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4.11.10 The semicircular ducts

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(3.33)

The three semicircular canals, and the membranous ducts they enclose, are perpendicular to each other. They're now named superior, lateral, and posterior. The superior and lateral canals were formerly called "anterior" and "horizontal".

The lateral canals on the left and right side lie in almost the same plane, and their two ampullae function together in response to rotational movement in that plane. From above, we can see that e ach superior canal lies in the same plane as the opposite posterior canal. Their two pairs of ampullae function together in response to rotational movement in their respective planes.

Here, after sculpting the bone around the semicircular canals, we've opened each canal so we can see the semicircular ducts. The dissection has been performed under water, and is shown here under water, so as to preserve the architecture of these delicate structures.

We're looking from an anterolateral viewpoint. Here's the superior semicircular duct, here's the lateral one. Their two ampullae converge, to enter the utricle together. The lateral semicircircular duct curls around out of sight. We'll see it again in a moment.

Going right around to a posteromedial view, here's the superior duct continuing backward and downward. This is the posterior duct, joining with the superior duct to form the common crus. Here's the lateral duct again. The lateral duct and the common crus enter the lower part of the utricle close together. The posterior duct opens into it here.

Each semicircular duct is about one third the diameter of its bony canal, except at the ampulla, where it widens to fill the canal. To see inside the ampulla we'll remove this part of its wall. This curved ridge in the wall of the ampulla is the ampullary crest. It's more easily seen in another ampulla that's been opened here, ...

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(3.33)

The three semicircular canals, and the membranous ducts they enclose, are perpendicular to each other. They're now named superior, lateral, and posterior. The superior and lateral canals were formerly called "anterior" and "horizontal".

The lateral canals on the left and right side lie in almost the same plane, and their two ampullae function together in response to rotational movement in that plane. From above, we can see that e ach superior canal lies in the same plane as the opposite posterior canal. Their two pairs of ampullae function together in response to rotational movement in their respective planes.

Here, after sculpting the bone around the semicircular canals, we've opened each canal so we can see the semicircular ducts. The dissection has been performed under water, and is shown here under water, so as to preserve the architecture of these delicate structures.

We're looking from an anterolateral viewpoint. Here's the superior semicircular duct, here's the lateral one. Their two ampullae converge, to enter the utricle together. The lateral semicircircular duct curls around out of sight. We'll see it again in a moment.

Going right around to a posteromedial view, here's the superior duct continuing backward and downward. This is the posterior duct, joining with the superior duct to form the common crus. Here's the lateral duct again. The lateral duct and the common crus enter the lower part of the utricle close together. The posterior duct opens into it here.

Each semicircular duct is about one third the diameter of its bony canal, except at the ampulla, where it widens to fill the canal. To see inside the ampulla we'll remove this part of its wall. This curved ridge in the wall of the ampulla is the ampullary crest. It's more easily seen in another ampulla that's been opened here, so we can look into it from this end.

Here's the opening that was made. This is the ampullary crest. It's covered by a layer of hair cells, which initiate our sensory input for rotational movement. The hair cells are acted on, by a structure that can't be seen in a preserved specimen, the cupula.

Here's the ampullary crest with its layer of hair cells. This is the cupula. It's a soft disk of non-cellular material that forms a flexible partition across the ampulla. The hair cells are embedded in its base.

When our head rotates in the plane of the duct, fluid inertia bends the cupula slightly, in a direction opposite to the rotational movement.

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