CELLS AND THE CELL THEORY 43 



2. Irritability. All forms of living protoplasm have the power 

 of reacting when stimulated. This phenomenon is called irrita- 

 bility and is produced by the action of a large variety of external 

 ibrces upon the protoplasm itself. Any external force which 

 serves to produce a reaction in the protoplasm is spoken of as 

 a stimulus. Almost any kind of stimulus has the power of 

 affecting protoplasm: mechanical, thermal, electrical, and chemi- 

 cal. Stimuli all "have their effect upon protoplasm and all pro- 

 duce certain reactions within it. Protoplasm is, in short, irri- 

 table to almost any external stimulus. While the different 

 forms of protoplasm show different degrees of irritability to 

 various stimuli, they have certain general reactions in common. 

 The activity of protoplasm increases directly with the heat to 

 a certain point, and then decreases, and finally ceases altogether 

 if the temperature continues to rise. 



Although some forms of protoplasm are much more irritable 

 to mechanical stimuli than others, nevertheless, all types of pro- 

 toplasm are influenced by external, mechanical force. Various 

 other factors, light, chemism, gravity, etc.,' mentioned upon 

 pages 57, 58, stimulate protoplasm. Various organic, internal 

 changes stimulate it as well. If the protoplasm is improperly 

 nourished it produces a condition that is in general known as 

 hunger, and this excites the irritability of protoplasm. The same 

 thing is true if there is insufficient water within the protoplasm, 

 producing an irritation called thirst. Protoplasm is also destroyed 

 by various chemicals called poisons, like chloroform, corrosive 

 sublimate, etc. 



3. Conductility. An irritation produced in any one part of a 

 bit of protoplasm is rapidly conducted throughout the whole 

 mass, a phenomenon known as conductility. In an ordinary 

 cell, this phenomenon of conductility does not have very much 

 meaning, because the bit of protoplasm is too small; but some 

 cells possess long protoplasmic fibers extending from their 

 bodies; and then this function of conducting impulses from one 

 end of the protoplasm to the other becomes of considerable 



