INTRODUCTION. 13 



expand like bellows. This I think easy of demonstration; and 

 indeed conceive that I have already proved it. Nevertheless, 

 in that book of Galen headed ' Quod Sanguis continetur in Ar- 

 teriis,' he quotes an experiment to prove the contrary: An artery 

 having been exposed, is opened longitudinally, and a reed or other 

 pervious tube, by which the blood is prevented from being lost, 

 and the wound is closed, is inserted into the vessel through the 

 opening. " So long," he says, " as things are thus arranged, the 

 whole artery will pulsate ; but if you now throw a ligature about 

 the vessel and tightly compress its tunics over the tube, you will 

 no longer see the artery beating beyond the ligature." I have 

 never performed this experiment of Galen's, nor do I think that 

 it could very well be performed in the living body, on account of 

 the profuse flow of blood that would take place from the vessel 

 which was operated on ; neither would the tube effectually 

 close the wound in the vessel without a ligature ; and I cannot 

 doubt but that the blood would be found to flow out between 

 the tube and the vessel. Still Galen appears by this experi- 

 ment to prove both that the pulsative faculty extends from the 

 heart by the walls of the arteries, and that the arteries, whilst 

 they dilate, are filled by that pulsific force, because they expand 

 like bellows, and do not dilate because they are filled like skins. 

 But the contrary is obvious in arteriotomy and in wounds ; for 

 the blood spurting from the arteries escapes with force, now 

 farther, now not so far, alternately, or in jets; and the jet always 

 takes place with the diastole of the artery, never with the systole. 

 By which it clearly appears that the artery is dilated by the 

 impulse of the blood ; for of itself it would not throw the blood 

 to such a distance, and whilst it was dilating; it ought rather 

 to draw air into its cavity through the wound, were those things 

 true that are commonly stated concerning the uses of the arte- 

 ries. Nor let the thickness of the arterial tunics impose upon 

 us, and lead us to conclude that the pulsative property pro- 

 ceeds along them from the heart. For in several animals the 

 arteries do not apparently differ from the veins ; and in extreme 

 parts of the body, where the arteries are minutely subdivided, as 

 in the brain, the hand, &c., no one could distinguish the arteries 

 from the veins by the dissimilar characters of their coats; the 

 tunics of both are identical. And then, in an aneurism pro- 

 ceeding from a wounded or eroded artery, the pulsation is pre- 



