Page:Collier's New Encyclopedia v. 02.djvu/582

CIRCULATION 508 CIRCUMFERENCE course of the blood from the heart to the most minute blood-vessels (the capil- laries), and from these back to the heart. The heart is situated nearly in the cen- ter 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 ob- liquely 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 vense cavse, 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, 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 mini- mum. 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 ai'teries, are found in nearly every tis- .«ue; they commence by minute plexuses (an anatomical term for a network- rke 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 to- ward the heart. If we accept certain venous structures (called sinuses) occur- ring in the interior of the skull, we may divide the veins into two sets — the super- ficial 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 gen- erally exist in pairs, one on each side of the artery, and are called venss comites, while the larger arteries have usually only one accompanying vein. The superficial veins occur immedi- ately 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 jugu- lars) 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. CIRCUMCISION, an operation con- sisting in removing circularly the pre- puce of infants. God commanded Abra- ham to use circumcision as a sign of his covenant; and in obedience to this order, the patriarch at 99 years of age was cir- cumcised, as also his son Ishmael, and all the males of his household (Gen. xvii: 10-12). God repeated the precept to Moses, and ordered that all who intend- ed to partake of the Paschal sacrifice should receive circumcision, and that this rite should be performed on children on the eighth day after their birth (Ex. xii: 44). The Jews and all the other nations sprung from Abraham, as the Ishmael- ites, the Arabians, etc., have always been very exact in observing this ceremony. At the present day it is an essential rite of the Mohammedan religion, and though not enjoined in the Koran, prevails wher- ever this religion is found. A similar operation is performed among the Egyptians, Arabians, and Persians, on the females. The Jews esteemed uncir- cumcision as a very great impurity; and the greatest offense they could receive was to be called uncircumcised. Paul frequently mentions the Gentiles under this term, not opprobriously (Rom. ii: 26), but in opposition to the Jews, whom he names "the circumcised." The feast of circumcision is a festival observed in the Roman Catholic Church and in some other denominations, in com- memoration of the circumcision of Jesus Christ. It is held on Jan. 1. CIRCUMFERENCE, or PERIPHERY the curve which incloses a circle, ellipse, oval, cardioid, or other plane figure. In figures bounded by straight lines, as the triangle, square, and polygon, the term perimeter is employed to designate the sum of all the bounding lines taken to- gether. The length of the circumference depends partly on the nature of the curve. See Circle, etc.