936 



CENTRAL NERVOUS SYSTEM 



the central end of an afferent root is stimulated in the lumbar region of the 

 spinal cord, the movement produced is distinctly rhythmic in character. 

 (2) Upon stimulating the central end of the sciatic nerve in a frog whose 

 spinal cord has been cut some days previously, a clonic action of the 

 contralateral foot occurs, and the rate of the rhythm is not affected by 

 variation in the frequency of the stimulus. 



Fig. 237. Diagram showing the reflex arcs involved in the scratch reflex. Ra and R0 represent 

 ihe afferent neurons connected with hairs on the skin of the back and flank. The afferent im- 

 pulses are transmitted by these fibers, and on entering the corresponding segments of the spinal 

 cord terminate by synapses on cells of the internuncial neurons, whose arrows Pa and PQ travel 

 down in the lateral columns to terminate similarly around the cells of the motor neurons that 

 innervate the muscles of the hind limb. Since afferent impulses coming from elsewhere, par- 

 ticularly from the skin of the leg (R and L), also terminate on these neurons and may excite 

 them to a different type of action, the motor neuron is called the final common path (F.C.). 

 (From Sherrington.) 



Fig. 238. The region of body of dog from which the scratch reflex can be elicited. (From 



Sherrington.) 



In all the above cases the refractory period may be held responsible 

 for the rhythmic nature of the contraction. In other reflexes it exists 

 for another purpose. In the case of the extensor thrust, which it will 

 be remembered is elicited by pressure applied to the pads of the plantar 

 aspect of the foot, the momentary extension of the leg lasts only for a 

 little less than two-tenths of a second, but is followed by a refractory 



