490 



PRINCIPLES OF GENERAL PHYSIOLOGY 



paragraph. It is also to be seen in the fact that when a reflex produced from two 

 different receptors employs the same final common path, simultaneous application 

 of stimuli to both receptors evokes a larger response than if applied to either alone. 

 This may also be called " reinforcement," and plays an important part in the 

 behaviour of an organism to the various stimuli playing upon it at one time. 

 These combinations of stimuli form, as Sherrington puts it, "constellations of 

 stimuli." It must also be remembered that a reflex not only takes possession of 



FC 



I" 



iAAAnnjiAJxruuuiAJU~iJUUvnJuuumr^^ 



FIG. 151. SUMMATION EFFECT (IMMEDIATE SPINAL INDUCTION) BETWEEN THE ARCS 



Ba. AND Rfi OF THE DIAGRAM GIVEN IN FlG. 146 (page 476). 



FC, Myogram of the flexor muscle of the hip. 



So,, Signal marking period of stimulation of the skin belonging to arc R& of the shoulder skin. The strength 



of the stimulus is subminimal, so that there is no reflex res)>onse. 



S/3, Signal marking stimulation, also subminimal, of a point of the shoulder skin 8 cm. from ./fa. 

 Though the two stimuli applied separately are each unable to evoke the reflex, when applied conU'inix r- 



aneously they quickly evoke the reflex. 



The two arcs Tfci and Rfl, .therefore, reinforce each other in their action on the final common ]>ath, FC. 

 Time in fifths of seconds. Read from left to right 



(Sherrington, 1906, pp. 119 and 121.) 



certain final common paths, but also of those whose muscles would oppose those of 

 the reflex itself. It inhibits the opposing neurones from being set into action by 

 other reflexes at the same time. 



Induction. Closely allied to the preceding are the phenomena of induction. 

 If the receptive skin area for the scratch reflex is stimulated at one point with 

 subminimal intensity, a reaction may be evoked if another point in the same area 

 be stimulated at the same time, also with subminimal intensity. Such reinforce- 

 ment is called by Sherrington (1906, p. 120), immediate spinal induction (see 

 Fig. 151). It appears that both stimuli act on the same set of neurones composing 



