THE LAWS OF LIVING PROTOPLASM 25 



numerous adaptations of organisms. The Law 

 of Sensation includes the reactions of living proto- 

 plasm to external stimuli. "Any external condi- 

 tion which modifies the activities of a living 

 organism may be called a stimulus." There are 

 positive stimuli, such as light and sound, and nega- 

 tive, such as darkness and silence. 



g. The Law of Maintenance of Life ( Metab- 

 olism). — Not all of the young plants and animals 

 grow to maturity nor do all adults continue to live 

 to old age. Death is one of the most obvious facts 

 connected with life, and it occurs at all ages. The 

 keeping of the living body alive is a process dis- 

 tinct from growth, for it continues long after 

 growth ceases. 



All biologists, physiologists and pathologists use 

 a rather exact and technical term to describe this 

 general condition which is true for all periods of 

 life, from the embryonic stage through youth to 

 maturity and old age. The term is Metabolism. 

 The Law of Metabolism, includes the chemical 

 changes that take place in food after it has been 

 eaten, the changes while it is being built up into 

 living protoplasm, and the changes through which 

 it passes in finally furnishing energy to the living 

 machine, or in being cast of as waste. These sev- 

 eral changes are engaging some of the best scien- 

 tific minds of our day, among both chemists and 

 biologists, and yet much remains unknown. We 



