178 THE HUMAN MECHANISM 



causing us to move about more briskly, or, if we do not, 

 by causing us to shiver, so that this really becomes a case 

 of muscular activity. The same thing is true of awakening 

 from sleep. We may therefore make the general statement 

 that muscular activity is the one important agent of life 

 which increases tissue respiration. 



And this increase is at times very great. Even the mus- 

 cular activity necessary to maintain the erect position in 

 sitting and standing, as compared with the complete relax- 

 ation of sleep, doubles the gaseous exchange ; gentle exercise 

 (a walk of three miles an hour) more than doubles that of 

 rest ; and vigorous, yet by no means excessive, exercise will 

 increase it tenfold. These increases mean corresponding, 

 though not absolutely proportionate, demands on the heart 

 and emphasize the importance of keeping that organ in an 

 efficient working condition. Breathlessness, for example, 

 usually indicates, in part at least, that the heart fails to 

 respond properly to the demands made upon it, these de- 

 mands being greater than it can meet without undue fatigue; 

 it is a warning that we are pushing the heart too hard, a 

 warning which we will do well to heed. Generally it is 

 also a warning that we are not getting sufficient muscular 

 activity; the heart fails to meet the emergency of some 

 unusual exertion because all along it has not been kept in 

 proper training; so that while we should, as stated, heed 

 the warning not to push the heart so hard for the time 

 being, we should also act upon the equally important warn- 

 ing that it needs practice or training a training which can 

 be given only by reasonable, regular, muscular activity. 



The training of muscular activity is therefore not only a 

 training of the muscles but also of the heart. But this is 

 not all. The work of the circulatory and respiratory mecha- 

 nisms must be adjusted or coordinated, the one to the other. 

 When, for example, the deepened breathing movements 

 accompanying muscular activity rush the blood back more 



