92 OF THE VESSELS 



The body, therefore, ccnitains two systems of blood-ves- 

 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 evi- 

 dent, that the impulse and pressure upon the sides of the 

 blood-vessels, will be much greater in one case than the 

 other. Accordingly, the arteries which carry out the blood, 

 are formed with 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 rup- 

 ture would be more dangerous, than in the veins ; these 

 vessels are defended from injury, not 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 fur- 

 rowed solely for the passage of these vessels. Sometimes 

 they proceed in channels, protected by stout parapets on 

 each side ; which last description is remarkable in the 

 bones of the fingers, these being hollowed out, on the un- 

 der side, like a scoop, and v/ith such a concavity that the 

 finger may be cut across to the bone without hurting the 

 artery which runs along it. At other times, the arteries 

 pass in canals wrought in the substance, and in the very 

 middle of the substance, of the bone ; this takes place in 

 the lower jaw, and is found where there would, otherwise, 

 be danger of compression by sudden curvature. All this 

 care is wonderful, yet not more than what the importance 

 of the case required. To those, who venture their lives 

 in a ship, it has been often said, that there is only an inch 

 board between them and death ; but in the body itself, es- 

 pecially in the arterial system, there is, in many parts, only 

 a membrane, a skin, a thread. For which reason this sys- 

 tem lies deep under the integuments ; whereas the veins, in 

 which the mischief that ensues from injuring the coats is 

 much less, lie in general above the arteries, come nearer 

 to the surface, are more exposed. 



It may be further observed concerning the two systems 

 taken together, that, though the arterial, with its trunks 

 and branches and small twigs, may be imagined to issue or 



