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

The utility of the circulation of the blood, I assume as an acknowledged point. One grand purpose is plainly answered by it; the distributing 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 proposed, than to discover a method of constantly stantly repairing the waste, and of supplying an accession of substance to every part, of a complicated machine at the fame time.

This system presents itself under two views: first, the disposition of the blood vessels, i. e. the laying of the pipes; and, secondly, the construction of the engine at the centre, viz, the heart, for driving the blood through them.

I. The disposition of the blood vessels, as far as regards the supply 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 still 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 serve a town, may represent the vessels, which carry the blood from the heart. But there is another thing necessary to the blood, which is not wanted for the water; and that is, the carrying of it back again to its source. For this office a reversed system of vessels is prepared, which, uniting at their extremities with the extremities of the first system, collects the divided and subdivided streamlets, first by capillary ramifications into larger branches, Secondly

condly by these branches into trunks ; and thus returns the blood (almost 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 systems of blo'od-vessels, arteries and veins. Between the constitution of the systems there are also two differences, suited to the functions which the systems have to execute. The blood, in going cut, 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 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 difN serence: 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 desended from injury, not only by M their 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 remarkable 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 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 r their lives in a ship, it has been bften said, that there is only an inch-board between them and death > but in the body itself, especially in the arterial system, there is, in many parts, only a membrane, a skin, a •.- ;'. thread. thread. For which reason this system lie3 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 trunk and branches and small twigs, may be imagined to issue 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 so, would be only to resolve one mechanism into another), yet the venal, the returning system, can never be formed in this manner. The arteries might go on mooting out from their extremities, i. e, lengthening and subdividing indefinitely; but an inverted system, continually uniting its streams, instead of dividing, and thus carrying back what the other system carried out, could not be referred to the fame process.

II. The next thing to be considered is the engine which works this machinery, viz. the heart. For our purpose, it is unnecessary to ascertain the principle upon which the heart M 2 acts.

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