THE MOTION OF THE BLOOD. 189 



have been convinced that the influence of the heart could not 

 reach the extreme arteries and the origins of the veins, they have 

 ascribed the progression of the blood in those vessels to a kind 

 of oscillation" 



These oscillations are quite imaginary, and now disallowed. 

 Although variations of dilatation must affect the course of the 

 blood through vessels, it is difficult to conceive how any regular 

 action of them can assist it, while the blood is propelled by and 

 drawn to the heart ; and the influence of the heart was seen by 

 Dr. Hastings, in some microscopical experiments in which partial 

 obstruction was produced, to extend to arteries, capillaries, and 

 veins, as the blood in them all received a sensible impulse at 

 each contraction of the ventricles. Indeed, we have ocular proof 

 that the capillaries do not contract on the blood in the ordinary 

 state of things ; for the blood in them, as well as in the arteries 

 and veins, may be seen for an hour together in the frog's foot, 

 under the microscope, to move in a stream unvarying neither 

 becoming finer alternately nor experiencing impulses. 1 



In foetuses without hearts u , it is not proved that the vascular 

 system carries on the circulation by its own power, because a 

 twin without a heart has never been seen, unless accompanied by 

 a perfect foetus, whose heart might circulate the blood of both ; 

 for placentae often communicate, so that one child has died 

 of haemorrhage from the chord of the other : and in the only 

 case where the matter was ascertained x , the akerious foetus was 

 actually injected by the navel-string of the perfect foetus, y 

 When, however, the blood is not moved by the heart, the capilla- 

 ries do impel it. Dr. Wilson Philip once saw it moving freely in 

 some mesenteric capillaries of a rabbit for an hour and a quarter 

 after the excision of the heart 2 ; and Haller and Bichat made 

 similar observations. 



Mr. Burns 3 , anxious to prove that the arteries are of more 

 importance than the heart, that they themselves circulate the 



1 Dr. Hastings, 1. c. p. 46. sq. Dr. Magendie, Journal de Physiol. t. i. 

 p. 107. sq. says that the blood streams in the arteries and veins of cold-blooded 

 animals, as if the vessels were motionless. 



u Hewson, Exp. Enquiry, v. ii. p. 15. Sir B. Brodie, Phil. Trans. 1806. 



* Phil. Trans. 1793. p. 155. 



y Dr. Young, Introduction to Med. Literature. 1823. 2d edit. p. 631. sq. 



z An Experimental Enquiry into the Laws of the Vital Functions. 3d ed. expt. 67. 



a Observations on some of the most frequent and important Diseases of the Heart, 

 &c. By Allan Burns. 1809. p. 117. sqq. 



O 



