CH. XLIX.j THE SCRATCH REFLEX 719 



But there is another form of stimulation which also throws 

 the same flexor muscles into action, although in rather a different 

 way, and that is stimulation of the sole of the foot. The foot 

 and leg are withdrawn, and the action is a steady one, and not 

 a succession of rhythmic discharges as in scratching. Both reflexes, 

 however, end in the same final common path ; and if while scratch- 

 ing is being elicited by stimulation of the shoulder, the foot is then 

 stimulated simultaneously, scratching immediately ceases; one set 

 of impulses has displaced the other from the final common path. 

 If then one ceases to stimulate the foot, the scratch reflex returns if 

 the irritation of the shoulder is kept up. This is well illustrated by 

 the tracing (fig. 443). 



But there is also another way in which the inhibition of reflexes 

 may be produced. The contraction of one set of muscles is usually 

 accompanied by relaxation of its antagonists, and the contraction of 

 the flexors in the scratch reflex may therefore be inhibited by making 

 the antagonistic muscles (the extensors) contract. Further, the 

 scratch reflex is unilateral, but this does not mean that the muscles 

 supplying the other legs are inactive, for they must act in such a 

 way as to support the dog on three legs, while it scratches with the 

 fourth. So if the right shoulder is stimulated, the right hind leg 

 scratches ; if the left shoulder is stimulated, the left hind leg 

 scratches ; but if both shoulders are stimulated together, only one or 

 the other leg scratches, not the two at once ; parts of the final paths 

 are common to both sides, and there is a struggle for their occupa- 

 tion. Some instances of reinforcing action were found ; for example, 

 if two points of the skin of one shoulder are stimulated with a very 

 feeble current, neither stimulus alone may be sufficient to evoke 

 the scratch reflex, but the two together may elicit it; in order 

 to attain this result the two points of skin must be fairly close 

 together. 



The afferent neurons (private paths) of the body are about five 

 times more numerous than the efferent (final common paths), and in 

 the struggle for the occupation of these public paths by the impulses 

 that enter the central nervous system by the more numerous 

 private paths, three factors are specially concerned : (1) Strength of 

 stimulus ; the stronger the stimulus the better chance the resulting 

 impulse has of getting round to the motor organ. (2) Character of 

 impulse; sensations of painful nature and sexual feelings win the 

 final path easily; it is a matter of common experience that such 

 sensations dominate and even exclude other sensations ; a man with 

 bad toothache is not likely to take much notice of anyone who pulls 

 his coat tails. (3) Fatigue; at the end of a long stimulation, a 

 stimulus applied to a fresh reflex arc has a better chance of capturing 

 the common path. 



