520 MANUAL OF PHYSIOLOGY. 



to be fuller than those in the web of the other limb, but if the 

 distal ends of the motor roots are stimulated, the dilated blood 

 vessels return to their normal calibre. By these experiments we 

 are shown that, together with fibres to the skeletal muscles, ef- 

 ferent fibres carrying impulses to the muscular walls of the vessels 

 are contained in the anterior roots of the spinal nerves. 



The fact that when the leg on the side where the anterior roots 

 have been severed is stimulated, the other moves, is sufficient to 

 show that the sensory connections between its surface and the 

 cord are not destroyed by cutting those anterior roots ; and we 

 may conclude taking the other facts just mentioned into account 

 that the afferent fibres are situated in the posterior roots. 



We can confirm this result by cutting the posterior roots on 

 one side of a recently-killed frog, and repeating the stimulation 

 of the feet. 



Pinching the limb whose posterior roots are cut, gives rise to 

 no response, because the impulses cannot reach the spinal cord ; 

 but stimulation of the sound foot causes obvious movements of 

 both legs. This shows that the section of the posterior roots of 

 one limb cuts off the afferent (sensory) communication on the 

 side operated on, but that the efferent (motor) impulses can pass 

 freely to the muscles, even when the posterior roots are divided, 

 for the limb moves on pinching the other foot. Further, if the 

 proximal ends of the cut posterior roots be stimulated, motions 

 are produced showing that the centres in the spinal cord are 

 influenced by the afferent impulses carried by those posterior 

 roots. On the other hand, if the distal ends of the cut roots be 

 stimulated no movement results. 



It has been sometimes found that stimulation of the anterior 

 roots seemed to cause pain, as shown by the motion of other parts 

 besides those to which this root was itself distributed ; and it was 

 believed that some sensory fibres must run in the anterior roots. 

 But it has been since found that if the posterior roots be first cut, 

 these signs of pain are not shown when the anterior roots are 

 stimulated. From this it has been concluded that the apparent 

 sensory channels of the motor roots are nothing more than some 

 sensory fibres which pass from the nerve trunk a little way up 



