CONTRACTILITY OF ARTERIES. 119 



to the air or cold, it gradually but manifestly contracts. Hun- 

 ter 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 ; for after death, when the vital muscular power 

 has ceased, and the mechanical elastic one alone operates, the 

 contracted artery dilates again. 



(2.) When aii 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 animals. In part this con- 

 traction is due to elasticity, but in part, no doubt, 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. Such irritation would not be followed by these effects, 

 if the arteries had no other power of contracting than that 

 depending upon elasticity. 



(3.) The contractile property of arteries continues many 

 hours after death, and thus affords an opportunity of distin- 

 guishing it from elasticity. When a portion of an artery, the 

 splenic, for example, 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 per- 

 manently retains the same size. If, while contracted, the ar- 

 tery be forcibly distended, its contractility is destroyed, and 

 it holds a middle or natural size. 



This persistence of the contractile property after death was 

 well shown in an observation of Hunter, which may be men- 

 tioned as proving, also, the greater degree of contractility pos- 

 sessed by the smaller than by the larger arteries. Having 

 injected the uterus of a cow, which had been removed from 

 the animal upwards 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 results of an experiment which Hunter made with the 

 vessels of an umbilical cord prove still more strikingly the 

 long continuance of the contractile power of arteries after 

 death. In a woman delivered on a Thursday afternoon, the 

 umbilical cord was separated from the foetus, having been first 

 tied in two places, and then cut between, so that the blood 



