7/6 



TIME-*- 



Figure 7/5/1 : Diagram of the changes in x, volume of urine in the bladder, 

 and y, activity in the centre for micturition, when automatic action has 

 been established after spinal section. 



'1 ~2 



Figure 7/5/2 : Field of the changes shown in Figure 7/5/1, 



determines a jump from to F. When it is at Y, its critical 

 state is x = X v y = Y, for the occurrence of this state determines 

 a jump from Y to 0. 



7/6. A common, though despised, property of every machine is 

 that it may 4 break '. This event is in no sense unnatural, since 

 it must follow the basic laws of physics and chemistry and is 

 therefore predictable from its immediately preceding state. In 

 general, when a machine ' breaks ' the representative point has met 

 some critical state, and the corresponding step-function has changed 

 value. 



As is well known, almost any machine or physical system will 

 break if its variables are driven far enough away from their usual 

 values. Thus, machines with moving parts, if driven ever faster, 

 will break mechanically ; electrical apparatus, if subjected to 

 ever higher voltages or currents, will break in insulation ; 

 machines made too hot will melt — if made too cold they may 

 encounter other sudden changes, such as the condensation which 

 stops a steam-engine from working below 100° C. ; in chemical 

 dynamics, increasing concentrations may meet saturation, or may 

 cause precipitation of proteins. 



Although there is no rigorous law, there is nevertheless a wide- 



85 G 



