120 CIRCULATION OF THE BLOOD. 



proceeding from the instrumentality of this common agent, viz., 

 the calidum innatum ; they farther regard these spirits as of a 

 sublime, lucid, ethereal, celestial, or divine nature, and the hond 

 of the soul ; even as the vulgar and unlettered, when they do 

 not comprehend the causes of various effects, refer them to the 

 immediate interposition of the Deity. Whence they declare 

 that the heat perpetually flowing into the several parts is in virtue 

 of the influx of spirits through the channels of the arteries ; as 

 if the blood could neither move so swiftly, nor penetrate so in- 

 timately, nor cherish so effectually. And such faith do they 

 put in this opinion, such lengths are they carried by their be- 

 lief, that they deny the contents of the arteries to be blood ! 

 And then they proceed with trivial reasonings to maintain that 

 the arterial blood is of a peculiar kind, or that the arteries are 

 filled with such aereal spirits, and not with blood ; all the while, 

 in opposition to everything which Galen has advanced against 

 Erasistratus, both on grounds of experiment and of reason. 

 But that arterial blood differs in nothing essential from venous 

 blood has been already sufficiently demonstrated ; and our senses 

 likewise assure us that the blood and spirits do not flow in the 

 arteries separately and disjoined, but as one body. 



We have occasion to observe so often as our hands, feet, or 

 ears have become stiff and cold, that as they recover again by 

 the warmth that flows into them, they acquire their natural 

 colour and heat simultaneously; that the veins which had be- 

 come small and shrunk, swell visibly and enlarge, so that 

 when they regain their heat suddenly they become painful; from 

 which it appears, that that which by its influx brings heat is the 

 same which causes repletion and colour ; now this can be and is 

 nothing but blood. 



When an artery and a vein are divided, any one may clearly 

 see that the part of the vein towards the heart pours out no 

 blood, whilst that beyond the wound gives a torrent; the di- 

 vided artery, on the contrary, (as in my experiment on the ca- 

 rotids,) pours out a flood of pure blood from the orifice next the 

 heart, and in jets as if it were forced from a syringe, whilst from 

 the further orifice of the divided artery little or no blood escapes. 

 This experiment therefore plainly proves in what direction the 

 current sets in either order of vessels towards the heart in the 

 veins, from the heart in the arteries; it also shows with what 



