32 THE FUNCTIONS OF ANIMALS. 



physical osmosis ; it is modified by the fact that the cells 

 are living. 



There are many processes going on in the body of 

 which a complete account can be given in chemical and 

 physical terms, but we cannot, at present at least, give in 

 chemical and physical terms an exhaustive account of any 

 distinctively vital action, nor of growth and development, 

 nor of a piece of behaviour. 



But though we cannot analyse living matter, nor 

 thoroughly explain the changes by which the material of 

 the body breaks down or is built up, we can trace, by 

 chemical analysis, how food passes through various trans- 

 formations till it becomes a usable part of the living body, 

 and we can also catch some of the waste products formed 

 when muscles or other parts are active. 



Living protoplasm is in a colloidal state, i.e. ultra- 

 microscopic solid particles and immiscible droplets are in 

 suspension and free movement in a fluid. There is a 

 complex mixture of proteins, carbohydrates, fats, and some 

 inorganic constituents, and 70-90 per cent, of water. In 

 this mixture there is a complex play of forces, such as those 

 of surface-tension .and electrical charge, and a great variety 

 of chemical changes, summed up in the term " metabolism." 

 Different kinds of chemical changes go on in close prox- 

 imity to one another, yet with some degree of separateness, 

 like eddies in a stream. Perhaps the localisation of par- 

 ticular processes within the cell depends on the deposition 

 of more stable, less labile constituents, forming a sort of 

 framework the furniture of the living laboratory. When 

 the substance of a cell is fixed and stained, it often shows 

 an intricate reticular, fibrillar, or alveolar structure, but 

 this seems to be mainly a post-mortem effect. 



It is highly probable that there is no one substance which 

 should be called protoplasm, but that vital phenomena 

 depend upon the interactions of several . complex sub- 

 stances. As Verworn says, "The life-process consists in 

 the metabolism of proteids." 



Generalising from his studies on colour sensation, Pro- 

 fessor Hering was led to regard .all life as an alternation of 

 two kinds of activity, both induced by stimulus, the one 

 tending to storage, construction, assimilation of material, 



