MUSCLE 249 



trical stimulation alone can be repeated without the substance 

 stimulated showing any evidence of injury in the process. 

 Mechanical, thermal, chemical stimuli destroy a portion of the 

 protoplasm upon which they act. Yet even the weakest of 

 electric currents is a gross disturbance as compared with 

 natural stimuli, such as touch, warmth, sound, light. The 

 essential and most distinguishing quality of living matter is its 

 return to its original state immediately after stimulation. It 

 does not even wait until the stimulator has ceased to act. An 

 effective influence is a sudden change in the environment. 

 It is answered by a sudden response, followed by a return of 

 protoplasm to the state in which it was before the impact of the 

 external force. The change progresses through the protoplasm 

 as a transitory alteration of state, the particles concerned in 

 conducting it returning to their original condition the moment 

 it has passed. No non-living matter responds to force in this 

 way. If a stone is dropped into a pond, a wave circles out- 

 wards from the spot it strikes ; but this is a wave of displace- 

 ment, not a change of state. Suppose the pond contained a 

 solution of sugar which the impact of the stone changed into 

 vinegar, and that the zone of vinegar spread outwards, the 

 liquor returning to the condition of sugar and water as it 

 passed. Here we should see some analogy to the progress of 

 an impulse. But no non-living matter behaves like this. A 

 product of the laboratory may be so unstable as to explode 

 when shaken, passing on the slightest provocation into a 

 more stable state. It does not return after the explosion to 

 its previous strained condition. Having thrown away its 

 energy, it continues on a lower plane. Protoplasm parts with 

 energy to recover it again. It returns to instability after 

 assuming a more stable form. 



If we are to form a conception of the cause of the irritability 

 of living matter, we must have a mental picture of the physical 

 conditions which distinguish life from death. All matter is in 

 a state of motion. It consists of separate molecules, each 

 moving in its orbit with vast rapidity. A molecule is a cluster 

 of atoms. The dimensions of its orbit depend upon the num- 

 ber and weight of the atoms in its cluster. If we could watch 

 the dance of the molecules of proteins and other substances 

 into which protoplasm breaks up on dying, we should see each 



