134 



THE MECHANISM OF LIFE 



which enables us to act upon the latter. It is not so muc 



prolonged or developed into consciousness of our environmen 



as into action upon the environment. 



We must deal further with this aspect of physiology. 



Stimulus and Response in General. 



We consider again a " mechanical model." Let there be 

 little compass, NS, placed in the neighbourhood of a wire int 

 which an electric current may be thrown by depressing a switch. 

 So long as the current does not flow 

 through the conductor, the needle remains 

 in the position NS that is, in the mag- 

 netic meridian; but whenever the switch 

 is put down and the current passes in the 

 (conventional) direction indicated by the 

 arrow, the compass needle deviates and 

 takes up the new position WE. When the 

 current is switched off the needle returns 

 to NS, and no matter how often we do 

 this the effect is always the same. Now 

 call the making contact in the switch a 

 "stimulus" and the deviation of the 

 needle the "response"; the latter, we 

 see, is invariable and inevitable. Given 

 that the same quantity of current flows 



FIG. 40. A COMPASS through the conductor, and that the in- 

 NEEDLE, NS, LAID . ., , ,, ,, , ,. ~ ,, 



ALONGSIDE A WIRE, 06, tensity of the earths magnetic field 



THROUGH WHICH A CUB- remains constant, then the needle always 

 KENT CAN BE PASSED. deviates in the same direction and to 



the same extent. There is strict determinism that is, the 

 " response " (a certain deviation) always follows upon the 

 " stimulus " (a current of a certain value). 



The Muscle-Nerve Preparation. Now let the prominent thigh 

 muscle (gastrocnemius) in the frog's leg be dissected out, leaving 

 its tendon still attached to the femur and an inch of its nerve 

 (the sciatic) attached to the muscle which carries a weight. 

 Place two electrodes carrying a current in contact with the nerve. 

 Throw a momentary electric current into the circuit containing 

 the electrodes, and the nerve will be stimulated at the place a. 

 The stimulus initiates an impulse which traverses the nerve 

 and releases potential energy in the muscle, whereupon the 



