THE BUILDING OF REFLEX ARCS 249 



not here if I could, because of their fundamental fixed and innate 

 character, and I think it simpler and safer to refer to such minor 

 reflex-arcs as those which govern the scratch -reflex in a dog, the 

 pinna reflexes in a cat, and a few smaller ones, on the principle of 

 ex uno disce omnes. Such minor nerve-mechanisms as these in a 

 pair of well-known domesticated animals will suffice for evidence 

 on behalf of initiative in evolution. 



The Scratch Reflex. 



The scratch -reflex in the dog, which like the tendon -reflex 

 in man was in my youth a subject for schoolboy tricks, has received 

 a vast amount of attention and research from physiologists to whom 

 it has brought valuable fruit. It is a familiar phenomenon in a 

 familiar friend of man. There is a saddle-shaped area on the back 

 of the dog over which it was found empirically that even a light 

 stimulus when applied rhythmically, produces the " scalptor- 

 reflex " or a reflex rhythmical action of the flexor muscles of the 

 leg on the same side, calculated to remove the irritating causes 

 of the stimulus. This includes a series of receptors in the skin 

 leading to a spinal segment in the region of the shoulder, a long 

 neurone in the cord, then a motor neurone, the axon from which 

 activates the flexor muscles of the leg and produces scratching. 

 It is described as an efferent arc from receptor to the motor neurone, 

 from which the Final Common Path supplies the motor apparatus 

 or effector. Professor Sherrington says that in this reflex a single 

 stimulus which is far below threshold intensity is found on its 

 fortieth repetition and nearly four seconds after its first application 

 to become effective and provoke the reflex and that its frequency 

 is about 4.5 per second. The reflex movement remains rhythmic 

 and clonic under the strongest as under weaker stimulation. When 

 it is easily elicitable the scratch -reflex can be evoked by various 

 forms of electrical as well as mechanical stimulation, but, when 

 not easily elicitable, electrical stimulation fails whereas rubbing 

 or other mechanical forms of stimuli still evoke it, though less 

 vigorously than usual. This reflex can also be set aside by the 

 '' nociceptive arc from the homonymous foot " or, in other words, 

 a nocuous stimulus to the leg of that side produces " interferences 

 which amounts to inhibition." Empirically it is easy to notice 

 also that if the " scalptor -reflex " can be elicited on both sides of 

 the body, the dog when standing will momentarily lose the power 

 in the hind legs. 



Note. — The rhythm of this reflex act is so special even to the 

 layman that lately I had a singular confirmation of its stereotyped 



