DESIGN FOR A BRAIN 8/4 



that flow in F when the needles are deviated to more than about 

 45 degrees from the central positions. The main variables are 

 continuous, and act and react on one another, giving the primary 

 feedback, which is complex, like A of Figure 4/15/1. The field 

 of the four main variables has only one state of equilibrium (at 

 the centre), which may be stable or unstable. Thus the system 

 is either stable and self-correcting for small impulsive displace- 

 ments to the needles, or unstable and self-aggravating, running 

 away to the limits of the troughs. Which it will be depends on 

 the quantitative details of the primary feedbacks, which are 

 dependent on the values on the step-mechanisms. 



The step-mechanisms of S. 7/12 can be made to correspond to 

 structures on the Homeostat in several ways, which are equivalent. 

 Perhaps the simplest way is to identify them with the twelve 

 values presented by the uniselectors at any given moment (three 

 on each). If the needle of a unit diverges for more than a few 

 seconds outside the limits of ±45°, the three values of its step- 

 functions will be changed to three new values. These new values 

 have no special relation either to the previous values or to the 

 problem in hand — they are just the values that next follow in 

 Fisher and Yates' table. 



It is easily seen that if any one, two, or three of the units are 

 used (as is often done for simplicity) this subsystem will still be 

 ultrastable. 



The Homeostat as adapter 



8/4. A remarkable property of the nervous system is its ability 

 to adapt itself to surgical alterations of the bodily structure. 

 From the first work of Marina to the recent work of Sperry, such 

 experiments have aroused interest and no little surprise. 



Over forty years ago, Marina severed the attachments of the 

 internal and external recti muscles of a monkey's eyeball and 

 re-attached them in crossed position so that a contraction of 

 the external rectus would cause the eyeball to turn not outwards 

 but inwards. When the wound had healed, he was surprised to 

 discover that the two eyeballs still moved together, so that 

 binocular vision was preserved. 



More recently Sperry severed the nerves supplying the flexor 

 and extensor muscles in the arm of the spider monkey, and re- 

 joined them in crossed position. After the nerves had regenerated, 



104 



