i88 THE WONDER OF LIFE 



movement and feeling, or contractility and irritability, 

 connected with the muscular and nervous systems respec- 

 tively, if these are differentiated. These two master- 

 activities make life worth living. To keep them a-going 

 there are the auxiliary functions of (a) nutrition including 

 the ingestion, digestion, and absorption or final incorpora- 

 tion of nutritive material ; (6) respiration, including the 

 absorption of oxygen, which may almost be called a 

 gaseous food, to keep the vital combustion a-going, and 

 the elimination of carbon dioxide, which is a gaseous waste ; 

 and (c) excretion, or the filtering out of the nitrogenous 

 waste. Now these five everyday functions are the condi- 

 tions of behaviour ; yet behaviour means something more. 

 In the same way, the periodic functions of growth and 

 reproduction may be said to condition behaviour, but they 

 do not necessarily involve it. 



The beating of the heart is a very vigorous activity, 

 going on, as we say, ' automatically '. It is in reality, of 

 course, very subtly controlled by the nervous system, and 

 can adjust itself to varying conditions within the body. 

 We may take it, however, as a good type of automatic 

 internal activities, such as the respiratory movements 

 and the quiet work of liver and kidneys also illustrate. 



Reflex Actions of Parts of the Body. When we 

 quickly draw away our finger from a hot surface, or close 

 our eye against an approaching ball, or cough when a crumb 

 of bread threatens to go the wrong way, we are illustrating 

 relatively simple reflex actions of parts of the body. A 

 stimulus from the outer world affects a sensory nerve, a 

 message passes to the central nervous system, and a 

 response quickly passes down a motor nerve, commanding 

 a muscle or several muscles to move. In essence, reflex 



