CONFORMITY WITH PLAN 351 



expressions describe what is done by the connecting portion 

 between reception and effect, an action which is sometimes 

 more simple and sometimes more complex. 



The three links in the function-chain of the individual 

 may be described as " reception — conduction of excitation — 

 effect," or as " observe — steer — act." 



Each individual must form an indivisible whole, because, 

 on the one side, it serves as the peg part of the machine, and, 

 on the other, as the socket, while at the same time it makes 

 the connection between the two. Any cell will serve as an 

 example of an elementary building-stone of the kind. The 

 simplest piece of framework we can think of consists of a peg 

 and a socket, with a single connecting-route between them. 

 The simple reflex-arcs, in spite of being built up of several 

 cells, nevertheless form simple pieces of framework, when, by 

 means of one single path through the centre, they effect the 

 connection between a nerve-cell and a muscle- or gland-cell. 



As soon as several receiving cells are connected through 

 a centre with several effector cells, the steering in the centre 

 comes into its own, and guides the excitation, now into one 

 route, and now into another. The steering, on its side, is 

 influenced by the " tone " of the whole, which may change 

 periodically. This decides the appearance of thresholds, 

 which divert the excitation, and so invade the steering. In 

 the same way, the threshold in the centre can be influenced 

 by the effector-organ. We know of muscles that are rhythmi- 

 cally " locked," and then " unlocked " (towards excitation). 

 In this case we speak of a " refractory period." 



The impulses, as we have seen, also influence the centre 

 and, within modest limits, enable the body to perform new 

 actions, which we call " plastic." The plastic actions also 

 are restricted within a predetermined frame, and never go 

 outside the limits set for the individual. 



This limitation, again, results from the congruity which 



