(From circulo, to compass about, moving as it were in a circle). Circulation. For what is understood by it in chemistry, see Circulato-rium and Digestio.

In anatomy it is the circulation of any fluid through the vessels destined for its conveyance. Strictly speaking, circulation is only applied to the blood, because it moves from the heart to return to it again; but the other fluids do not return to the organ from which they were first discharged.

The honour of the discovery of the circulation is undoubtedly due to Dr. Harvey; but it has been claimed for Servetus, Columbus, and Caesalpinus.

Servetus was an opponent of Calvin, and persecuted by him. He was a Spanish physician; but was not the author of any known medical work. In a theological tract, by way of allusion, he mentions the circulation of the blood through the lungs, rather indeed as an hypothesis than as an established fact. It is of more importance, in another view, to remark, that he considers the object of the circulation through the lungs to be the inhaling a spirit from the air, and the escape of a fuliginous vapour. He was unacquainted, however, with the structure of the heart, or the uses of the valves; and, with Galen, confines the blood to the liver and veins, while he supposes the heart and arteries filled with a spirit. Columbus, in 1569, followed him in describing this lesser circulation, and first explained the structure and use of the sigmoid and tricuspid valves; but with little consistency adopted, also, the fancies of Galen first mentioned.

Caesalpinus published about twelve years after Columbus, viz. in about 1681; and had not the authority of Aristotle and Galen possessed his imagination so strongly as to shut out the most obvious consequences of the best established facts, the honour of the discovery must 3L

C Hi 442 C I R have been his own. But his claims to genius of the highest rank are undisputed without this additional laurel. Aristotle first misled him by distinguishing two kinds of blood; one for the increase and the other for the nourishment of the body: the first he supposed to be derived from the liver, and poured into the vena cava, attracted by the heat of the heart. From the right ventricle he traces the blood, with Columbus or Serve-uis, to the lungs, where he supposes it to be cooled only. The blood, now become spirituous and alimentary, in successive periods, according to this system, causes, by the fermentation excited, the succession of pulsations, while the aliment destined for increase is elicited from the veins; yet in sleep this effervescing blood, he admits, is returned by the veins, the valves of which had been described by J. B. Cannanus, and, more accurately, by Fabricius ab Aquapendente.

Such were the opinions which Harvey found in the schools; and he need only have recollected that simplicity was the criterion of truth, to suppose that blood, which circulated in the night, might also circulate in the day. This was the foundation of our remark in the history of surgery (see Chiruhgia), that the facts were already established, and that it required only "patient thinking"to connect them. The claim to this quality distinguished both Harvey, Newton, and Columbus; nor, if we know any thing of the human mind, does this representation diminish their credit. Peaches had for ages fallen from the tree; the structure of the valves of the heart been for years known; and the Indies long discovered by an eastern course; when the calm dispassionate examination of these three first of philosophers drew consequences which had escaped all their predecessors. They have received their reward; for they have demonstrated how high human intellect can soar: it is for their opponents to show how low it can descend.

With regard to the circulation, however, it is thus clearly described. The blood is conveyed from the left ventricle of the heart, by the aorta and its branches, to the minutest and most remote parts of the body; and then passing from the extremities of the smallest arteries into the incipient veins, whether continuous or not anatomists have not decided, circulates through them into their larger branches into the right auricle of the heart, and in succession to the right ventricle. It is forced with the fresh supplies that it receives from the chyle, passing into the subclavian vein, from thence into the pulmonary artery; and after circulating through the lungs, in its passage, is returned by the pulmonary vein into the left auricle, and thence into the left ventricle. The same round recurs until death concludes the progress.

When Harvey promulgated this doctrine is uncertain. It has been supposed, that he delivered his new system in the Lumley lectures, 1615. It is, however, singular, that a discovery so important should have passed unnoticed; though little doubt can be entertained that this important fact was established in his own mind early in the following year. This appears clearly from his Ms. De Anatomia Universa. In the year 1619 this great discovery was promulgated; for, if we are not mistaken, in that year his Exercitatio Anatomica de Cordis, and Sanguinis Motu, appeared at Frankfort; a choice probably dictated by the convenience of circulation on the continent. This treatise is a masterpiece of simple, but cogent and decisive argument. After shortly confuting the errors of his predecessors, he describes the motion of the heart as it appears in a living animal; points out the alternate dilatations and contractions of its different auricles and ventricles, and their effect as regulated by the various valves. He then shows, by calculation, that the blood flows faster into the arteries than it can be supplied by aliment imbibed by the veins; and, as the arteries can receive no supplies but from the veins, the former must be gradually more distended, or the latter more emptied, unless the veins and arteries anastomose, which was supposed less improbable, as this communication takes place in the lungs. A few simple experiments illustrate this idea, and establish it beyond contradiction.