ILLUSTEATIONS OF KEFLEX ACTION. 195 



pected contrasts of ideas, should there be a contraction of 

 particular facial muscles, and particular muscles of the 

 chest and abdomen ? Such answer to this question as may 

 be possible, can be rendered only by physiology. 



Every child has made the attempt to hold the foot still 

 while it is tickled, and has failed ; and probably there ia 

 scarcely any one who has not vainly tried to avoid wink- 

 ing, when a hand has been suddenly passed before the eyes. 

 These examples of muscular movements which occur inde- 

 pendently of the will, or in spite of it, illustrate what phy- 

 siologists call reflex-action ; as likewise do sneezing and 

 coughing. To this class of cases, in which involuntary 

 motions are accompanied by sensations, has to be added 

 another class of cases, in which involuntary motions are 

 unaccompanied by sensations ; — instance the pulsations of 

 the heart ; the contractions of the stomach during diges- 

 tion. Further, the great mass of seemingly-voluntary acts 

 in such creatures as insects, worms, molluscs, are consid- 

 ered by physiologists to be as purely automatic as is the 

 dilatation or closure of the iris under variations in quantity 

 of light ; and similarly exemplify the law, that an impres- 

 sion on the end of an afferent nerve is conveyed to some 

 ganglionic centre, and is thence usually reflected along an 

 efferent nerve to one or more muscles which it causes to 

 contract. 



In a modified form this principle holds with voluntary 

 acts. Nervous excitation always te^ids to beget muscular 

 motion ; and when it rises to a certain intensity, always 

 aoL'S beget it. Not only in reflex actions, whether with or 

 witliout sensation, do we see that special nerves, when 

 raised to a state of tension, discharge themselves on special 

 muscles with which they are indirectly connected ; but 

 those external actions through which we read the feehngs 

 of others, show us that under any considerable tension, the 



