INTRODUCTION 9 



Observe that the mechanical act of pinching 

 caused the resting muscle to become active. Its 

 stored energy was transformed into external, 

 mechanical work, i. e. the moving of the foot. 

 Not all of the energy set free takes this easily 

 visible form. It will be shown later that much 

 of it is made active as molecular motion, in the 

 form of heat, chemical action, and electricity. 

 Agents which occasion a transformation of 

 energy within the living body are termed stim- 

 uli, and tissues which convert energy of one 

 form into energy of another in consequence of 

 stimulation are said to be irritable. All living 

 tissues are alike irritable, but the form in which 

 their kinetic or active energy appears differs 

 with the nature of the tissue. The contrast 

 between muscle and nerve in this respect is 

 especially instructive. 



Pinch the end of the nerve, i 



No change will be seen in the nerve, but the 

 muscle will contract. 



Thus, while the most conspicuous form which 

 the energy of muscle takes, when set free, is 

 mechanical, the active nerve does not alter its 

 form, but spends its energy in a molecular 

 change, the nerve impulse, which passes from 

 point to point along the nerve to the muscle, 

 or gland, or other structure connected function- 



