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filled, contracts, and as the blood cannot return into the auricle, it is driven along the shaded vessel, 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 semi-lunar 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 the special vessels through which it flows in different parts of the body; the actual number of the pulmonary veins is four-viz., two from each lung. The blood, now fitted for the various purposes of nutrition, enters the left auricle, c, which by its contraction propels it into the left ventricle, v', through the left auriculo-ventricular opening. This opening, like the corresponding one in the right heart, is guarded by a valve which, from its form, is termed the mitral valve, and which entirely prevents the reflux of the blood. The left ventricle, e, contracts and drives its contents into the large artery, a, which represents the aorta-the great trunk which, by means of its various branches (none of which are indicated in the diagram), supplies every portion of the body, from the crown of the head to the soles of the feet, with pure arterial blood. From the aorta and its various subdividing branches the blood passes into the capillaries, e, which occur in every part of the system; in these capillaries it undergoes important changes, which may be considered as almost exactly the reverse of those which occur in the pulmonary capillaries; it parts with its oxygen, becomes charged with carbonic acid, and, as it leaves the capillaries, and enters the minute veins formed by their union, presents all the characters of venous blood. The veins gradually unite till they form two large trunks, termed the superior and inferior vena cara, which pour their contents into the right auricle- the point from which we started. Only one of these great veins, d, is indicated in the diagram. Thus there is a complete double C.--a less C. effected by the blood in its passage from the right to the left heart through the lungs; and a greater C. effected by the blood in its passage from the left heart through the system generally to the right heart.

From the above simple ideal scheme, we proceed to the

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consideration of the more complicated arrangements by which the C. is actually effected in man and the higher animals.

The heart is situated in very nearly the centre of the cavity of the chest, or thorax, as it is termed in anatomy, between the lungs. behind the breast-bone, or sternum, in front of the vertebral column, and above the diaphragm, on which it obliquely rests. Its form is somewhat conical, the lower end tapering almost to a point, and directed rather forward and to the left. This lower portion alone is movable, and, at each contraction of the heart, it is tilted forward, and strikes against the walls of the chest between, in man, the fifth and sixth ribs, or a little below the left nipple. All the large vessels connected with the heart-the cena cave, the pulmonary artery, and the aorta-arise from its base (see fig. 2), and serve, from their attachment to the

Fig. 2.-The Lungs, Heart, and principal Blood-vessels in Man: a, h. veins from the right and left arms; b, f, right and left jugular veins, returning the blood from the head and neck-these four veins unite to form a single trunk, the vena cava superior, which enters the right auricle, 1; c, e, the right and left carotid arteries, the latter rising directly from the arch of the aorta, a', the former from a short trunk called the arteria innominata; g, the left subclavian artery, rising directly from the aorta, while the right subclavian springs from the arteria innominata; d, the trachea or windpipe; i, the right and left lungs; 1, ', the right and left auricles; p. the right ventricle, o, the apex of left ventricle; m, the inferior or ascending vena cava; n, the descending aorta, emerging from behind the heart; q, the pulmonary artery.

neighboring parts, to keep that portion of it fixed. Indeed, these vessels may be regarded as suspending the heart in the cavity, which is lined by a smooth serous membrane which, near the top, is reflected downward over the roots of the great vessels, and covers the whole of the outer surface of the heart. These two smooth serous surfaces--one lining the cavity, the other investing the heart-are kept moist by a fluid which they secrete, and by this arrangement friction may be regarded as reduced to its minimum. The cavity

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or sac in which the heart lies is called the pericardium. Like all serous membranes, it is a closed sac, and, as it may not be easy for the non-professional reader to understand the relative position of the heart, which is at the same time surrounded by and external to this membrane, we may observe that the head in an old-fashioned double night-capwhich is a closed bag --is in much the same position as the heart in the pericardium; it is inside the night-cap, but not in the cavity which intervenes between its two layers. The substance of the heart is essentially muscular, The fibres run in difierent directions, longitudinally and transversely, but most of them obliquely; many pass over the apex from one side of the heart to the other; and all are so involved as to render it very difficult to unravel them. In consequence of this arrangement the fibres, by their contraction, seem simultaneously to diminish each cavity in all directions, and thus serve most efficiently to drive the blood onward. The size of the heart has been estimated as about that of the closed fist of the same individual. Its weight, as compared with that of the body, was deter

Fig. 3.-Theoretical Section of the human Heart: f, b, the two venæ cavæ, opening into d, the right auricle; c, the tricuspid valve; a, the right ventricle, from which proceeds the pulmonary artery, dividing into branches g and i, going to the right and left lung respectively; e, e', the pulmonary veins (two from either lung), entering into the left auricle, k; 1, the mitral valve; m, the left ventricle, from which proceeds the aorta, whose arch is indicated by h, and the descending portion by n, none of its branches being indicated in this figure; o, the partition, or septum, between the right and left hearts.

mined by Dr. Clendinning to be 1:160 in the male, and 1:150 in the female. The same physician carefully examined nearly 400 hearts of persons of both sexes, and determined the average weight at about nine oz. avoirdupois, while Dr. John Reid found the average weight of the male heart to be a little more than 11 oz., and that of the female heart to be a little above 9 oz.

In our ideal sketch of the organs of C. (fig. 1), we have indicated the different cavities into which the heart is divided. In fig. 3 there is represented a section of the human heart, which is sufficiently like the reality to give the