4 ELEMENTS OF BIOLOGY 



are sensitive to their environment and any change in the surround- 

 ings aflFects the organism in such a way as to modify its manner of 

 living. This response to environmental incidents may be a violent 

 reaction, as in the case of the response of an animal to injury; it 

 may be the production of chemical agents which respond to and 

 combat the bacteria of disease, or a slow change of position or direc- 

 tion of growth as in the response to light of a growing plant. Irri- 

 tability, therefore, as a characteristic common to all living objects 

 includes the entire range of adjustment of a plant or animal to the 

 events which it constantly encounters. Non-living objects are also 

 responsive and change with changes in the surroundings. For exam- 

 ple, a metal rod becomes shorter when the temperature is lowered. 

 A distinction between the effect of the environment on living and 

 on non-living objects is that living objects tend to react, that is, 

 attempt by some change in their activities to overcome or to adjust 

 to the changed conditions. When the temperature is lowered the 

 organism responds by some activity which tends to lessen the effect 

 of the change; the metal rod is helpless. 



Contractility: This property is apparent to us if we note the 

 change in dimensions of the muscles of the arm when the elbow is 

 flexed or extended. It is noted that the flexing of the joint is 

 brought about by the shortening of certain large muscles which 

 can be felt in the upper arm. It is also noted that as the muscle is 

 shortened it at the same time becomes much thicker. Non-living 

 objects also contract; a metal rod contracts with a decrease in tem- 

 perature. The distinction is that in the living body contraction in- 

 volves a decrease in one dimension with a corresponding increase 

 in another, in reality a change in shape, while the metal rod de- 

 creases in all dimensions; it actually occupies less space. The prop- 

 erty of contractility is concerned in many of the activities of living 

 bodies, in the capture of food, in escape from enemies, in the pro- 

 pelling of fluids within the body. It is a less general property than 

 irritability, for some organisms exhibit it to a very limited degree. 



