semicircular

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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.

sem·i·cir·cu·lar

(sem'ē-sir'kyū-lăr),
Forming a half circle or an incomplete circle.
Synonym(s): semiorbicular

semicircular

(sĕm″ē-sŭr′kū-lăr) [″ + circulus, a ring]
In the form of a half circle.

semicircular

shaped like a half-circle.

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.
References in classic literature ?
Very solemnly the red-headed brats rosc from the floor and formed a semicircle.
We drew up chairs and sat in a semicircle in the window.
The next time Jerry dabbed at him with a paw, barked, and scurried away in an enticing semicircle, Michael started involuntarily though slowly after him.
Moving the opera-glass slowly along the semicircle formed by the seats, I suddenly stopped when I reached the middle.
We sat all three in a semicircle, with our heads advanced, and our chins upon our hands, while the strange, jerky little fellow, with his high, shining head, puffed uneasily in the centre.
The children were arranged in a semicircle round the new master, and he was soon listening to their dull, drawling, hesitating recital of those stories of engrossing interest which are to be found in the more antiquated spelling-books.
When the Carrancha is quietly seated on the branch of a tree or on the ground, the Chimango often continues for a long time flying backwards and forwards, up and down, in a semicircle, trying each time at the bottom of the curve to strike its larger relative.
This is a semicircle cut into the lawn and edged with privet, and in this semicircle are eleven beds of different sizes bordered with box and arranged round a sun-dial, and the sun-dial is very venerable and moss-grown, and greatly beloved by me.
He was seated on a cushioned chair, and had a garland on his head, for he had been sacrificing in the court; and there were some other chairs in the room arranged in a semicircle, upon which we sat down by him.
In many successive visits we came to know his stage well--the purple semicircle of hills, the slim trees leaning over houses, the yellow sands, the streaming green of ravines.
When it subsided, I saw them all drawn up in four semicircles, before four chairs, placed at the four tables; all held books in their hands, and a great book, like a Bible, lay on each table, before the vacant seat.
An audience would be demanded of the governor-general, who would hold the conference with becoming state, seated in an elbow-chair, with the Indians ranged in semicircles before him, seated on the ground, and silently smoking their pipes.