14 THE ANIMAL ORGANISM 



out is a distinct process, carried out by an exercise of the activity 

 of the Hving substance of the body. No real distinction can be 

 drawn between the two cases, but the process is called excretion 

 when the substances cast out are purely waste, as in the urine, 

 and secretion when they are of some further use to the body, as 

 in the gastric juice. Finally, an expenditure of energy is involved 

 in the conveyance of impulses which bring about events in the 

 body, from the localities where the impulses are started by 

 stimuli (p. 6) to the localities in which the events take place. 

 Thus, when a drop of water which has fallen upon the skin is 

 to be brushed off, an impulse is started in the skin and conveyed 

 along those tracts of the body which we know as nerves till it 

 causes such movements of the muscles of the arms as are necessary 

 to brush off the drop. This property in living matter of conveying 

 impulses is known as conductivity, and it involves the evolution 

 of energy by disintegration in the conducting substance. 



It should be noted that the forms in which the energy of the 

 body is used in these and other processes are very different. 

 Besides mechanical movement, the exhibition of molar energy, 

 it may bring about chemical changes, or become heat, as is shown 

 by its warming the human body, or light, as in the glow-worm, 

 or electricity, as in the well-known electric eel, and less conspicu- 

 ously in many events in the human and other living bodies ; 

 and there are other processes, such as secretion, its action in 

 which has not yet been certainly compared with any event in 

 the lifeless world. 



METABOLISM 



It will be seen that disintegration and its complementary 

 assimilation constitute a series of chemical changes, continually 

 taking place in the body, whereby there is kept up a continual 

 evolution of energy. These changes, regarded as a whole, are 

 known as metabolism, the disintegrative changes being known as 

 katabolism and the assimilative as anabolism. 



THE STRUCTURE OF LIVING MATTER 



The characteristics of living matter with which we have dealt 

 so far have been functional, that is, they have been concerned 

 with processes or actions, but there are also others which are 

 structural, or concerned with the form which living matter takes. 



