SEC. 5. THE REGULATION AND ADAPTATION OF 

 THE VASCULAR MECHANISM. 



The Regulation of the Beat of the Heart. 



150. So far the facts with which we have had to deal, with 

 the exception of the heart's beat itself, have been simply physical 

 facts. All the essential phenomena which we have studied may 

 be reproduced on a dead model. Such an unvarying mechanical 

 vascular system would however be useless to a living body whose 

 actions were at all complicated. The prominent feature of a living 

 mechanism is the power of adapting itself to changes in its in- 

 ternal and external circumstances. In such a system as we have 

 sketched above there would be but scanty power of adaptation. 

 The well-constructed machine might work with beautiful regu- 

 larity; but its regularity would be its destruction. The same 

 quantity of blood would always flow in the same steady stream 

 through each and every tissue and organ, irrespective of local and 

 general wants. The brain and the stomach, whether at work and 

 needing much, or at rest and needing little, would receive their 

 ration of blood, allotted with a pernicious monotony. Just the 

 same amount of blood would pass through the skin on the hottest 

 as on the coldest day. The canon of the life of every part for the 

 whole period of its existence would be furnished by the inborn 

 diameter of its blood vessels, and by the unvarying motive power 

 of the heart. 



Such a rigid system however does not exist in actual living 

 beings. The vascular mechanism in all animals in which it is 

 present is capable of local and general modifications, adapting it 

 to local and general changes of circumstance. These modifications 

 fall into two great classes : 



1. Changes in the heart's beat. These, being central, have of 

 course a general effect ; they influence or may influence the whole 

 body. 



F. 18 



