5/11 ADAPTATION AS STABILITY 



The two aspects are not incompatible, for the constancy of some 

 variables may involve the vigorous activity of others. A good 

 thermostat reacts vigorously to a small change of temperature, 

 and the vigorous activity of some of its variables keeps the others 

 within narrow limits. The point of view taken here is that the 

 constancy of the essential variables is fundamentally important, 

 and that the activity of the other variables is important only in 

 so far as it contributes to this end. (The matter is discussed 

 more thoroughly in /. to C, Chapter 10.) 



Stability and co-ordination 



5/11. So far the discussion has traced the relation between the 

 concepts of ' adaptation ' and of ' stability '. It will now be 

 proposed that ' motor co-ordination ' also has an essential con- 

 nexion with stability. 



4 Motor-co-ordination ' is a concept well understood in physio- 

 logy, where it refers to the ability of the organism to combine the 

 activities of several muscles so that the resulting movement 

 follows accurately its appropriate path. Contrasted to it are the 

 concepts of clumsiness, tremor, ataxia, athetosis. It is suggested 

 that the presence or absence of co-ordination may be decided, in 

 accordance with our methods, by observing whether the move- 

 ment does, or does not, deviate outside given limits. 



Figure 5/11/1. 



The formulation seems to be adequate provided that we measure 

 the limb's deviations from some line which is given arbitrarily, 

 usually by a knowledge of the line followed by the normal limb. 

 A first example is given by Figure 5/11/1, which shows the line 

 traced by the point of an expert fencer's foil during a lunge. 

 Any inco-ordination would be shown by a divergence from the 

 intended line. 



A second example is given by the record of Figure 5/11/2. The 

 subject, a patient with a tumour in the left cerebellum, was asked 



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