The disposition of the blood vessels is like that of the water pipes in a city, namely, 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 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 set of vessels is prepared, which are united at their extremities with the extremities of the first set, and collect from them the divided streamlets into branches, and these branches are again collected into trunks, and through them, the blood is returned to the fountain whence its motion proceeded, namely, to the heart.

The body thus contains two sets of blood vessels. The arteries which send the blood out from the heart, and the veins which carry the blood back to it.

The blood in going out passes always from wider into narrower tubes, and in

coming back, it passes from narrower into wider; the arteries which carry out the blood are formed of much rougher and stronger coats than the veins which bring it back.

And therefore it is, that in the arteries, by reason of the greater force by which the blood is urged along them, a wound or rupture would be more dangerous than in the veins.

Now we find that the arteries are des fended from injury, not only by their texture but by their situation, and by every advantage of situation which can be given to them.

Sometimes they creep along groves made for them in the bones: for instance, the under edge of the ribs is sloped and furrowed, as if solely for the passage of these vessels.

Sonfetimes they proceed in channels protected by stout parapets of bone or muscle on each side, which last descup tion is remarkable in the bones of the fingers, they being hollowed out in the underside, like a scoop, in such a manner that the finger may be cut across to the one without hurting the artery which runs along it.

In other instances, the arteries pass in canals, which are in the very middle of the substance of the bone; this takes place in the lower jaw.

All this care is wonderful, yet not more than the importance of the case requires.

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 system of arteries, there is, in many parts, only a film or a thread.

For which reason, the arteries lie deep under the skin, whereas the veins, in which the mischief that ensues from injuring the coat is much less, lie in general above the arterie ies, they come nearer to the surface, and are more exposed.

The next thing to be considered is the engine which works this machinery of the blood vessels, namely, the heart.

'There is found in the central part of the body a hollow muscle, called the heart, bedded in strong fibres.

By the contraction of these fibres, the sides of the heart are necessarily squeezed together, so as to force out the blood which it contained; and again, by the relaxation of the same fibres, the cavity is

in its turn dilated, and of course prepared to admit the blood when poured back into it by the veins.

This is a general account of the apparatus, and the simplest idea of its action is, that by each contraction of the heart a portion of blood is forced, by a sort of syringe, into the arteries, and at each dilatation of the heart, an equal portion is received back into it from the veins.

This produces, at every stroke, a motion and change in the mass of blood, equal to the amount of what the heart contains, which, in the heart of a full grown person, is about an ounce, or two table spoonsful.

We may, any of us, feel this beating of the heart, and also the pulse at our wrists; they move exactly together and depend upon the same cause; they are each of them the action of squirting the blood through the arteries.

The heart contracts about four thousand times in one hour; from which it follows, that there passes through the heart, every hour, four thousand ounces of blood. Now the whole mass of blood is said to be about twenty-five pounds, or three hundred ounces, so that a quantity equal to the whole mass of blood passes through the

heart fourteen times in one hour, which is about once every four minutes.

Consider what an affair this is when we come to very large animals.

The great artery of a whale is larger in the bore than the main pipe of the water works at London bridge; and the water roaring in its passage through that pipe, is inferior in force and velocity to the blood gushing from the whale's heart.

The heart performs this office in conjunction with another office of equal curiosity and importance: it is necessary that the blood should be continually brought into contact with the air.

So that the air, by some means or other, must be introduced into a near communication with the blood.

The lungs of animals are constructed for this purpose; they consist of blood vessels and air vessels lying close to each other.

The internal surface of these vessels in the lungs is so great, that, if collected and spread out, they would be, in a man, equal to a surface of fitteen square feet.

As soon as the blood is received by the heart from the veins, and before it is sent out again into its arteries, it is carried by