42 Foster, Physical Basis of Psychical Events. 



of the delicately varied and exquisitely cared for nutrition 

 of the nervous unit, in view of the molecular labours, 

 other than those issuing in change of form, which it has 

 to carry out. 



I have just urged that the fine differentiation of 

 nervous material entails the suppression of the funda- 

 mental primeval power of change of form. I may now 

 add that the differentiation entails the loss of another 

 fundamental property, that of reproduction. While in 

 other tissues, notably in the skin and muscles, old units, 

 old cells worn out by use or by divers influences are 

 replaced by new ones, a fact broadly but incorrectly ex- 

 pressed in the popular statement that the whole body is 

 renewed every seven years, the evidence which we poss ess 

 goes to show that such a regeneration, that the appearance 

 of new units by birth from old ones, comes to an end at 

 an early period of life in the case of the nervous system. 

 Take away a piece of skin, of bone, or of connective 

 tissue, new stuff of the same kind makes good the loss ; 

 but a piece of brain once lost is lost for ever ; the cells, 

 the units which made it up, are never replaced. Examine 

 almost any tissue of the body other than that of the 

 nervous system, at almost any period of life, and you will 

 find evidence in the changes going on in the nuclei that 

 old cells are giving birth to new ones. Examine in the 

 same way the nervous system for like signs, and after a 

 certain date, which we may probably fix as that of very 

 early youth, you will find none. At a very early date the 

 installation of nervous units is completed ; after that no 

 new units make their appearance. So far as the number 

 of units is concerned, the machinery is a finished one. 

 All the growth, all the increase of complexity, hence- 

 forward possible, is accomplished not by the introduction 

 of new units, but by the extension and development of 

 those already installed. 



