CHAP. II.] CO-CONSTRUCTION — CIRCULATION. 127 



or viscid — call it which we like — causes heat, which 

 being general, is fever ; the rapidity of the circula- 

 tion increases violently, and the blood now becomes 

 more fluid than when the animal was in good health. 

 Arteries may be distinguished from veins m the 

 dead subject, by the property they possess, of re- 

 taining their tubular shape after the blood is dis- 

 charged ; whereas, the veins collapse, when empty. 

 43. If the arteries are plain tubes, lying for the 

 most part concealed, the veins, on the contrary, are 

 more frequently found exposed to the sight under 

 the skin, next to the muscle. But, more delicate 

 and more numerous, the veins perform their part of 

 the circulation by a totally different means than is 

 found in the propulsion and contraction of the 

 arteries. One of the means of effecting this pur- 

 pose is by the obvious and simple movement of the 

 body and limbs, as well as by the act of breathing, 

 whereby the blood is pressed out of some one part 

 or other of the veins into the adjacent part ; but 

 these vessels being furnished with innumerable 

 valves, within an inch, or less, of each other, the 

 doors whereof open only towards the heart, the 

 blood must necessarily force itself out that way, 

 and no other. If we suppose that any given por- 

 tion of the vein is hereby emptied, what follows 

 from the circumstance, but that the next-door valve, 

 which kept back the blood contained in the adjoin- 

 ing portion below, while it was pressed upon from 

 above, being thus relieved from the pressure, will 

 now open? Then, in rushes the blood, and the 



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