THE SPINAL CORD 



677 



3. The scratch reflex. Light stimulation of the " saddle " area call* 

 forth movements corresponding to the familiar scratching movements 

 indulged in by dogs removing a flea or other irritant from this area. 



The first point to be observed in regard to such reflexes is that 

 a given stimulus always evokes the same response. Thus, a pin-prick 

 applied to the foot always evokes the flexor 

 and not the extensor response. The reflex 

 is localized. 



Another important point is that only one 

 reflex can have charge of the final effector 

 path at once. Just as it would be very 

 embarrassing if the telephone exchange 

 permitted several callers to speak at once 

 down the same final transmitting wire, so 

 would it be if various afferent calls had 

 possession of the final effector path at the 

 same time. All kinds of inco-ordinate move- 

 ments would be evoked. Any such inco- 

 ordination is therefore prevented by the law 

 that only one reflex at a time can have 

 charge of the final common path. This is 

 exemplified as follows: If the stimuli in- 

 ducing the flexor and extensor reflexes be 

 applied to the foot at the same time, one 

 or other reflex is evoked, and not a com- 

 bination of the reflexes. The same final 

 common path is used in both reflexes, and 

 is taken possession of by the stronger 

 stimulus. If the stimuli be of equal inten- 

 sity, but entering different levels of the cord, 

 then there is a further code of rules. For 

 instance, if, while the animal is performing 

 the scratch reflex as the result of a stimulus 

 applied to the saddle area (Ra), a stimulus 

 of equal intensity be applied to the scratch- 

 ing foot (L), the scratching ceases, and the 

 foot is withdrawn from the harmful stimulus 

 applied to the foot (Fhj. 402). Or, again, 

 if, during the performance of the scratch 

 reflex, a stimulus be applied to the hind- 

 limb of the opposite side (R), the scratching 

 ceases, and the opposite hind-limb is with- 

 drawn out of harm's way (Fig. 403). It will be seen, therefore, that 

 an impulse entering the same segment of the cord, whether from 

 the same or the opposite side, has priority over an impuls3 entering 

 the cord in a more remote segment. 



So, also, can it be shown that for the same level of the cord i\:i 

 impulse entering the same side has priority over an impulse entering 

 at the same level on the opposite side. 



Fiu. 403. SCRATCH RE- 

 FLEX CUT SHORT BY 

 EXCITATION OF THE SKIN 

 OF A DIGIT OF OPPOSITK 

 HIND FOOT. 



Below upper signal mark 

 marks period of applica- 

 tion of stimulus to 

 opposite hind foot ; lower 

 signal marks application 

 of stimulus exciting 

 scratch reflex. Time in 

 fifths of second. (From 

 Sherrington's " Initegra- 

 tivc Actionof the Nervous 

 System," by permission 

 of Yale University Press 

 and Messrs. Constable 

 and Co., Ltd.) 



