circuit or electric circuit Path that transmits electric current. A circuit includes a battery or a generator that gives energy to the charged par¬ ticles; devices that use current, such as lamps, motors, or electronic com¬ puters; and connecting wires or transmission lines. Circuits can be classified according to the type of current they carry (see alternating cur¬ rent, direct current) or according to whether the current remains whole (series) or divides to flow through several branches simultaneously (par¬ allel). Two basic laws that describe the performance of electric circuits are Ohm's law and Kirchhoff's circuit rules. See also tuned circuit.
circuit, printed See printed circuit circuitry, computer See computer circuitry
circuit riding In the U.S., the act, once undertaken by a judge, of trav¬ eling within a judicial district (or circuit) to facilitate the hearing of cases. The practice was largely abandoned with the establishment of permanent courthouses and laws requiring parties to appear before a sitting judge.
circulation Process by which nutrients, respiratory gases, and metabolic products are transported throughout the body. In humans, blood remains within a closed cardiovascular system composed of the heart, blood ves¬ sels, and blood. Arteries carry blood away from the heart under high pres¬ sure exerted by the heart’s pumping action. Arteries divide into smaller arterioles, which branch into a network of tiny capillaries with thin walls across which gases and nutrients diffuse. Capillaries rejoin into larger venules, which unite to form veins, which carry blood back to the heart. (See artery; capillary; vein.) The right and left heart chambers send blood into separate pulmonary and systemic circulations. In the first, blood is carried from the heart to the lungs, where it picks up oxygen and releases carbon dioxide; in the second, blood is carried between the heart and the rest of the body, where it carries oxygen, nutrients, metabolic products, and wastes. See illustration above.
right common
internal jugular vein
right pulmonary artery
right subclavian artery
right subclavian vein
superior vena cava right veins
ventricle right inferior vena
vessels liver
renal blood vessels
superior mesenteric artery
ascending colon
common iliac
external iliac vein
brachiocephalic (innominate) artery
left common carotid artery left subclavian artery -aorta
left pulmonary artery
pulmonary veins
left atrium
ventricle
aorta
gastric arteries
spleen
splenic artery
stomach
-inferior
mesenteric artery
descending
colon
external iliac artery
Human circulatory system. Oxygen-rich blood is shown in red, oxygen-poor blood in blue. The pulmonary circulation consists of the right ventricle and the exiting pul¬ monary artery and its branches, the arterioles, capillaries, and venules of the lung, and the pulmonary vein. Unlike the other arteries and veins, the pulmonary arteries carry deoxygenated blood and the pulmonary veins carry oxygenated blood. The aorta arises from the left ventricle. The brachiocephalic artery arises from the aorta and divides into the right common carotid and right subclavian arteries. The left and right common carotids extend on either side of the neck and supply much of the head and neck. The left subclavian artery (arising from the aorta) and the right subclavian artery supply the arms. In the lower abdomen, the aorta divides into the common iliac arteries, which give rise to external and internal branches supplying the legs.
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circumcision Cutting away of all or part of the foreskin (prepuce) of the penis. The practice is known in many cultures. It is performed either shortly after birth (e.g., among Muslims and Jews), within a few years of birth, or at puberty. For Jews it represents the fulfillment of the covenant between God and Abraham (Genesis 17:10-14). That Christians were not obliged to be circumcised was first recorded biblically in Acts 15. Evi¬ dence regarding the purported medical benefits of circumcision (e.g., reduced risk of cancer) is inconclusive, and the practice persists mainly for cultural reasons. See also clitoridectomy.
circumstantial evidence In law, evidence that is drawn not from direct observation of a fact at issue but from events or circumstances that surround it. If a witness arrives at a crime scene seconds after hearing a gunshot to find someone standing over a corpse and holding a smoking pistol, the evidence is circumstantial, since the person may merely be a bystander who picked up the weapon after the killer dropped it. The popular notion that one cannot be convicted on circumstantial evidence is false. Most criminal convictions are based, at least in part, on circum¬ stantial evidence that sufficiently links criminal and crime.
circus Entertainment or spectacle featuring animal acts and human feats of daring. The modem circus was founded in England in 1768 by the bareback rider Philip Astley (1742-1814), who built stands around his performance ring and opened Astley’s Amphitheatre. One of his riders later established the Royal Circus (1782), the first modem use of the term.
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Cirenaica ► city I 409
The first U.S. circus opened in Philadelphia in 1793. Horse acts were later joined by wild-animal acts. After the invention of the flying trapeze by Jules Leotard (1859), aerial acts were featured. P.T. Barnum expanded the traditional circus by adding two rings to create the three-ring circus (1881) and augmented it with sideshow performers. Circuses traveled through¬ out the U.S., Europe, and Latin America, performing in a tent (the Big Top) into the 1950s. Today circuses usually perform in permanent build¬ ings, though small troupes still travel with tents in some regions. By the late 20th century, notable circuses also had developed in Africa, India, Spain, Brazil, and Mexico. Perhaps the most innovative trend in circuses at the turn of the 21st century was the establishment of companies such as the Cirque du Soleil; these companies employed no animals, instead emphasizing acts of human skill and daring and integrating elements of contemporary music and dance into the overall performance.
Cirenaica See Cyrenaica
cirrhosis \s9-'ro-s9s\ Degeneration of functioning liver cells and their replacement with fibrous connective tissue, leading to scarring. The most common cause is alcohol abuse with malnutrition. Others include bile duct obstruction, viral infection, toxins, iron or copper accumulation in liver cells, and syphilis. Jaundice, edema, and great abdominal swelling are com¬ mon in all. Death usually results from internal bleeding or hepatic coma due to blood chemical imbalance.
Cisalpine \sis-'al-pln\ Republic Former republic, northern Italy. Cre¬ ated by Napoleon in 1797 from conquered territories, it was centred in the Po River valley and included the lands around Milan and Bologna. It was incorporated into the Napoleonic kingdom of Italy in 1805.
Ciskei \'sis-,kl\ Former black enclave. South Africa. Inhabited princi¬ pally by Xhosa-speaking peoples, it bordered the Indian Ocean and South Africa. In the late 18th century the Xhosa peoples living in the area came into conflict with European settlers; the ensuing wars resulted in the region’s incorporation into the Cape Colony by the end of the 19th cen¬ tury. Ciskei became an administratively distinct territory within South Africa in 1961. In 1972 it was declared a self-governing black state, with its capital at Bisho. In 1981 it became nominally independent. Under the new South African constitution, it was reincorporated (1994) into South Africa as part of the new Eastern Cape province.