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THIS work, although based upon Chambers's Encyclopædia, whose distinguished
merit is widely known, differs from it in important respects. It could scarcely be
expected that an Encyclopædia, edited and published for a foreign market, would give
as much prominence to American topics as American readers might desire. To supply
these and other deficiencies the American Editors have inserted about 15,000 titles,
arranging the whole, inc uding Chambers's Supplement, in a single alphabet. The
total number of titles is now about 40,000. The additions give greater fullness in the
departments of biography, geography, history, natural history, and general and applied
science. Scrupulous care has been taken not to mutilate or modify the original text of
the edition of 1880; no changes have been made except such verbal alterations as are
required by the omission of the wood-cuts. The titles of articles from Chambers's
Encyclopædia, either from the main work or from the Supplement, are printed in bold-
faced type-AMERICA. The titles of the American additions, whether of new topics or
of enlargements of the old, are printed in plain capitals-AMERICA. Should it appear
that an article from the English work and its American continuation disagree in any
points, the reader will readily refer the conflicting statements to their proper sources.

The labor of consultation will be much reduced by the catch-words in bold-faced
type at the top of the page, being the first and last titles of the pages which face each
other; and by the full title-words on the back of the volume, being the first and last
titles contained therein.

The word ante refers to Chambers's Encyclopædia, as represented in this issue.
Whenever the word (ante) follows a title in the American additions, it indicates that
the article is an enlargement of one under the same title in Chambers's Encyclopædia-
usually to be found immediately preceding.

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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.




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

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