10 THE MECHANISTIC THEORY OF LIFE 



system. Thus the nervous mechanism by 

 which the breathing is so regulated as to bring 

 to the lungs just the requisite quantity of air, 

 in spite of rapid and enormous changes in the 

 volume of air required, is actuated by chemical 

 stimuli conveyed to the respiratory centre by 

 the blood which bathes it. The nervous or 

 other mechanisms by means of which the 

 activity of the heart and the distribution of 

 blood in different parts of the body are pro 

 perly regulated have also been to a large 

 extent elucidated. 



It is not, however, in connection with 

 the nervous system only that co-ordinating 

 mechanisms have been discovered. It has 

 been known for long that chemical substances 

 produced by the activity of one organ are 

 conveyed by the blood to other organs, in 

 which they excite activity of such a nature as 

 to maintain the normal structure, composition 

 and functional activity of the body as a whole. 

 Thus the secretory action of the kidneys 

 varies according to the nature and amount 

 of the various substances discharged into the 

 blood, or withdrawn from it, by other organs. 

 If, for instance, an excess of water is taken up 



