142 HAND-BOOK OF PHYSIOLOGY. 



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 

 assists, 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. Simple 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 ani- 

 mals. 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 irritating 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 ex- 

 posed, 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. 



This persistence of the contractile property after death was well shown 

 in an observation of Hunter, which may be mentioned as proving, also, 

 the greater degree of contractility possessed by the smaller than by the 

 larger arteries. Having injected the uterus of a cow, which had been 

 removed from the animal upward of twenty-four hours, he found, after 

 the lapse of another day, that the larger vessels had become much more 

 turgid than when he injected them, and that the smaller arteries had 

 contracted so as to force the injection back into the larger ones. 



THE PULSE. 



If one extremity of an elastic tube be fastened to a syringe, and the 

 other be so constricted as to present an obstacle to the escape of fluid, 

 we shall have a rough model of what is present in the living body: The 

 syringe representing the heart, the elastic tube the arteries, and the con- 

 tracted orifice the arterioles (smallest arteries) and capillaries. If the 

 apparatus be filled with water, and if a finger-tip be placed on any 

 part of the elastic tube, there will be felt with every action of the syringe, 

 an impulse or beat, which corresponds exactly with what we feel in the 

 arteries of the living body with every contraction of the heart, and call 

 the pulse. The pulse is essentially caused by an expansion wave, which 

 is due to the injection of blood into an already full aorta; which blood 



