DESIGN FOR A BRAIN 9/7 



9/7. Every cell contains many variables that might change in 

 a way approximating to the step-function form, especially if the 

 time of observation is long compared with the average time of 

 cellular events. Monomolecular films, protein solutions, enzyme 

 systems, concentrations of hydrogen and other ions, oxidation- 

 reduction potentials, adsorbed layers, and many other constituents 

 or processes might act as step-mechanisms. 



If the cell is sufficiently sensitive to be affected by changes of 

 atomic size, then such changes might be of step-function form, 

 for they could change only by a quantum jump. But this source 

 of step-functions is probably unavailable, for changes of this 

 size may be too indeterminate for the production of the regular 

 and reproducible behaviour considered in this book (S. 1/14). 



Round the neuron, and especially round its dendrons and axons, 

 there is a sensitive membrane that might provide step-functions, 

 though the membrane is probably wholly employed in the trans- 

 mission of the action potential. Nerve ' fibrils ' have been des- 

 cribed for many years, though the possibility that they are an arte- 

 fact cannot yet be excluded. If they are real their extreme 

 delicacy of structure suggests that they might behave as step- 

 functions. 



The delicacy everywhere evident in the nervous system has 

 often been remarked. This delicacy must surely imply the 

 existence of step-functions ; for the property of being ' delicate ' 

 can mean little other than ' easily broken ' ; and it was observed 

 in S. 7/19 that the phenomenon of something ' breaking ' is the 

 expression of a step-function changing value. Though the argu- 

 ment is largely verbal, it gives some justification for the opinion 

 that step-mechanisms are by no means unlikely in the nervous 

 system. 



' The idea of a steady, continuous development ', said 

 Jacques Loeb, ' is inconsistent- with the general physical 

 qualities of protoplasm or colloidal material. The colloidal 

 substances in our protoplasm possess critical points. . . 

 The colloids change their state very easily, and a number of 

 conditions . . . are able to bring about a change in their 

 state. Such material lends itself very readily to a discon- 

 tinuous series of changes. ' 



126 



