When the blood leaves the heart, it is sent with con siderable force into the large trunks of the arteries ; these vessels soon begin to ramify in various directions, to all parts of the body, until at length they ara reduced to ves sels too small to be traced by the eye, or even by the microscope. There are in the body two distinct sets of arteries, which are distributed to different organs, and serve very different purposes in the animal economy ; one of these is sent to the lungs, the other to all the remain ing parts of the body ; the former constitutes the pulmo nic, the latter the systemic circulation. On this subject we shall at present only observe, that before the blood can be employed in the various functions of the body, it is ne cessary for it to undergo a certain change, which is effect ed in the lungs. After it has undergone this change, it is brought back to the heart, and is again propelled from this organ, along the arteries of the systemic circulation, to all parts of the body. The arteries which perform this office of conveying the blood from the heart, are flexible elastic tubes, principally composed of membrane, but, as is generally supposed, containing also a quantity of mus cular fibres, which give them, to a certain extent, the power of contracting, and consequently of propelling their contents. When the blood has been transmitted by the arteries towards the extremities, it is returned to the heart by the veins, being first received into their minute ex tremities, and carried from larger to smaller branches, contrary to what takes place in the arteries, until at length it arrives at the large trunks, and is poured from them into the heart. The veins resemble the arteries in being membranous tubes, but they differ from them in possess ing a less firm texture, in being without muscular fibres, and in having a number of valves in different parts of their course, whereas the arteries have no valves, except at their commencement.
We shall now trace the progress of the blood through the heart and along the vessels ; and it will be perceived, that every portion of it makes two complete circuits, be fore it arrives again at the same point in the circulation. We may conveniently begin with that part where the blood enters the heart, when it leaves the systemic veins, or those which belong to the greater circulation. It is re ceived into the right auricle, and is poured from this into the right ventricle ; when the ventricle becomes distended, it contracts, and the valves which are at its month closing i up the passage into the auricle, it is necessarily forced into the pulmonary artery. The pulmonary artery carries the blood through the lungs, along what is called the lesser or pulmonic circulation, where it undergoes that peculiar change which fits it for the support of life ; it is brought back in this altered state to the left auricle, and from this into the left ventricle. The left ventricle, when filled with blood, contracts; and the valves, preventing the fluid from returning into the auricle, it is sent into the aorta, or the grand trunk of the systemic arteries. It is transmitted by this through the minute arterial branches, to all parts of the body ; from these it is received by the extremities of the veins, passes from them into the large veins, and so to the vena cava, the main trunk of the systemic veins, whence it is finally delivered into the right auricle of the heart.
A very casual observation of the phenomena of the liv ing body was sufficient to prove that the blood is perpetu ally in motion ; but the nature of this motion, or the course which it pursues, was unknown to the ancients. They had many unfounded speculations upon the subject, which it is not necessary to relate, although they are sanctioned by very high authority. Some approaches to the true theory of the circulation were made by Cesalpini, who flourished in the sixteenth century ; but the honour of this great discovery, which is generally regarded as the great est that ever was made in anatomy or physiology, is due to our illustrious countryman Harvey. He completed the
discovery about the year 1620, but, with a rare degree of philosophical forbearance, he spent eight years in digest ing and maturing his ideas, when they were at length given to the world, in a short tract, written with remarkable clear ness and perspicuity, which has been justly characterized " as one of the most admirable examples of a series of ar guments, deduced from observation and experiment, that ever appeared on any subject." The manner in which this discovery was received by the public forms a curious and instructive occurrence in the history of philosophy. Harvey, for some time, scarcely obtained a single convert, and he was assailed by a violent clamour for having ven tured to call in question the authority of Aristotle and Ga len. After some time, however, it was found that the doc trine was true, and his opponents were then driven to a new species of attack ; they asserted that what had been brought forward as a new discovery was in fact only bor rowed from the writings of the ancients; and passages were produced, although warped and strained in a thousand ways, to meet this allegation. It is even said that Harvey suffered in his medical practice, in consequence of the pre judice that was excited against him ; but he lived long enough to witness the triumph of truth over the cavils of his enemies, and finally saw his doctrine almost universally adopted.
Although it might seem unnecessary to adduce a formal train of ai guments in support of a theory which is no longer opposed, yet it may be useful to review the nature of the proofs which were brought forward by Harvey, as they constitute a series of important facts, which throw consi derable light upon the structure and properties of the san guiferous system. The proof that the circulation proceeds in the course which has been described above, is derived partly from observations and experiments made on living animals, and partly from the anatomical structure of the organs concerned. If we open the chest of a cold-blooded animal during life, and bring the heart into view, we may observe its alternate contraction and dilatation proceeding with considerable regularity. - For a short space of time, the heart lies at rest, and suffers itself to be distended with blood ; then it is suddenly seen to rise up on its basis, to shorten its fibres, and to expel its contents. During this period it strikes the ribs, and produces what is called the beating of the heart. From an inspection of the mecha nism of the valves, we perceive that it is impossible for the blood to return into the auricles, but that there will be no obstacle to its entrance into the arteries ; they therefore become distended with blood. The valves, which are at the commencement of the arteries, will prevent any blood which has once entered them from returning to the heart ; it must consequently be carried forwards into the minute extremities of the arteries, and from these it must pass into the veins, proceeding from the smaller to the larger ves sels, until it arrives at the vena cava, whence it is deliver. ed into the right side of the heart. As there is no direct communication between the two auricles and the two ven tricles, it cannot pass from the right to the left cavities without traversing the pulmonic circulation, which may be regarded as a smaller circle, interposed, as it were, be tween the right ventricle and the right auricle. The valves of these parts point out the current to be, as we have described it in the systemic circulation, from the ventricle, first along the artery, and afterwards through the veins.