452 A MANUAL OF VETERINARY PHYSIO LOGY 



to the grey matter of leg segments. (3) The motor neurone 

 (Fig 134, fc), from the spinal segment of the leg to the flexor 

 muscles. This last is the final common path. The chain thus 

 consists of three neurones. It enters the grey matter twice — 

 that is, it has two neuronic junctions, two synapses. It is a 

 disynaptic arc. 



Now if, while stimulation of the skin of the shoulder is 

 evoking the scratch reflex, the skin of the hind-foot is stimulated, 

 the scratching is arrested. Stimulation of the skin of the hind- 

 foot causes the leg to be flexed, drawing the foot up. This is the 

 foot reflex. The drawing up of the foot is effected by strong 

 tonic contraction of the flexors of hock, stifle, and hip. In this 

 reaction the reflex arc is : (1) The receptive neurone or nerve path 

 (Fig. 134, l) from the foot to the spinal segment ; (2) perhaps a 

 short intraspinal neurone ; and (3) the motor neurone or nerve 

 path (Fig. 134, fc) to the flexor muscle — e.g., of hip. Here, 

 therefore, we have an arc which embouches into the same final 

 common path as sa. The motor neurone fc is a nerve path 

 common to it and to the scratch reflex aics ; both arcs employ 

 the same effector organ, a hip-flexor. 



The channels for both reflexes finally embouch upon the same 

 common path. The flexor effect specific to each differs strikingly 

 in the two cases. In the scratch reflex the flexor effect is an 

 intermittent contraction of the muscle ; in the foot reflex it is 

 steady and maintained. The accompanying tracing (Fig. 135) 

 shows the result of conflict between the two reflexes. The one 

 reflex displaces the other from the common path. There is no 

 compromise. The scratch reflex is set aside by that of the 

 reflex arc provoked from the foot. The stimulation which pre- 

 viously sufficed to evoke the scratch reflex is no longer effective, 

 though it is continued all the time. But when the stimulation 

 of the foot is discontinued the scratch reflex returns. In that 

 respect, although there is no enforced inactivity, there is in- 

 hibition. There is interference between the two reflexes, and 

 the one is inhibited by the other. Though there is no cessation 

 of activity in the motor neurone, one form of activity that was 

 being impressed upon it is cut out and another takes its place. 



Suppose, again, during the scratch reflex, stimuli are applied to 

 the foot, not of the scratching, but of the opposite side (Fig. 134, r). 

 This stimulation of the foot causes flexion of its own leg 

 and extension of the opposite. If, when the left leg is executing 

 the scratch reflex, the right foot is stimulated, the scratching, 

 involving as it does the left leg's flexors, is cut short. This 

 inhibition of the flexor scratching movement occurs sometimes 

 when the contraction of the extensors is minimal or hardly 

 perceptible. 



