ON THE MOTION OF THE BLOOD. 79 



various obstacles opposed to its progress, render it unlikely that 

 the mere propulsive power of the heart is sufficient to maintain the 

 circulation perpetually. But great 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 rapid but quiet motion 

 of the blood in the veins is thus accounted for and would other- 

 wise be inexplicable. The situation of the valves of the heart is 

 also accounted for. There are valves between the auricles and 

 ventricles, and at the mouths of the two great arteries, because 

 behind each of these four openings is a cavity of the heart, alter- 

 nately dilating and affording a vacuum, into which, without 

 valves, the blood would be drawn retrograde. At the venous 

 openings of the auricles no valves exist, because they do not 

 open from a cavity of the heart, from a part ever experiencing 

 a vacuum, and therefore the blood cannot, when the auricles 

 contract, move retrograde, but will necessarily pass forwards into 

 the ventricles, which at that moment are offering a vacuum. 

 The inferior elasticity and irritability of the veins are also ex- 

 plained. If veins were capable of contracting equally with arteries, 

 on the diminution of their contents, the suction influence of the 

 heart would constantly reduce their cavities to a smaller capacity 

 than is requisite for their functions. The collapse of the veins 

 by pressure, during the suction of the heart, is prevented by the 

 fresh supply of blood afforded by the vis a tergo, which does exist, 

 although it cannot be considered as of itself adequate to convey 

 the blood back to the right auricle. The reason appears why a 

 tied vein is emptied in the part nearest the heart ; its blood is 

 drawn forwards by suction. We see why a punctured vein does 

 not bleed, if there are other veins to convey the blood discharged 

 from the arteries. The puncture necessarily removes the suction 

 influence of the heart, and the great cause of the progress of the 

 blood in the vein is taken away, while it exists in full force in 

 the other veins of the limb. Were it not for this circumstance, 



