184 PHYSIOLOGY FOR BEGINNERS CHAP. 



of the fibres of the posterior roots do not go into the grey 

 matter of the posterior horns at the level where they spring, 

 but run up the cord in the posterior columns of white matter, 

 and go into the grey matter of the cord higher up, or even 

 pass to the spinal bulb. 



Function of the Nerve Boots. Some of the nerve 

 fibres in a spinal nerve trunk carry impulses from the skin to 

 the spinal cord, and some carry impulses from the spinal cord 

 to the muscles ; that is, some of the fibres are sensory and 

 some are motor. These two kinds of fibres run together 

 down the nerve trunk, parting company at length to go to the 

 particular portion of skin and the particular muscles supplied 

 by that nerve. As the nerve trunk is traced back to the 

 spinal cord, it is found that at the junction of the two roots 

 the sensory and motor fibres are sorted out, and that all the 

 sensory fibres pass into the cord by the posterior root, and all 

 the motor fibres by the anterior root. The posterior is 

 therefore called the sensory root and the anterior the 

 motor root of the spinal nerve. If the posterior root of a 

 spinal nerve has been severely injured or cut across, the prick 

 of a pin or the heat of a burning coal applied to that portion 

 of the skin to which the nerve goes is not felt at all. The 

 connection of the sensory fibres of the nerve with the spinal 

 cord has been broken ; hence no sensory nervous impulses 

 can reach the spinal cord, and so the brain, from the nerve. 

 Motor impulses, which pass from the spinal cord along the 

 anterior root, can be transmitted just as well as before the 

 posterior root was cut, and lead as before to the contraction 

 of the muscles to which the nerve goes. If, on the other 

 hand, the anterior root and not the posterior be injured or 

 cut across, these motor impulses cannot pass and the cor- 

 responding muscles cannot be set in action ; they are said 

 to be paralysed. In this case, however, sensory impulses 

 can pass from the skin to the cord, and so to the brain, 

 as well as before ; pain can be felt in the part of the body to 

 which the nerve goes, but no movement can be produced 

 there. What has just been said of sensory and motor fibres 

 applies also to all afferent and efferent fibres. 



Functions of the Spinal Cord. When a man "breaks 

 his back "-he receives some injury to the vertebral column, which 



