the act of moving round, or in a circle: thus we say, the circulation of the blood, &c.

Circulation of the blood, the natural motion of the blood in a living animal, whereby that fluid is alternately carried from the heart into all parts of the body, by the arteries, from whence it is brought back to the heart again by the veins.

This motion is chiefly caused by the dilatation and contraction of this organ, and is the principle on which life depends; for when it ceases in any part, it dies; when it is diminished, the operations are weak; and, when it ceases totally, life is extinguished.

All the veins discharge themselves into the ventricles of the heart; from hence all the arteries arise: the blood expelled out of the right ventricle must be carried, through the pulmonary artery, into the lungs; from which it must be returned, by the pulmonary veins, to the left ventricle; from the left ventricle the blood, thus imported, is, by the constriction of that part, again expelled into the aorta, and by it distributed all over the rest of the body, and thence is returned again to the right ventricle by the cava, which completes the circulation.

This circulation becomes actually visible, with the assistance of a microscope, especially in fish, frogs, &c., wherein the inoculation, or union of the extremities of the arteries with those of the veins, together with the globules of the blood flowing from the one into the other, may be plainly seen.

The auricles of the heart being large hollow muscles, furnished with a double series of strong fibres, proceeding with a contrary direction to the opposite tendons, the one adhering to the right ventricle, the other to the sinus venosus; as also with innumerable veins veins and arteries; by the contractile force of these auricles, the blood will be vigorously expressed and driven into the right ventricle, which, upon this contraction, is rendered flaccid, empty, and disposed to admit it.

Now, if the right ventricle, thus full of blood, by the contraction of its fibres, press the blood towards the aperture again, the venous blood at the same time pouring in, will drive it back again into the cavity, and mix it more intimately, till rising up against the parietes, it raise the valvulae tricupidides, which are so connected to the fleshy columns extended on the opposite side, as that, when laid quite down, these cannot close the parietes of the right ventricle; these it thrusts towards the right auricle, till being there joined, they stop the passage very closely, and prevent any return.

By the same means, the same blood rises into the three semilunar valves, placed in the extremity of the other mouth, and lying open to the pulmonary artery; these it shuts close against the sides of the artery, and leaves a passage into the artery alone; the blood carried by this artery into the lungs, and distributed by its branches through the whole substance thereof, is first admitted into the extremities of the pulmonary vein, called arteria venosa; whence passing into four large vessels, which unite together, it is brought to the left sinus venosus, or trunk of the pulmonary vein, by the force of whose muscular structure it is driven into the left ventricle, which, on this occasion, is relaxed, and by that means prepared to receive it.

Hence, as before, it is driven into the left ventricle, which is relaxed by the same means; and by the valve mitrales opening, admits it into the left ventricle, and hinders its flux into the pulmonary vein; from hence it is forced into the aorta, at whose orifice there are three semilunar valves, which also prevent a reflux, by closing the same.

The motion of the blood in living animals is attended with the following phenomena: 1. Both the venous sinuses are filled, and grow turgid at the same time. 2. Both auricles grow flaccid at the same time, and both are filled at the same time with blood, impelled by the contractile force of its correspondent muscular venous sinus. 3. Each ventricle contracts and empties itself of blood at the same time; and the two great arteries are filled and dilated at the same time. 4. As soon as the blood, by this contraction, is expelled, both ventricles being empty, the heart grows larger and broader. 5. Upon which the muscular fibres of both venous sinuses contract, and express the blood contained in them into the ventricle of the heart. 6. In the meantime the venous sinuses are again filled as before, and the auricles, &c., return into their former habitue. 7. This alteration continues till the animal begins to languish under the approach of death, at which time the auricles and venous sinuses make several palpitations, for one contraction of the ventricle.

In a fetus, the apparatus for the circulation of the blood is somewhat different from that in adults. The septum, which separates the two auricles of the heart, is pierced through with an aperture, called the foramen ovale, and the trunk of the pulmonary artery, a little after it has left the heart, sends out a tube into the descending aorta, called the communicating canal. The fetus being born, the foramen ovale closes by degrees, and the canal of communication dries up, and becomes a simple ligament.

As to the velocity of the circulating blood, and the time wherein the circulation is completed, several computations have been made. By Dr Keil's account, the blood is driven out of the heart into the aorta with a velocity which would carry it twenty-five feet in a minute; but this velocity is continually abated in the progress of the blood, in the numerous sections or branches of the arteries, so that before it arrive at the extremities of the body, its motion is greatly diminished. The space of time wherein the whole mass of blood ordinarily circulates, is variously determined. Some state it thus: Supposing the heart to make two thousand pulses in an hour, and that at every pulse there is expelled an ounce of blood; as the whole mass of blood is not ordinarily computed to exceed twenty-four pounds, it must be circulated seven or eight times over in the space of an hour.

The circulation of the blood was first discovered in England, in the year 1728, by Dr Harvey.

Circulation of the sap of vegetables. See Vol. I. p. 45.

chemistry, is an operation whereby the same vapour, raised by fire, falls back, to be returned and distilled several times.

Circulation of money. See Commerce, and Money.

Circulus, in chemistry, an iron instrument in form of a ring, which being heated red hot, and applied to the necks of retorts and other glass vessels, till they grow hot, a few drops of cold water thrown upon them, or a cold blast, will make the necks fly regularly and evenly off.

Another method of doing this, is to tie a thread, first dipped in oil of turpentine, round the place where you would have it break; and then setting fire to the thread, and afterwards sprinkling the place with cold water, the glaas will crack exactly where the thread was tied.

Circumajentes Musculi, or Obliqui Musculi, in anatomy. See Vol. I. p. 290.