II. THE VITAL PHENOMENA OF THE CIRCULATION. 



So far the facts with which we have had to deal, with the ex- 

 ception of the heart's beat itself, have been simply physical facts. 

 All the essential phenomena which we have studied may be re- 

 produced 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 which possess one 

 is capable of local and general modifications, adapting it to local 

 and general changes of circumstances. These modifications fall 

 into two great classes : 



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

 course a general effect. 



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 or general. 



