THE NERVOUS SYSTEM 451 



ment of the leg. The movement is rhythmic flexion at hip, 

 stifle, and hock. It has a frequency of about four per second. 

 The stimuli provocative of it are mechanical, such as rubbing 

 the skin, or pulling lightly on a hair. The nerve-endings which 

 generate the reflex lie in the surface layer of the skin, about the 

 roots of the hairs. A convenient way of exciting these is by 

 feeble faradisation. 



Prominent among the muscles active in this reflex are the 

 flexors of the hip. If we record their rhythmic contraction we 

 obtain tracings, as in Fig. 135. A series of brief contractions 

 succeed one another at a certain rate, whose frequency is inde- 

 pendent of that of the stimulation. The contractions are pre- 

 sumably brief tetani. The stimulus to the hair-bulbs of the 



Fig. 134. — Spinal Arcs involved in Scratch Reflex (Sherrington). 



Diagram of the spinal arcs involved in Fig. 133. l, Receptive or afferent nerve 

 path from the left foot ; r, receptive nerve path from the opposite foot ; 

 sa, s/3, receptive nerve paths from hairs in the dorsal skin of the left side ; 

 fc, the final common path, in this case the motor neurone (nerve path) to a 

 flexor muscle of the hip ; pa, p/3, neurones originating within the cord. 



shoulder throws into action a lumbar spinal centre, innervating 

 the hip-flexor, much as the bulbar respiratory centre drives the 

 spinal phrenicus centre. In the case of the respiratory muscle 

 the frequency of the rhythm is, however, much less. 



The reflex is unilateral : stimulation of the left side of the back 

 evokes scratching by the left leg, not by the right. In the 

 lateral column of the spinal cord fibres exist directly con- 

 necting the spinal segments of the shoulder with the spinal 

 segments containing the motor neurones for the flexor muscles 

 of the hip, and knee, and ankle. We thus arrive at the follow- 

 ing reflex chain for the scratch reflex : (1) The receptive neurone 

 (Fig. 134, sa), from the skin to the spinal grey matter of the 

 corresponding spinal segment in the shoulder. This is the 

 exclusive or private path of the arc. (2) The long descending 

 neurone within the cord (Fig. 134, Pa), from the shoulder segment' 



