GENERAL PHENOMENA OF LIFE 35 



to stimuli. Thus, while a certain stimulus may at first produce a 

 maximal reaction, it often loses its stimulating qualities altogether 

 in the course of time. This state of adaptation should be sharply- 

 differentiated from a somewhat similar one which is known as the 

 refractory state. It has been previously emphasized that every 

 activity of protoplasm incurs a certain destruction of material which 

 must first be overcome by assimilation before another reaction can 

 take place. Thus, if the dissimilation has been severe, or if the as- 

 similation has been hindered in some way, the living substance sud- 

 denly finds itself unable to receive stimuli, or to develop them into a 

 reaction. This period during which living matter remains, so to speak, 

 impermeable to stimuli, is known as the refractory period. 



The property of protoplasm to receive stimuli and to undergo in 

 consequence of them characteristic chemical and physical changes, is 

 known as irritability. Most generally, however, these alterations are 

 not confined to the seat of the excitation but are propagated to other 

 parts of its mass. This transmission of the waves of irritabihty is 

 dependent upon its property of conductivity. In the multicellular 

 forms, conduction between widely separated parts is greatly facilitated 

 by the interposition of nervous tissue which is peculiarly suited for 

 this function. The impulses leave these conducting paths eventually 

 to be transferred to the constituents of the motor organ. The recep- 

 tion of an impulse by the cell is usually followed by the shifting of its 

 constituents which in turn leads to a change in its form and position. 

 This behavior of living matter is dependent upon its property of 

 contractility. 



