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CHAPTER X.

OF THE VESSELS OF ANIMAL BODIES.

THE circulation of the blood, through the bodies of men and quadrupeds, and the apparatus by which it is carried on, compofe a fyftem, and testify a contrivance, perhaps the best understood of any part of the animal frame. The lymphatic fyftem, or the nervous system, may be more fubtile and intricate; nay, it is poffible that in their structure they be even more artificial than the fanguiferous; but we do not know fo much about them.

The utility of the circulation of the blood, I affume as an acknowledged point. One grand purpose is plainly answered by it; the diftributing to every part, every extremity, every nook and corner, of the body, the nourishment which is received into it by one aperture. What enters at the mouth, finds its way to the fingers' ends. A more difficult mechanical problem could hardly I think be propofed, than to discover a method of con

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ftantly repairing the waste, and of supplying an acceffion of substance to every part, of a complicated machine at the fame time.

This fyftem prefents itfelf under two views: firft, the difpofition of the blood vessels, i. e. the laying of the pipes; and, fecondly, the conftruction of the engine at the centre, viz. the heart, for driving the blood through

them.

I. The difpofition of the blood veffels, as far as regards the fupply of the body, is like that of the water pipes in a city, viz. large and main trunks branching off by smaller pipes (and these again by ftill narrower tubes) in every direction, and towards every part, in which the fluid, which they convey, can be wanted. So far, the water pipes, which ferve a town, may represent the veffels, which carry the blood from the heart. But there is another thing neceffary to the blood, which is not wanted for the water; and that is, the carrying of it back again to its fource. For this 'office a reverfed fyftem of veffels is prepared, which, uniting at their extremities with the extremities of the first system, collects the divided and fubdivided ftreamlets, first by capil lary ramifications into larger branches, fe

condly

condly by thefe branches into trunks; and thus returns the blood (almoft exactly inverting the order in which it went out) to the fountain from whence its motion proceeded. All which is evident mechanism.

The body, therefore, contains two fyftems of blood-veffels, arteries and veins. Between the conftitution of the fyftems there are alfo two differences, fuited to the functions which the fyftems have to execute. The blood, in going out, paffing always from wider into narrower tubes; and, in coming back, from narrower into wider; it is evident, that the impulfe and preffure upon the fides of the blood-veffels, will be much greater in one cafe than the other. Accordingly, the arteries which carry out the blood, are formed with much tougher and ftronger coats, than the veins which bring it back. That is one difference: the other is ftill more artificial, or, if I may so speak, indicates, ftill 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 veffels are defended from injury, not only by their

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their texture, but by their fituation; and by every advantage of fituation which can be given to them. They are buried in sinuses, or they creep along grooves, made for them, in the bones; for inftance, the under edge of the ribs is floped and furrowed folely for the paffage of these veffels. Sometimes they proceed in channels, protected by ftout parapets on each fide; which last description is remarkable in the bones of the fingers, these being hollowed out, on the under fide, like a scoop, and with fuch 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 pafs in canals wrought in the fubftance, and in the very middle of the fubftance, of the bone: this takes place in the lower jaw; and is found where there would, otherwife, be danger of compreffion by fudden curvature. All this care is wonderful, yet not more than what the importance of the cafe required. To thofe, who venture their lives in a fhip, it has been bften faid, that there is only an inch-board between them and death; but in the body itself, especially in the arterial fyftem, there is, in many parts, only a membrane, a skin, a thread.

thread. For which reason this fyftem lies deep under the integuments; whereas the veins, in which the mischief that enfues from injuring the coats is much lefs, lie in general above the arteries; come nearer to the surface; are more exposed.

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may be further obferved concerning the two fyftems taken together, that, though the arterial, with its trunk and branches and small twigs, may be imagined to iffue or proceed, in other words to grow from the heart, like a plant from its root, or the fibres of a leaf from its foot stalk (which however, were it fo, would be only to refolve one mechanism into another), yet the venal, the returning system, can never be formed in this manner. arteries might go on fhooting out from their extremities, i. e. lengthening and fubdividing indefinitely; but an inverted fyftem, continually uniting its ftreams, instead of dividing, and thus carrying back what the other system carried out, could not be referred to the fame process.

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II. The next thing to be confidered is the engine which works this machinery, viz. the heart. For our purpose, it is unneceffary to ascertain the principle upon which the heart acts.

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