Circulation of the Blood 163 



arterial system, and then absorbed and imbibed from 

 every part by the veins, it returns through these in a 

 continuous stream. That all this is so, sense assures 

 us ; and necessary inference from the perceptions of 

 sense takes away all occasion for doubt. Lastly, this is 

 what I have striven, by my observations and experi- 

 ments, 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 violence, which sight and touch make us aware of in 

 the heart and greater arteries ; and to the systole and 

 diastole constituting 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, pulsate ; 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 dis- 

 tending 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, 



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



