THE MOTION OF THE BLOOD. 181 



The influence of a vacuum, pointed out by Rudiger 1 , enlarged 

 upon by Dr. Andrew Wilson, and mentioned as probable by 

 Haller m , John Hunter , &c., has been very ably displayed by 

 Dr. Carson of Liverpool. 



The quantity of the blood, the length of its course, and the 

 various obstacles opposed to its progress, render, in his opinion, 

 the mere propulsive power of the heart insufficient to maintain 

 the circulation perpetually. But assistance must be given by the 

 vacuum which takes place in all the cavities of the organ, when 

 the contraction of the muscular fibres is over. The blood is thus 

 drawn into each relaxed cavity, and the heart performs the double 

 office of a forcing and a suction pump. The situation of the valves 

 of the heart is thus explained. There are valves at the mouths 

 of the two great arteries, because behind each of these openings 

 is a cavity of the heart, alternately dilating and affording a va- 

 cuum, into which, were there no valves, the blood would be drawn 

 retrograde. There are valves between the auricles and ventricles, 

 because the contraction of the ventricles tends to impel the blood 

 back into the auricles, as well as into the pulmonary artery and 

 aorta. At the venous openings of the auricles no valves exist, 

 because they do not open from a part ever experiencing a va- 

 cuum and the blood does not appear to leave the sinuses of the 

 auricles so much by their contraction, which would impel it in 

 all directions, like the ventricles, as by the vacuum offered it in 

 the dilated ventricle ; and therefore the blood of the auricles will 

 not move retrograde, but will necessarily pass forwards into the 

 ventricles, which are offering a vacuum. The inferior elasticity and 

 irritability of the veins are also explained. If veins were capable 



1 Quoted by Haller, El. Physiol. t. ii. lib. vi. p. 325. 



m His words are " Sanguinem in auriculam dextram, tanquam in vacuum cas- 

 tellum approperare, ne id quidem videtur absque specie veri dici." 1. c. An idea 

 of the same kind appears to have been entertained before the time of Rudiger, 

 whose work, De Regressu Sanguinis per Venas mechanico, was published at Leipsig 

 in 1 704. For in Pecquet's Experimenta nova Anatomica, published in 1 651, argu- 

 ments are adduced against those who conceived that the diastole sucked the blood 

 towards the heart, (" num, ut quibusdam placuit, ATTRAHENDO pelliciat EXUGATVE, 

 investigandum." Chap. vii. sqq.) At that time suction was not generally known 

 to be merely a means of removing or diminishing the resistance to the pressure 

 of air, but supposed to be an occult principle. He details experiments to show 

 its true nature, but urges nothing against suction in the proper acceptation of the 

 term, and his adversaries were right in their fact, though ignorant of its true nature. 



n A Treatise on the Blood, &c. p. 185. 



