700 REFLEX ACTIONS. [Book in. 



call forth quite a different movement. When a decapitated 

 snake or newt is suspended and the skin of the tail lightly 

 touched with the finger, the tail bends towards the finger ; when 

 the skin is pricked or burnt, the tail is turned away from the 

 offending object. And so in many other instances. It must be 

 remembered of course that a difference in the intensity of the 

 stimulus entails a difference in the characters of the afferent im- 

 pulses ; gentle contact gives rise to what we call a sensation of 

 touch, while a sharp prick gives rise to pain, consciousness being 

 differently affected in the two cases because the afferent impulses 

 are different. Hence the instances in question are in reality 

 fuller illustrations of the dependence of the characters of a reflex 

 movement on the characters of the afferent impulses. 



§ 460. Further, the movement, forming part of a reflex 

 action, varies in character according to the particular part of 

 the body to which the stimulus is applied. The reflex actions 

 developed by stimulation of the internal viscera are different 

 from those excited by stimulation of the skin. We have reason 

 to think that the contraction of or other changes in a skeletal 

 muscle may produce, by reflex action, contractions of other mus- 

 cles ; and such reflex actions also differ from those started by 

 stimulation of the skin. In reflex actions started by applying 

 a stimulus to the skin the movements vary largely according to 

 the particular area of the skin which is affected. Thus, pinch- 

 ing the folds of skin surrounding the anus of the frog produces 

 different effects from those witnessed when the flank or the toe 

 is pinched ; and, speaking generally, the stimulation of a par- 

 ticular spot calls forth particular movements. In the case of 

 the simpler reflex movements, it appears to be a general rule 

 that a movement started by the stimulation of a sensory surface 

 or region on one side of the body, is developed on the same side 

 of the body, and if it spreads to the other side, still remains most 

 intense on the same side ; the movement on the other side more- 

 over is symmetrical with that on the same side. 



§ 461. From these and similar phenomena it is obvious that 

 when we allow ourselves to speak of the grey matter of the cord 

 as supplying central mechanisms for carrying out reflex move- 

 ments we must not regard these several mechanisms as rigidly 

 separate from each other or indeed as anatomically distinct. 

 On the contrary without any anatomical change, as the result 

 simply of physiological changes, the afferent impulses reaching 

 the cord along a set of afferent fibres, and it may be any set, will 

 under certain conditions give rise to efferent impulses, not re- 

 stricted to a definite set of efferent fibres and so leading to a 

 definite purposeful movement, but overflowing along almost all 

 the efferent fibres leaving the cord, and bringing about con- 

 tractions of almost all the skeletal muscles. Thus when a frog 

 is poisoned with strychnia, a mere touch of the skin at almost 



