26 THE HUMAN BODY 



plant touching one leaflet will excite regular movements of the 

 whole leaf, and many of the lower aquatic plants exhibit move- 

 ments as active as those of animals. On the other hand, no one of 

 these four faculties is absolutely distinctive of living things in the 

 way that growth by intussusception and reproduction are. Irritabil- 

 ity is but a name for unstable molecular equilibrium, and is as 

 marked in nitroglycerin as in any living cells; in the telephone the 

 influence of the voice is conducted as a molecular change along a 

 wire, and produces results at a distance; and many inanimate ma- 

 chines afford examples of the coordination of movements for the 

 attainment of definite ends. 



Spontaneity. There is, however, one character belonging to 

 many of the movements exhibited by amoeboid cells, in which they 

 appear at first sight to differ fundamentally from the movements 

 of inanimate objects. This character is their apparent spontaneity 

 or automaticity . The cells frequently change their form inde- 

 pendently of any recognizable external cause, while a dead mass at 

 rest and unacted on from outside remains at rest. This difference 

 is, however, only apparent and depends not upon any faculty of 

 spontaneous action peculiar to the living cell, but upon its nutritive 

 powers. It can be proved that any system of material particles in 

 equilibrium and at rest will forever remain so if not acted upon by 

 an external force. Such a system can carry on, under certain con- 

 ditions, a series of changes when once a start has been given ; but it 

 cannot initiate them. Each living cell in the long run is but a com- 

 plex aggregate of molecules, composed in their turn of chemical 

 elements, and if we suppose this whole set of atoms at rest in 

 equilibrium at any moment, no change can be started in the cell 

 from inside; in other words, it will possess no real spontaneity. 

 When, however, we consider the irritability of amoeboid cells, or, 

 expressed in mechanical terms, the unstable equilibrium of their 

 particles, it becomes obvious that a very slight external cause, 

 such as may entirely elude our observation, may serve to set going 

 in them a very marked series of changes, just as pressing the trigger 

 will fire off a gun. Once the equilibrium of the cell has been dis- 

 turbed, movements either of some of its constituent molecules or 

 of its whole mass will continue until all the molecules have again 

 settled down into a stable state. But in living cells the reattain- 

 ment of this state is commonly indefinitely postponed by the re- 



