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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, by these branches into trunks; and thus returns 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 blood-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 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-vessel, will be much greater in one case than the other. Accordingly, the arteries which carry out the blood, are formed 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 ves. sels 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 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 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, especially in the arterial system, there is, in many parts, only a membrane, a skin, a thread. For which reason, this system 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 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 shooting 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 same 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
Whether it be irritation excited by the contact of the blood, by the influx of the nervous fluid, or whatever else be the cause of its motion, it is something which is capable of producing, in a living muscular fibre, reçi
procal contraction and relaxation. This is | the power we have to work with: and the
inquiry is, how this power is applied in the instance before us. There is provided, in the central
part of the body, a hollow muscle, invested with spiral fibres, running in both di, rections, the layers intersecting one another ; in some animals, however, appearing to be semicircular rather than spiral. By the contraction of these fibres, the sides of the muscular cavities are necessarily squeezed together, so as to force out from them any. fluid which they may at that time contain ; by the
relaxation of the same fibres, the cavities are in their turn dilated, and, of course, prepared to admit every
may be poured into them. Into these cavities are inserted the great trunks, both of the arteries which
carry out the blood, and of the veins which bring it back. This is a general account of the apparatus ; and the simplest idea of its action is, that, by each contraction, a portion of blood is forced by a syringe into the arteries: and, at each dilatation, an equal portion is received from the veins. This, produces at each pulse, a motion, and change in the mass of blood, to the amount of what the cavity contains, which in a full-grown human heart I understand is about an ounce, or two tablespoons full.' How quickly these changes succeed one another, and by this succession how sufficient they are to support a stream or circulation throughout the system, may be understood by the following computation, abridged from Keill's Anatomy, p. 117, ed. 3: “ Each ventricle will at least contain one ounce of blood. The heart contracts four thousand times in one hour; from which it follows, that there pass through the heart, every hour, four thousand ounces, or threehundred-and-fifty pounds of blood. Now the whole mass of blood is said to be about