INTRODUCTORY : THE ANIMAL ORGANISM 5 



certain of its actions consist in the chemical manufacture of 

 materials which are not purely waste but have their uses to 

 the body. The casting out from the substance of the 

 glands of the materials formed in these two cases, and of 

 the water in which they are dissolved, is a necessary part of 

 the working of the bodily machine. The waste products 

 are got rid of because they are poisonous, and the products 

 of chemical manufacture are removed in order to be of use 

 elsewhere. Both kinds of material are accordingly shed, 

 sometimes upon the surface of the body, but usually into 

 vessels which conduct them to the required locality. This 

 shedding out is a distinct process, carried on by an exercise 

 of the activity of the living 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 1 when they are 

 of some further use to the body, as in the gastric juice. 

 Finally, it is possible that an expenditure of energy is 

 involved in the conveyance of the impulse which starts any 

 response from the spot where the stimulus is received to the 

 locality where the main part of the response takes place. 

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

 is brushed off, an impulse is started in the skin and con- 

 veyed 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 conduc- 

 tivity. The mode of conduction is not understood, but it 

 may be that it involves the evolution of energy by the 

 disintegration of the conducting substance. 



It should be noted that the forms in which the energy 

 of the body is used in these various 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, 2 as in the well- 



1 The term "secretion" is sometimes loosely used to include the 

 formation as well as the removal of chemical products, or even applied 

 to the substances secreted, for which secreta is a better name. 



2 Small electrical changes accompany many of the processes of the 

 body. 



