VESSELS OF ANIMALS. 105 



a reversed system of vessels is prepared, which, umtiiig ai 

 their extremities with the extremities of the first system, 

 collect the divided and subdivided streamlets, first, by capil- 

 lary ramifications into larger branches, secondly, by these 

 branches into trunks ; and thus return the blood — almost 

 exactly inverting the order in which it went out — to the 

 fountain whence its motion proceeded. All which is evident 

 mechanism. 



The body, therefore, contains two systems of bloodves- 

 sels, arteries and veins. Between the constitution of the 

 systems there are also two differences, suited to the func- 

 tions which the systems have to execute. The blood, in 

 going out, passing always from wider into narrower tubes, 

 and in coming back, from narrower into wider, it is evident 

 that the impulse and pressure upon the sides of the blood- 

 vessels will fee much greater in one case than, the other. 

 Accordingly the arteries, which carry out the blood, are form- 

 ed of much tougher and stronger coats than the veins, which 

 bring it back. That is one difference ; the other is still more 

 artificial, or, if I may so speak, indicates still more clearly 

 the care and anxiety of the Artificer. Forasmuch as, in the 

 arteries, by reason of the greater force with which the blood 

 is urged along them, a wound or rupture would be more 

 dangerous than in the veins, these vessels are defended from 

 injury, riot only by their texture, but by their situation, and 

 by every advantage of situation which can be given to them. 

 They are buried in sinuses, or they creep along grooves made 

 for them in the bones ; for instance, the under edge of the 

 ribs is sloped and furrowed solely for the passage of these 

 vessels. Sometimes they proceed in channels, protected by 

 stout parapets on each side ; which last description is remark- 

 able in the bones of the fingers, these being hollowed out 

 on the under side like a scoop, and with such a concavity 

 that the finger may be cut across to the bone without hurt- 

 ing the artery, which runs along it. At other times the ar- 

 teries pass in canals wrought in the substance, and in tha 

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