CHAP, vm THE FUNCTIONS OF THE BODY 139 



corporated in (or absorbed into) the living framework. 

 As to respiration, vital activity implies many kinds of 

 chemical change, but pre-eminently combustion or 

 oxidation ; and respiration is to be regarded as the 

 taking in of oxygen to keep the fire of life burning, and 

 the getting rid of the poisonous waste-product of com- 

 bustion, carbon dioxide. The beginner w r ho has got 

 hold of the idea that respiration means " purifying the 

 blood " should dismiss it, for it does not get at the gist 

 of the business ; and there are many animals without 

 any blood. Moreover, plants have to " respire " as well 

 as animals, though the respiratory function is disguised 

 during the day in green plants by a counteractive nutri- 

 tive process of utilising carbon dioxide in the photo- 

 synthesis of carbon compounds. As to excretion, the 

 functions of the muscular and nervous tissues involve the 

 production of nitrogenous waste-products, and there are 

 other subtle waste-products that are formed in the 

 various laboratories within the body ; the filtering out 

 or elimination of these nitrogenous waste-products (by 

 the kidneys especially) is called excretion. The beginner 

 should note that although the undigested and indigestible 

 remains of the food are called excreta (jceces) when ex- 

 pelled from the food-canal, physiologists mean by the 

 function of excretion the getting rid of nitrogenous waste- 

 products. 



We may say, then, that there are two master activities 

 in the animal body those of muscular and nervous tissues, 

 and that there are three main subsidiary activities 

 nutrition, respiration, and excretion. The subsidiary 

 functions are not ends in themselves as the master 

 activities may be said to be, but they are the indispens- 

 able conditions of the muscular and nervous functions. 

 It is useful to keep by themselves the periodic junctions 

 of growth and reproduction, though, as we shall after- 

 wards see, they must not be separated off in any rigid 

 way, since they are linked to the everyday processes of 

 accumulating energy and repairing worn-out parts. 



2. Functions of the Nervous System. It is through 

 the nervous system that the animal receives stimulation 



