Circulation of the Blood 135 



dilate as bellows, are not filled as sacs. This experi- 

 ment is spoken of by Vesalius, the celebrated anatomist ; 

 but neither Vesalius nor Galen says that he had tried 

 the experiment, which, however, I did. Vesalius only 

 prescribes, and Galen advises it, to those anxious to 

 discover the truth, and for their better assurance, not 

 thinking of the difficulties that attend its performance, 

 nor of its futility when done ; for indeed, although 

 executed with the greatest skill, it supplies nothing in 

 support of the opinion which 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 r 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 cannot imagine that the pulsa- 

 tion 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 



