DESIGN FOR A BRAIN 



7/19 



when it has developed an automatic intermittently-emptying 

 action after spinal section. The bladder fills steadily with urine, 

 while at first the spinal centres for micturition remain inactive. 

 When the volume of urine exceeds a certain value the centres 

 become active and urine is passed. When the volume falls below 



— TIME— *- 



Figure 7/18/1 : 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. 



a certain value, the centre becomes inactive and the bladder refills. 

 A graph of the two variables would resemble Figure 7/18/1. The 

 two-variable system is state-determined, for it has the field of 

 Figure 7/18/2. The variable y is approximately a step-function. 



o x, x 2 



Figure 7/18/2 : Field of the changes shown in Figure 7/18/1. 



When it is at 0, its critical state is x = X 2 , y = 0, for the occur- 

 rence of this state determines a jump from to Y. When it is at 

 Y, its critical state is x = X lt y = Y, for the occurrence of this 

 state determines a jump from Y to 0. 



7/19. A common, though despised, property of every machine is 

 that it may ' 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 



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