454 A MANUAL OF VETERINARY PHYSIOLOGY 



It is obvious from this that the final common path, fc, to 

 the flexor muscle can be controlled by, in addition to the before- 

 mentioned arcs, others that actuate the extensor muscles, for 

 it can be thrown out of action by them. The final path, fc, is 

 therefore common to the reflex arcs, not only from the same 

 side foot (Fig. 134, l) and shoulder skin (sa, s/3), but also 

 to arcs from the opposite foot (r), in the sense that it is 

 in the grasp of all of them. In this last case we have a conflict 

 for the mastery of a common path, not, as in the previous 

 instance, between two arcs, both of which use the path in a 

 pressor manner, although differently, but between two arcs 

 that, though both of them control the path, control it differently, 

 one in a pressor manner heightening its activity, the other in a 

 depressor manner lowering or suppressing its activity. 



We said that the scratch reflex is unilateral. If the right 

 shoulder be stimulated, the right hind-leg scratches ; if the left 

 shoulder be stimulated, the left hind-leg scratches. If both 

 shoulders be stimulated at the same time, one or the other 

 leg scratches, but not the two together. The one reflex that 

 takes place prevents the occurrence of the other. The reason is 

 that, although the scratch reflex appears unilateral, it is not 

 strictly so. Suppose the left shoulder stimulated. The left leg 

 then scratches. If the right leg is then examined, it is found 

 to present steady extension, with some abduction. This exten- 

 sion of the leg which accompanies the scratching movement 

 of the opposite leg contributes to support the animal on three 

 legs, while it scratches with the fourth. 



Suppose now we stimulate the left shoulder, evoking the 

 scratching movement of the left leg, and that the right shoulder 

 is at the same time appropriately and strongly stimulated. This 

 latter stimulus often inhibits the scratching movement in the 

 opposite leg, and starts it in its own. In other words, the 

 stimulus at the right shoulder not only sets the flexor muscles 

 of the leg of its own side into scratching action, but it inhibits 

 the flexor muscles of the opposite leg. It throws into contrac- 

 tion the extensor muscles of that leg. In the previous example 

 there was a similar co-ordination. The motor nerve to the 

 flexor muscle is therefore under the control not only of the arcs 

 of the scratch reflex from the shoulder on the same side, but of 

 those from the opposite shoulder as w"ell. But in regard to 

 their influence upon this final common path, the arcs from the 

 homonymous shoulder and the opposite shoulder are opposed. 



The scratch reflex occurs in many anirnals besides the dog — 

 e.g., the cat, guinea-pig, sheep, rabbit, rat, parrot. In small 

 animals its rate of rhythm is greater than in large ; the scratch 

 is quicker in small dogs than in large. 



