CIRCULATION OF THE BLOOD. Ill 



maintains that the coats of the vessel are the cause of the pulse ; 

 it much rather proclaims that this is owing to the impulse of the 

 blood. For the moment you have thrown your ligature around 

 the artery upon the reed or tube, immediately, by the force of 

 the blood thrown in from above, it is dilated beyond the circle 

 of the tube, by which the flow is impeded, and the shock is 

 broken ; so that the artery which is tied only pulsates obscurely, 

 being now cut off from the full force of the blood that flows 

 through it, the shock being reverberated, as it were, from that 

 part of the vessel which is above the ligature ; but if the artery 

 below the ligature be now divided, the contrary of what has been 

 maintained will be apparent, from the spurting of the blood 

 impelled through the tube; just as happens in the cases of 

 aneurism, referred to in my book on the Motion of the Blood, 

 which arise from an erosion of the coats of the vessel, and 

 when the blood is contained in a membranous sac, formed not 

 by the coats of the vessel dilated, but preternaturally produced 

 from the surrounding tissues and flesh. The arteries beyond 

 an aneurism of this kind will be felt beating very feebly, 

 whilst in those above it and in the swelling itself the pulse will 

 be perceived of great strength and fulness. And here we can- 

 not imagine that the pulsation and dilatation take place by the 

 coats of the arteries, or any power communicated to the walls 

 of the sac ; they are plainly due to the shock of the blood. 



But that the error of Vesalius, and the inexperience of those 

 who assert their belief that the part below the tube does not 

 pulsate when the ligature is tied, may be made the more appa- 

 rent, I can state, after having made the trial, that the inferior 

 part will continue to pulsate if the experiment be properly per- 

 formed ; and whilst they say that when you have undone the 

 ligature the inferior arteries begin again to pulsate, I maintain 

 that the part below beats less forcibly when the ligature is 

 untied than it did when the thread was still tight. But the 

 effusion of blood from the wound confuses everything, and ren- 

 ders the whole experiment unsatisfactory and nugatory, so that 

 nothing certain can be shown, by reason, as I have said, of the 

 hemorrhage. But if, as I know by experience, you lay bare 

 an artery, and control the divided portion by the pressure of 

 your fingers, you may try many things at pleasure by which 

 the truth will be made to appear. In the first place, you will 



