ELECTRONS AND ATOMS 45 



the cycle of life of the individual. There is 

 a popular fallacy in lay minds that the whole 

 human body is replaced by fresh material 

 in a period which by some whimsical fancy 

 has been fixed at seven years. As a matter of 

 fact, some cells are formed, pass to maturity, 

 and perish almost daily, while others last 

 as long as the animal itself. Many of these 

 latter cells form master-cells of the body, of 

 vital importance, and their decay determines 

 the downfall of the whole vast community. 

 Such cells are to be found in the brain and 

 other parts of the central nervous system, in 

 arterial walls, and in mechanisms which 

 control the heart. As age advances the work 

 of these, and other cells, alters, and chemical 

 products are thrown out and accumulate 

 which sow the seeds of decay. Decline and 

 death are accordingly part of a normal 

 process just as much as birth and growth are 

 at the early parts of the course of life, and, so 

 far as our present knowledge leads us, all we 

 can hope for is by watchful care to prevent 

 earlier decay by seeing to it that these delicate 

 mechanisms are not clogged and poisoned by 

 effete products of a poisonous nature manu- 

 factured by wrongful habits of life, either in 

 the body itself, or added from without by 

 unsuitable nutrition, or the invasions of 



