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OF THE LAST (1880) EDINBURGH AND LONDON EDITION
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LIBRARY OF UNIVERSAL KNOWLEDGE.
IRCULATION, in anatomy and physiology, is the term used to designate the course of the blood from the heart to the most minute blood-vessels (the capillaries, q. v.), and from these back to the heart.
To simplify the consideration of the subject, we shall consider-1. The anatomy of the organs of circulation-and, 2. The physiology of the circulation.
FIG. 1.-MODE OF CIRCULA
1. The organs of C. consist of the heart, arteries, veins, and capillaries. The course of the blood through these organs will be best elucidated by the aid of a diagram, which is equally applicable for all other mammals as well as for man, and for birds. The shaded part of fig. 1 represents structures filled with impure or venous blood, while the unshaded portion represents structures in which pure, oxygenated, arterial blood occurs. In this diagram we observe a dotted circle, representing a closed bag or sac, termed the pericardium, and inclosing the four cavities c, v, c', o', of which the heart is composed. Two of these cavities, cand c', are for the purpose of receiving the blood as it flows into the heart, and are termed the auricles; while the two cavities v and v' are for the purpose of propelling the blood through the lungs and general system respectively, and are a termed the ventricles. The vessels that transport blood into the auricles are termed veins, and the vessels through which the blood is driven onwards from the ventricles are known as arteries (q.v.). The diagram further shows that what we commonly term the heart, is in reality two distinct hearts in apposition with each other-one, shaded in the figure, which is called the right, or venous, or pulmonary heart; and the other, unshaded, which is called the left, or arterial, or systematic heartthe last name having been given to it, because the blood is sent from it to the general system; just as the right heart is termed pulmonary from its sending blood to the lungs. We will now trace the course of the blood as indicated by the arrows in this TION IN MAN AND OTHER diagram, commencing with the right auricle, c. The right auMAMMALS, AND IN BIRDS. ricle contracting upon the venous or impure blood with which h, heart; v, right ventri- we suppose it to be filled, drives its contents onwards into the cle: ', left ventricle; c, right ventricle v, through an opening between these two cavities, right auricle; c', left au- called the right auriculo-ventricular opening, which is guarded a, aorta; d, vena cava; e, greater circula- by a valve named the tricuspid-from its being composed of tion; b, smaller circula three-pointed membranous expansions-which almost entirely tion; f, pulmonary artery: g,pulmonary veins. prevents the regurgitation or reflux of the blood from the ventricle into the auricle. The ventricle v being now filled, con. tracts, and as the blood cannot return into the auricle, it is driven along the shaded ves sel, the dividing branches of which are indicated by f. This vessel is known as the pulmonary artery, and conveys the blood to the lungs. At its commencement, it is guarded by valves, termed, from their shape, the semilunar pulmonary valves, which entirely prevent the blood which has once been propelled into the pulmonary artery from re-entering the ventricle. The pulmonary artery gradually divides into smaller and smaller branches, which ultimately merge into capillaries. In these capillaries, which are freely distributed over the interior of all the air-cells (of which the lung is mainly composed), the venous blood is brought in contact with atmospheric air, gives off its carbonic acid gas (which is its principal impurity), and absorbs oxygen, by which processes it is converted into pure or arterial blood. The capillaries, b, in which the blood is arterialized, gradually unite to form minute veins, which, again, join to form larger vessels, until finally the blood is collected into a small number of vessels known as pulmonary veins, which pour their contents into the left auricle. Only one such vessel, g, is shown in the figure, because the main object of this diagramatic scheme is to illustrate the mode and general direction in which the blood circulates, not to indicate