44 Foster, Physical Basis of Psychical Events. 



grow in course of time into a very complex figure, and, 

 moreover, that the extent and character of its growth is 

 to a large extent determined by the demands for action 

 made upon the cell by the incidents of life. In all the 

 tissues of the body we may trace the influence of exercise 

 and the stimulating effect of the demand for action ; but 

 these are seen in their highest form in nervous tissue and 

 most conspicuously perhaps in the nerve cells of the 

 cerebral cortex. The nutrition of every tissue of the body 

 is determined by two factors, by the quantity and quality 

 of the blood supply and by the activity of the tissue itself 

 But whereas in most tissues the former is the predom- 

 inating factor, in nervous tissue and especially in the 

 cortical cells, it is the latter which rules. As we have 

 incidentally seen, the nerve unit, so soon as it is cut off 

 from the play of its fellows upon it, begins to droop, how- 

 ever rich and full the blood stream which reaches it ; it 

 droops the sooner and the more, the quicker was its life 

 before. The potentialites of the unit may have their 

 source in the blood supply which bathes it, but its 

 actualities are determined by the calls made upon it. 



This, in fact, is the physical basis of education. The 

 material of the nerve cell renouncing the grosser duties of 

 bodily movement, of secretion, that is to say the production 

 of chemical substances destined for the good of units 

 other than itself, and of reproduction, concentrates its 

 energies of nutrition on making itself responsive by inner 

 molecular movement to the finest changes in its sur- 

 roundings. It is the seat of a finely balanced nutrition 

 wholly exceeding that of any other tissue. Every inci- 

 dent of its life stirs that nutrition, which indeed is directed 

 to being so stirred ; and, moreover, while producing an 

 immediate effect, leaves a trace behind. When the 

 blows of external influences are repeated often enough, 

 especially if they be heavy enough, the effects are mani- 



