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. See Anatomy, p. 125.

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 curious, in microscopic observations, have found an easy method of seeing the circulation of the blood in the bodies of animals; for these inquiries it is necessary to choose such animals as are small, and easily manageable, and which are either wholly or in part transparent. The observations made by this means are preferable to any others we can have recourse to; since, in dissections, the animal is in a state of pain, or dying; whereas in animals small enough to be thus viewed, all is left in its usual course, and we see what nature does in her own undisturbed method. In these creatures also, after viewing, as long as we please, the natural state and current of the blood, we may, by pressure, and several other ways, impede its course; and by putting various mixtures into the creature's water, induce a morbid state, and finally see the creature die, either by means of this or by any other method; and we may thus accurately observe all the changes it undergoes, and see what occasions the trembling pulse, &c. of dying people.

The current of the blood in small animals, that is, its passing on through the vessels, either to or from the heart, is very easily seen by the microscope; but its circulation, that is, its running to the extremities of the parts, and thence returning, is more difficult; because the vessels where this should be seen are so extremely minute, as not easily to come under observation. The larger arteries are easily distinguished from the veins by the motion of the blood through them, which in the veins is always smooth and regular; but in the arteries by several pulsations after the manner of pulsation. But this difference is not to be found in the more minute vessels; in all which, as well arteries as veins, the motion of the blood is even and regular.

The transparent membrane, or web between the toes of a frog's hinder foot, is a very proper object to observe the circulation of the blood in. The tails or fins of fishes are also very fine objects; and when the fish is very small, these are manageable, and afford a view of a great number of veins and arteries, with a very quick and beautiful succession of blood through them. The tail of a flounder may be very conveniently placed before the double microscope on a plate of glass; and its body being supported by something of equal height, the fish will lie still, and the circulation may be seen very agreeably. In the minutest vessels thus examined, the blood always appears pale. Circulation, pale or colourless, but in the large ones it is manifestly red. The arteries usually branch out extremely before they join the veins to carry the blood back to the heart; but this is not always the case; for Mr. Lewenhoeck has observed, that on each side of the little gill-filaments which give a stiffness to the tail of a flounder, there may be seen a very open communication of the veins and arteries; the blood running towards the extremities through arteries, and returning back again through veins, which were evidently a continuation of those arteries, and of the same diameter with them. The whole fish on the tail of which this examination was made, was not more than half an inch in length; it is easy to conceive, therefore, how small the tail must be; and yet in it there were 68 vessels which carried and returned the blood; and yet these vessels were far from being the most minute of all. How inconceivably numerous then must the circulations in the whole human body be? Mr. Lewenhoeck is of opinion, that a thousand different circulations are continually carried on in every part of a man's body in the breadth of a finger nail.

The tail of a newt or water-lizard affords also a very entertaining prospect of the circulation of the blood through almost numberless small vessels; but no object shows it so agreeably as one of these animals while so young as not to be above an inch long; for then the whole body is so very transparent, that the circulation may be seen in every part of it, as well as in the tail; and, in these objects, nothing is more beautiful than the course of the blood into the toes and back again, where it may be traced all the way with great ease. Near the head there are also found three small fins which afford a very delightful prospect; these are all divided like the leaves of polypody; and in every one of the branches of these, the blood may be very accurately traced, running to the end through the artery, and there returning back again by a vein of the same size, and laid in the same direction; and as the vessels are very numerous and large in this part, and the third or fourth magnifier may be used, there are sometimes seen 30 or 40 channels of running blood at once; and this the more as the globules of blood in the newt are large, and are fewer in number, in proportion to the quantity of serum, than in any other animal; and their figure, as they are protruded through the vessels, changes in a very surprising manner. The impetus occasioning the circulation, is great enough in some animals to raise the blood six, seven, or eight feet high from the blood-vessel it springs out at; which, however, is far exceeded by that of the sap of a vine in bleeding time, which will sometimes rise forty feet high.

Circulation of the sap of Plants. See Plants, and Sap.

Circulation of the Spirits, or Nervous Fluid. See Anatomy, No. 136.

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.

Subterranean Circulation. See Springs.