356 THE CIRCULATION OF THE BLOOD. [CHAP. XXVIII. 



wards or towards the heart. It is by reason of the existence of 

 such an apparatus at the mouth of the aorta that the elastic coat of 

 the arteries by its reaction propels the blood. But the muscular 

 coat would not contract simultaneously at all points as the elastic 

 coat does. It would, as in the oesophagus and intestines, act in 

 successive portions and the artery would, as in those tubes, be 

 almost or altogether obliterated at the point of contraction. It is 

 easy, however, to show that no such vermicular action takes place 

 in the arteries, nor can it occur in tubes which, like them, are 

 always and at all points filled. 



It seems most probable that the contractile power of the arteries 

 exercises a regulating influence upon the flow of blood through them. 

 Its influence in this respect has long been recognized by practical 

 men, under the name of tone or tonic power. It restrains within 

 due bounds the distension of the arteries, limits the quantity of 

 blood in each artery, adapts the size of the artery to the volume of 

 its contents, and offers a certain amount of opposition or antagonism 

 to the force of the heart. 



It is owing to the resistance afforded by this contractile power 

 of arteries to the passage of fluid into them, that the anatomist will 

 fail to inject a tissue completely, if he attempt it too soon after 

 death. The well-known experiment of John Hunter, on the pla- 

 centa, shows how long the contractile power will remain in the 

 arteries of a part after its separation from the system, or after 

 death. In a woman delivered on Thursday, the navel string was 

 separated from the foetus in the usual way, by tying the cord in 

 two places, and dividing it between them thus the blood was 

 retained in the vessels of the cord and placenta. On the Friday 

 morning, a ligature was placed an inch below the lowest of those 

 ligatures, and that inch was cut off. The blood immediately gushed 

 out, and, on watching the cut ends of the vessels, Hunter observed 

 the arteries contracting with the whole of their elastic power, 

 which took place immediately. The next morning (Saturday), on 

 examining the mouths of these arteries, they were found quite closed 

 up, so that in twenty-four hours the muscular coat had contracted 

 to such a degree as to close up the area of the artery. On Saturday 

 morning the experiment of Friday was repeated with another inch 

 of the cord, with precisely the same results, but after its repetition 

 on the Sunday, it was found on the Monday that the mouths of the 

 arteries remained open, their muscular coat having by that time 

 lost its contractility.* 



* Hunter, Joe. cit., p. 11 6. The muscular fibres of the arteries of a part 



