Circulation
in anatomy and physics, the term used to designate the CIRCULATION, in anatomy and CIRCULATION course of the blood from the heart to the most minute blood-vessels (the capillaries), and from these back to the heart. The heart is situated nearly in the center of the cavity of the chest, or thorax, as it is termed in anatomy, between the lungs, behind the breastbone, or sternum, in front of the vertebral column, and above the diaphragm, on which it obliquely rests. In form it 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 venæ cavæ, the pulmonary artery and the aorta-arise from its base, and serve, from their attachment to the 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, brane, 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 or sac in which the heart lies is called the pericardium.
Since all the arterial blood leaves the heart through the aortic opening, in tracing its course to the different parts of the system we obviously have only to follow the aorta to its final branches.
The arteries distribute the arterial blood to the capillaries, which pervade every part of the body. The veins, like the arteries, are found in nearly every tissue; they commence by minute plexuses (an anatomical term for a networklike arrangement), which communicate with the capillaries. Branches from these plexuses uniting together form small venous trunks, which by joining increase in size as they pass onward toward the heart. If we accept certain venous structures (called sinuses) occurring in the interior of the skull, we may divide the veins into two sets the superficial or cutaneous and the deep veins. The deep veins accompany the arteries, and are usually inclosed in the same sheath of cellular tissue with them. In the case of the smaller arteries, they generally exist in pairs, one on each side of the artery, and are called venæ comites, while the larger arteries have usually only one accompanying vein.
CIRCUMFERENCE The superficial veins occur immediately beneath the integument; they not only return the blood from the skin and adjacent structures, but communicate with the deep veins. All the veins finally unite into two large trunks, termed the superior and inferior vena cava, which open into the right auricle of the heart; the superior vena cava being formed by the union of the veins which return the blood from the head and neck (the jugulars) with those which convey it from the arms (the subclavians), while the inferior vena cava receives the blood from the lower extremities, the trunk, and the abdominal and pelvic viscera.
See HEART.