CHAPTER XIX 

 THE REGULATION OF THE CIRCULATION 



We have treated the subject of the circulation thus 

 far from a mechanical point of view. We have dealt 

 with average conditions as they may be visualized for 

 the human being, and we have said little or nothing of 

 the adaptive changes which take place from time to 

 time. But the reader has learned that adaptive changes 

 are to be looked for in any living system and perhaps 

 they are nowhere more striking than in the heart and 

 the blood-vessels. The facts have been hinted at in 

 Chapter VIII; a fuller exposition is now called for. 



The regulation of the blood-flow may be considered 

 under two heads. We must give an account of the 

 influence exerted through the nerve centers upon the 

 heart, and we must later consider the vasomotor mechan- 

 ism. Under the first head we deal with the total 

 quantity of blood pumped and under the second we 

 attend to its varying distribution. 



We have properly emphasized the automatic character 

 of the heart. But we must not ignore the influence of 

 the nervous system upon its rate and force. The heart 

 is like a horse in harness: it is seen to be working at 

 a certain rate and there is no doubt of its independent 

 power to act, but we cannot say that it is doing quite 

 what it would do if left free from all guidance. To 

 destroy the brain is not to stop the heart, but a change 

 in its rate under the circumstances is to be expected. 

 In the same way a horse may go on his way when his 

 driver has fallen from his seat, but a change in his pace 

 is highly probable. 



The statement has already been made that the 

 severing of connections between the nervous system 



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