138 THE CIRCULATION. 



selves to the different movements of the several parts of 

 the body. 



We have already referred to the fact that the middle 

 coat of the arteries is composed of unstriped muscular 

 fibres, mingled with fine elastic filaments. The evidence 

 for the muscular contractility of arteries may, however, be 

 given briefly for the sake of the physiological facts on 

 which it hinges. 



(i.) 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; for after death, 

 when the vital muscular power has ceased, and the 

 mechanical elastic one alone operates, the contracted 

 artery dilates again. 



(2.) When an artery is cut across, its divided ends con- 

 tract, 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 contraction 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 dis- 

 tinguishing 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 





