PEOTOPLASM 5 



are on the one hand destructive (katabolic), on the other 

 constructive (anabolic). Living matter thus differs from 

 dead matter simply in this respect, that side by side with 

 destructive changes, constructive changes are always going 

 on, whereby its amount is maintained or increased. 



Hence our conception of living matter is not of a definite 

 chemical substance, but of a substance constantly undergoing 

 internal changes. It might be compared to a whirlpool con- 

 stantly dragging things into its vortex, and constantly throw- 

 ing them out more or less changed, but itself continuing 

 apparently unchanged throughout. Hoppe-Seyler expresses 

 this by saying : " The life of all organisms depends upon, or, 

 one can almost say, is identical with, a chain of chemical 

 changes." Foster puts the same idea in more fanciful 

 language : " We may speak of protoplasm as a complex sub- 

 stance, but we must strive to realise that what we mean by 

 that is a complex whirl, an intricate dance, of which, what 

 we call chemical composition, histological structure, and gross 

 configuration are, so to speak, the figures." 



The rate of these chemical processes may be quickened or 

 slowed by changes in the surroundings, and such changes are 

 called stimuli. If the stimulus increases the rate of change, it 

 is said to excite ; if it diminishes the rate of change, it is said 

 to depress. Thus the activity of the changes in yeast may 

 be accelerated by a slight increase of the temperature of the 

 surrounding medium, or it may be depressed by the addition 

 of such a substance as chloroform water. 



While the continuance of these chemical changes in proto- 

 plasm is life, their stoppage is death. For the continuance 

 of life the building-up changes must be in excess of or equal 

 to the breaking-down. The evolution of energy must be 

 sufficient for growth and maintenance. It is only the surplus 

 over this which is available for external work. In youth the 

 surplus energy is largely used for growth, in manhood for 

 work. When failure in the supply or in the utilisation of the 

 energy-yielding material occurs, the protoplasm dwindles and 

 disintegrates. Death is sudden when the chemical changes 

 are abruptly stopped, slow when the anabolic changes are 

 interfered with. The series of changes which occur between 

 the infliction of an incurable injury and complete disintegration 



