semicircular

semicircular [sem″e-ser´ku-lar]

shaped like a half-circle.

semicircular canals the passages in the bony labyrinth of the inner ear that control the sense of equilibrium. Each ear has three semicircular canals (anterior, lateral, and posterior), at approximately right angles to each other, which are filled with fluid and have ampullae (enlarged portions) at one end that contain nerve endings. (See also Plates.)

The semicircular canals respond to movement of the head. When the head changes position in any direction, the fluid in the canal that lies in the plane of movement also moves, but because of its inertia the fluid flow lags behind the head movement. Thus the fluid presses against the delicate hairs of the nerves in the ampulla, and the nerves then register the fact that the head is turning in a given direction so that the body can seek to maintain its equilibrium.

It is the fluid movement in the semicircular canals that causes the feeling of dizziness or vertigo after spinning. When the spinning stops, the fluid in the horizontal canal continues to move for a moment in the direction of the spin, giving a temporary false reading that the head is turning in the other direction. motion sickness is caused by the unusual and erratic motions of the head in an airplane, car, or ship, and the resulting stimulation of the semicircular canals.

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semicircular

shaped like a half-circle.

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semicircular canals

the passages in the inner ear, in the bony labyrinth concerned with the sense of balance, especially the detection of movement. Each ear has three semicircular canals (anterior, lateral and posterior) situated approximately at right angles to each other. They contain the semicircular ducts.

semicircular ducts

the membranous tubes housed within the semicircular canals.They have enlarged portions at one end, called ampullae, which contain nerve endings, and which are filled with fluid.

The semicircular ducts respond to movement of the head. When the head changes position, the fluid in the duct that lies in the plane of movement also moves but, because of its inertia, the fluid flow lags behind the head movement. Thus the fluid presses against the delicate hairs of the nerves in the ampulla, and these nerves then register the fact that the head is turning in such a direction.

It is the fluid movement in the semicircular ducts that causes the feeling of dizziness or vertigo after spinning. When the spinning stops, the fluid in the horizontal canal continues to move for a moment in the direction of the spin, giving a temporary false reading that the head is turning in the other direction. Motion sickness is caused by the unusual and erratic motions of the head in an airplane, car or ship, and the resulting stimulation of the semicircular ducts. Many animals, even fish, can experience motion sickness.