DESIGN FOR A BRAIN 



9/5 



We must beware, for instance, of asking where they are; for 

 this question assumes that they must be somewhere, and then the 

 1 where ' is apt to be interpreted anatomically, or histologically, 

 or in some other way that is not appropriate to the actual variable. 

 Some calculating machines, for instance, carry their records in 

 the form of a train of pulses that circulates around a cyclic path 

 that includes a long column of mercury. Each pulse behaves as a 

 step-function in that it has only two values: present or absent. 

 It is localised by its position in the sequence, but it has no localisa- 

 tion in any particular part of the column. (This is the sort of 

 4 localisation ' that the Fraunhofer lines have as sunlight comes to 

 us: they occupy a definite place in the spectrum but no unique 

 place in three-dimensional space.) 



With these warnings in mind, a brief review will now be given 

 of some of the possibilities. (The list is almost certainly not 

 complete, and at the present time it is probably most important 

 that we should be alert for forms not yet considered.) 



9/5. A possibility early suggested by Young was that a closed 

 circuit of neurons might carry a stream of impulses and be self- 

 maintaining in this excited state. Unexcited it would stay at 

 rest, excited it would be active maximally; such a system could 

 carry permanently the effects of some event. 



Lorente de No has provided abundant histological evidence that 

 neurons form not only chains but circuits. Figure 9/5/1 is taken 



Figure 9/5/1 : 

 reflex arc. 



Neurons and their connexions in the trigeminal 

 (Semi-diagrammatic ; from Lorente de No.) 



from one of his papers. Such circuits are so common that he has 

 enunciated a ' Law of Reciprocity of Connexions ' : * if a cell- 

 complex A sends fibres to cell or cell-complex B, then B also sends 

 fibres to A, either direct or by means of one internuncial neuron '. 



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