THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS. 25 



the lower aquatic plants exhibit movements 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. Irritability 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 machines afford examples of the co-ordination 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 automat icity. The cells frequently 

 change their form independently 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 pe- 

 culiar to the living cell, but upon its nutritive powers. It 

 can be proved that any system of material particles in equi- 

 librium and at rest will forever remain so if not acted upon 

 by an external force. Such a system can carry on, usder cer- 

 tain conditions, 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 complex aggregate of molecules, 

 composed in their turn of chemical elements, and if we sup- 

 pose 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, how- 

 ever, we consider the irritability of amoeboid cells, or, ex- 

 pressed 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 disturbed, movements either of some of its constitu- 

 ent 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 reattainment of this state is commonly in- 

 definitely postponed by the reception of new particles, food 

 in one form or another, from the exterior. The nearest ap- 



