8/7 



THE HOMEOSTAT 



stop the shocks. After the tenth trial, the application of the 

 shock would usually cause the rat to go straight to the pedal and 

 depress it. These, briefly, are the observed facts. 



Consider the internal linkages in this system. We can suffi- 

 ciently specify what is happening by using six variables, or sets 

 of variables: those shown in the box-diagram below. By con- 



sidering the known actions of part on part in the real system we 

 can construct the diagram of immediate effects. Thus, the excita- 

 tions in the motor cortex certainly control the rat's bodily move- 

 ments, and such excitations have no direct effect on any of the 

 other five groups of variables; so we can insert arrow 1, and 

 know that no other arrow leaves that box. (The single arrow, of 

 course, represents a complex channel.) Similarly, the other arrows 

 of the diagram can be inserted. Some of the arrows, e.g. 2 and 4, 

 represent a linkage in which there is not a positive physical action 

 all the time; but here, in accordance with S. 2/3, we regard them 

 as permanently linked though sometimes acting at zero degree. 



Having completed the diagram, we notice that it forms a 

 functional circuit. The system is complete and isolated, and 

 may therefore be treated as state-determined. To apply our 

 thesis, we assume that the cerebral part, represented by the boxes 

 around arrow 6, contains step-mechanisms whose critical states 

 will be transgressed if stimuli of more than physiological intensity 

 are sent to the brain. 



We now regard the system as ultrastable, and predict what 

 its behaviour must be. It is started, by hypothesis, from an 

 initial state at which the voltage is high. This being so, the 

 excitation at the skin and in the brain will be high. At first 



109 



