8 HUMAN PHYSIOLOGY 



constant chemical changes, metabolism, and these changes 

 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 changes 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, and when failure in the supply or in 

 the utilisation of the material used in construction 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 of the tissue constitute 

 the processes of Necrobiosis, and their study is of importance 

 in pathology. 



