SEC. 5. THE REGULATION AND ADAPTATION OF 

 THE VASCULAR MECHANISM. 



The Regulation of the Beat of the Heart. 



132. 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 unvary- 

 ing mechanical vascular system would however be useless to a 

 living body whose actions were at all complicated. The promi- 

 nent feature of a living mechanism is the power of adapting itself 

 to changes in its internal and external circumstances. And 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. 



2. Changes in the peripheral resistance, due to variations in 

 the calibre of the minute arteries, brought about by the agency of 

 their contractile muscular coats. These changes may be either 

 local, affecting a particular vascular area only, or general, affecting 

 all or nearly all the blood vessels of the body. 



These two classes of events are chiefly governed by the 

 nervous system. It is by means of the nervous system that the 

 heart's beat and the calibre of the minute arteries are brought 

 into relation with each other, and with almost every part of the 

 body. It is by means of the nervous system acting either on 

 the heart, or on the small arteries, or on both, that a change of 



