134 CIRCULATION OF THE BLOOD. 



perceptions of sense takes away all occasion for doubt. Lastly, 

 this is what I have striven, by my observations and experiments, 

 to illustrate and make known ; I have not endeavoured from 

 causes and probable principles to demonstrate my propositions, 

 but, as of higher authority, to establish them by appeals to sense 

 and experiment, after the manner of anatomists. 



And here I would refer to the amount of force, even of vio- 

 lence, which sight and touch make us aware of in the heart 

 and greater arteries ; and to the systole and diastole constitut- 

 ing the pulse in the large warm-blooded animals, which I do 

 not say is equal in all the vessels containing blood, nor in all 

 animals that have blood; but which is of such a nature and amount 

 in all, that a flow and rapid passage of the blood through the 

 smaller arteries, the interstices of the tissues, and the branches 

 of the veins, must of necessity take place ; and therefore there 

 is a circulation. 



For neither do the most minute arteries, nor the veins, pul- 

 sate ; but the larger arteries and those near the heart pulsate, 

 because they do not transmit the blood so quickly as they 

 receive it. 1 Having exposed an artery, and divided it so that 

 the blood shall flow out as fast and freely as it is received, you 

 will scarcely perceive any pulse in that vessel; and for the 

 simple reason, that an open passage being afforded, the blood 

 escapes, merely passing through the vessel, not distending it. 

 In fishes, serpents, and the colder animals, the heart beats so 

 slowly and feebly, that a pulse can scarcely be perceived in the 

 arteries ; the blood in them is transmitted gradually. Whence 

 in them, as also in the smaller branches of the arteries in man, 

 there is no distinction between the coats of the arteries and 

 veins, because the arteries have to sustain no shock from the 

 impulse of the blood. 



An artery denuded and divided in the way I have indicated, 

 sustains no shock, and therefore does not pulsate ; whence it 

 clearly appears that the arteries have no inherent pulsative 

 power, and that neither do they derive any from the heart ; but 

 that they undergo their diastole solely from the impulse of the 

 blood ; for in the full stream, flowing to a distance, you may see 

 the systole and diastole, all the motions of the heart their order, 



1 Vide Chapter III, on the Motion of the Heart and Blood. 



