 

 them are fibres of the spinal nerves that enter the cord. 



Between every two adjacent vertebrae one spinal nerve goes 

 out from the cord on each side. But each spinal nerve has two 

 roots — dorsal (to the back) and ventral (towards the belly). 

 The dorsal root consists of nerve fibres that carry impulses from 

 the skin, muscles, tendons, joints, and viscera into the spinal 

 cord, while the ventral root contains fibres that carry impulses 

 out from the cord to the muscles and viscera. And, therefore, if 

 the dorsal roots of several adjacent spinal nerves are severed, 

 there is loss of sensation in some part of the trunk or limbs, for 

 sensory impulses are thus prevented from entering the cord and 

 passing up into the brain. And if several adjacent ventral roots 



Posterior tracts 



Yentrai or/^-'-^^ ^'•■^-->^\^hite ma-fter 



motor rooT l (Commissural barr) 



r" ^ 4.J. _ Nerve-fibres. 



ijhey matrer ' 



(Ganjf/ionic p>a.rt) 



nerve-cells. 



Fig. 23. — A Diagrammatic Section through the Spinal Cord, 



SHOWING THE ORIGIN OF THE SpINAL NeRVES. 



are severed, there is paralysis of the muscles in some part of the 

 body. Several roots must be cut through to produce these 

 effects, for, by a very beautiful arrangement, every part of the 

 body receives nervoub supply from several segments of the cord, 

 and this overlapping of nervous and bodily segments minimises 

 the effect of an accident. Allowing for the complications of the 

 overlapping, we may say that each group of muscles in the trunk 

 and each area of skin is under the control of one or more segments 

 of the cord. The arms and legs, because of their great import- 

 ance in locomotion and general acting, are supplied by a number 

 of spinal nerves which unite together to form the limb " plexuses." 

 To a great extent the spinal cord is a path of conduction of 

 nervous impulses from the brain to the body, and vice versa. 

 But it is also a nervous controlling centre in itself, and when it is 



