314 PHYSIOLOGY CHAP. 



involves antagonistic sets of motor cells alternately, e.g. the 

 alternate movement of flexion and extension. 



5. The groups of motor nerve -cells contemporaneously dis- 

 charged by spinal reflex action innervate synergic and not 

 antergic muscles. 



6. The reflex movements that may be elicited in and from any 

 one spinal region exhibit much uniformity despite considerable 

 variety of the locus of incidence of the exciting stimulus. Approxi- 

 mately the same movement, e.g. in the hind-limb flexion of the 

 three great joints, will result, whatever piece of the limb surface 

 be irritated. The seat of incidence of the stimulus will only 

 influence the movement in so far that the flexion will tend to 

 occur predominantly at that joint, the flexor muscles of which are 

 innervated by motor cells segment ally near to the entrance of the 

 afferent fibres from the particular piece of skin which is the seat of 

 application of the stimulus. 



The laws, or rather the rules which govern the course of irradia- 

 tion in long spinal reflexes, were formulated by Pfliiger in 1853. 

 They can be stated as follows : 



(a) Law of homonymous conduction for unilateral reflexes. If 

 a stimulus applied to a sensory nerve provokes muscular move- 

 ments solely on one side of the body, that side is without exception 

 that which is the seat of application of the stimulus. This 

 statement, as already known to Johannes Miiller, does not 

 completely express the facts. For instance, when the tail is 

 touched on one side, it is in many animals, from the fish to the 

 mammal, moved towards the opposite side, i.e. the reflex is dis- 

 charged by the musculature of the side opposite to the seat of 

 excitation. 



(&) Law of the bilateral symmetry of the reflex action. When 

 the excitation evokes movements on both sides, those muscles of 

 the opposite side first come into play which are symmetrical with 

 those already excited in the homonymous half of the cord. This 

 statement, although true of a number of instances, fails to conform 

 with fact in many others. The important crossed reflex from the 

 hind-limb of the bird and mammal does not conform to it, and 

 Luchsinger observed on narcotised dogs that excitation of a front 

 limb evokes reflexes from the hind -limb on the opposite side. 

 This crossed reflex, which occurs very frequently in mammals 

 (Sherrington), is probably connected with the co-ordination of the 

 spinal centres for progression. 



(c) Law of unequal intensity of bilateral reflexes. When 

 excitation of a sensory nerve elicits bilateral reflexes of unequal 

 intensity, the side of stronger contractions is always homonymous 

 with the seat of application of the stimulus. This law also 

 .suffers exceptions. The abduction of the tail from the side 

 stimulated, referred to above, and the " torticollis " reflex towards 



