THE CIRCULATION OF THE BLOOD. 137 



escape of blood. It is only in consequence of such contraction and 

 coagulation that we are free from danger through even very slight 

 wounds; for it is only when the artery is closed that the processes for the 

 more permanent and secure prevention of bleeding are established. 



(4) There appears no reason for supposing that the muscular coat as- 

 sists, to more than a very small degree, in propelling the onward current 

 of blood. 



(1.) "When a small artery in the living subject is exposed to the air or 

 cold, it gradually but manifestly contracts. Hunter observed that the 

 posterior tibial artery of a dog when laid bare, became in a short time so 

 much contracted as almost to prevent the transmission of blood; and the 

 observation has been often and variously confirmed. Simply elasticity 

 could not effect this. 



(2.) When an artery is cut across, its divided ends contract, and the 

 orifices may be completely closed. The rapidity and completeness of 

 this contraction vary in different animals; they are generally greater in 

 young than in old animals; and less, apparently, in man than in the 

 lower animals. This contraction is due in part to elasticity, but in part, 

 also, to muscular action; for it is generally increased by the application 

 of cold, or of any simple stimulating substances, or by mechanically irri- 

 tating the cut ends of the artery, as by picking or twisting them. 



(3.) The contractile property of arteries continues many hours after 

 death, and thus affords an opportunity of distinguishing it from their 

 elasticity. When a portion of an artery of a recently killed animal is 

 exposed, it gradually contracts, and its canal may be thus completely 

 closed; in this contracted state it remains for a time, varying from a few 

 hours to two days; then it dilates again, and permanently retains the 

 same size. 



The Pulse. 



If we place our fingers upon the radial artery at the wrist, or upon 

 any artery of the body which is sufficiently superficial, we experience a 

 sensation as if our fingers were alternately lifted or raised up from the 

 artery and allowed to fall again, and this action is repeated very fre- 

 quently in the course of a minute. In other words we feel the pulse of 

 the artery. 



The pulse is generally described as an expansion of the artery pro- 

 duced by the wave of blood set in motion by the injection of blood into 

 the already full aorta at each ventricular systole. 



As the force of the left ventricle, however, is not expended in dilat- 

 ing the aorta only, the wave of blood passes on, expanding the arteries 

 as it goes, running as it were on the surface of the more slowly travelling 

 blood already contained in them, and producing the pulse as it pro- 

 ceeds. 



The distention of each artery increases both its length and its diam- 

 eter. In their elongation, the arteries change their form, the straight 

 ones becoming slightly curved, and those already curved becoming more 



