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. See ANATOMY Index.

In a fetus, the apparatus for the circulation of the blood is somewhat different from that in adults. The leptom, 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 arrives 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 prejudice, 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 puffing 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 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 Leuwenhoeck has observed, that on each side of the little gills 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 Leuwenhoeck 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 objects 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 Circulation 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 polydipsy; 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 thirty or forty channels of running blood at once; and this the more as the globules of blood in the newt are large, and 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 five, 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 40 feet high.

**Circulation of the Sap of Plants.** See Plants and Sap.

**Circulation of the Spirits, or Nervous Fluid.** See Anatomy Index.

**Circulation, in 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.

**Circulus,** in Chemistry, an iron instrument in form of a ring, which being heated red hot, and applied to the neck 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 glass will crack exactly where the thread was tied.

**Circumambient,** an appellation given to a thing that surrounds another on all sides; chiefly used in speaking of the air.

**Circumcelliones.** See Circoncilliones.

**Circumcision,** the act of cutting off the prepuce; a ceremony in the Jewish and Mahometan religions, wherein they cut off the foreskin of their males, who are to profess the one or the other law.

Circumcision commenced in the time of Abraham; and was the seal of a covenant stipulated between God and him. It was in the year of the world 2178 that Abraham, by divine appointment, circumcised himself and all the males of his family; from which time it became an hereditary practice among his descendants.

The ceremony, however, was not confined to the Jews. Herodotus and Philo Judaeus observe, that it obtained also among the Egyptians and Ethiopians, Herodotus says, that the custom was very ancient among each people; so that there was no determining which of them borrowed it from the other. The same historian relates, that the inhabitants of Colchis also used circumcision; whence he concludes, that they were originally Egyptians. He adds, that the Phenicians and Syrians were likewise circumcised; that they borrowed the practice from the Egyptians; and, lastly, that a little before the time when he wrote, circumcision had passed from Colchis to the people inhabiting near Thermopylae and Parthenius.

Martham is of opinion, that the Hebrews borrowed circumcision from the Egyptians; and that God was not the first author thereof, citing Diodorus Siculus and Herodotus as evidences on his side. This latter proposition seems directly contrary to the testimony of Moses, who affirms us (Gen. xvii.), that Abraham, though 99 years of age, was not circumcised till he had the express command of God for it. But as to the former position of Martham, it will admit of more debate. The arguments on both sides may be seen in one view in Spencer de Legibus Hebræorum, l. 2. c. 4.

Be this as it will, it is certain the practice of circumcision among the Hebrews differed very considerably from that of the Egyptians. Among the first it was a ceremony of religion, and was performed on the eighth day after the birth of the child. Among the latter, a point of mere decency and cleanliness; and, as some will have it, of physical necessity; and was not performed till the 13th year, and then on girls as well as boys.

Among the Jews, the time for performing this rite was the eighth day, that is, six full days, after the child was born. The law of Moses ordained nothing with respect to the person by whom, the instrument with which, or the manner how, the ceremony was to be performed; the instrument was generally a knife of stone. The child is usually circumcised at home, where the father or godfather holds him in his arms, while the operator takes hold of the prepuce with one hand, and with the other cuts it off; a third person holds a porringer, with sand in it, to catch the blood; then the operator applies his mouth to the part, and, having sucked the blood, spits it into a bowl of wine, and throws a styptic powder upon the wound. This ceremony was usually accompanied with great rejoicings and feasting; and it was at this time that the child was named in presence of the company. The Jews invented several superstitious customs at this ceremony, such as placing three stools, one for the circumcisor, the second for the person who holds the child, and the third for Elia, who, they say, assists invisibly at the ceremony, &c.

The Jews distinguished their proselytes into two sorts, according as they became circumcised or not: those who submitted to this rite were looked upon as children of Abraham, and obliged to keep the laws of Moses; the uncircumcised were only bound to observe the precepts of Noah, and were called Noachidae.

The Turks never circumcise till the seventh or eighth year, as having no notion of its being necessary to salvation. The Persians circumcise their boys at 13, and their girls from 9 to 15. Those of Madagascar cut the flesh at three several times, and the most zealous of the relations present catches hold of the preputium and swallows it.

Circumcision is practised on women by cutting off the foreskin of the clitoris, which bears a near resemblance and analogy to the preputium of the male penis. We are told that the Egyptian captive women were circumcised; and also the subjects of Prester John.

CIRCUMCISION is also the name of a feast, celebrated on the first of January, in commemoration of the circumcision of our Saviour.