xvi THE NERVOUS SYSTEM 301 



What happens during a simple reflex action of the cord, 

 then, is this : Impulses set up by a stimulation of the sensory 

 nerve endings of the skin travel along the sensory fibers of a 

 spinal nerve to the cord, entering it through a dorsal root. 

 The afferent fibers as they enter the cord divide and run for 

 a distance both anteriorly and posteriorly, and then give off 

 collaterals, which branch in the gray matter. Some of these 

 form connections with the cells of the ventral roots of the 

 spinal nerves of the same side of the cord. In the simplest 

 case impulses reaching these cells through the collaterals are 

 transmitted to the axis cylinders of the motor nerves arising 

 from them and pass out through the ventral root and down 

 the same spinal nerve to the muscles of the leg, causing 

 them to contract. The spinal cord here serves as a medium 

 of communication between sensory and motor nerves. If a 

 dorsal nerve be cut, stimulation of a sensory nerve produces 

 no effect. If, however, the distal end of the part in connec- 

 tion with the cord is irritated, muscular contractions will be 

 produced. If the ventral or motor end of the nerve be cut, 

 reflex action in the part supplied by that nerve is destroyed. 

 If the cut end of the nerve is irritated, the muscles to which 

 it is distributed will contract, but stimulation of the end 

 connected with the spinal cord will have no effect. 

 ^The impulses passing through the cord are not limited to 

 a single path, but they may take any one or more of several 

 routes. They may cross by way of the gray commissures to 

 the opposite side of the cord and become transferred through 

 the branches of the gray commissural fibers to the ventral 

 roots of that side, or they may pass across by means of the 

 white commissures. They may also pass backward or forward 

 along the cord either in the gray matter or in the white. In 

 this way a stimulus applied to any part of the body may give 

 rise to movements not only on the two sides, but also in 



