CONTRACTILITY OF THE ARTERIES. 659 



Hunter demonstrated the existence of this contractility in portions of 

 the moderate-sized arteries, which, he showed, went on contracting, 

 for a time, after death, by a sort of rigor mortis, and then dilated again, 

 owing to the resiliency of the elastic coat. Poiseuille found that, after 

 subjection to an equal distending force, an artery, which still retained 

 its vital contractility, contracted more than a perfectly dead one ; he 

 also observed that when a living artery was injected with a certain 

 force, it recoiled with a greater force, a result implying more than the 

 reaction of mere elasticity, which could only be equal to the original 

 force. The vital contractility of the smaller arteries, has been demon- 

 strated in the mesenteric arteries of toads and frogs, by means of cold 

 (Schwann), by the application of magneto-electricity to the frog's web 

 (Weber and Kolliker), and in still smaller vessels in the mouse, bat, 

 and frog, by various chemical, irritant, and mechanical stimuli (Whar- 

 ton Jones, Lister, and many others). Weber found that minute arteries 

 begin to contract in two or three seconds after stimulation; in five to 

 ten seconds, they are diminished to half their original area, and, the 

 stimulus being continued, become completely closed, after which, the 

 electrical current being removed, they slowly dilate again to their 

 original size. The vital contractility of the arteries may be excited 

 through the nervous system, either directly, or in a reflex manner; for 

 they undergo changes in diameter, through the contraction or relaxa- 

 tion of their muscular coat, induced by division or irritation of the 

 vasi-motor nerves or nervous centres (p. 808). Two purposes are ful- 

 filled by this vital contractility of the arteries; first, that of slowly 

 adapting the capacity of the entire arterial system, to the quantity of 

 blood circulating through it; and secondly, under the control of the 

 nervous system, that of modifying the relative quantity permitted to 

 flow to any given organ. Moreover, if. during life, a small artery be 

 cut quite across, its contractility closes its orifice, and so arrests fur- 

 ther hemorrhage. This fact, indeed, is quoted as a proof of its con- 

 tractility ; for the elasticity of the arterial walls is insufficient to account 

 for the perfect contraction of a wounded vessel, and would rather tend 

 to keep it partly open, as we see happens in a dead artery. 



It has been supposed that the contractility of the arteries might 

 serve, as well as their elasticity, to adapt them to the intermittent and 

 variable pressure of the blood projected into them by the heart; but 

 there is no evidence of this, and the characteristic slowness of action 

 of organic muscular fibres, renders it doubtful whether the arteries 

 could alternately relax and contract, concurrently with the rapid action 

 of the heart. 



The so-called tone of the arterial system, seems to depend on a healthy 

 contraction of the muscular coat the so-called property of tonicity be- 

 ing a continued exercise of muscular contractility. This tonicity is 

 shown by the contraction which takes place in the portion of an artery 

 included between two ligatures, when it is punctured to allow of the 

 escape of its contained blood; also by the gradual emptying of an ar- 

 terial trunk beyond any point at which it has been tied a contraction 

 much more complete than elasticity can explain ; and, again, by the 

 almost complete obliteration of the canal of a portion of artery removed 



