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Incus


Type

irregular

Description

Similar in size to the malleus, this is the middle of the three ear ossicles. Although its name suggests an anvil, this is probably not from its similarity to an anvil in shape, but from the fact that the hammer hits it. It has a cube shaped body with two divergent processes projecting from it. The bone has an interesting evolutionary history. It arises from the first or mandibular gill bar. In earlier vertebrates it was a posterior element, the quadrate bone, in the jaw apparatus. The mammals were the first vertebrates to incorporate this bone into the ear anatomy.

Etymology

Incus comes from the Latin word for anvil. This arises from the Latin term in plus the verb cudere meaning to strike. The malleus and incus were discovered in 1503 by the anatomist, Alessandro Achillini. In 1543 Andreas Vesalius illustrated and named these bones in his writings.

Articulations

The incus articulates with two bones: the malleus and the stapes. It forms a deep concavo-convex facet on the anterior surface of the body that articulates with malleus. At the end of its long crus it forms a delicate lenticular process that forms a synovial joint with the stapes.

Ossification

The incus ossifies endochondrally from a primary center in the cartilaginous precursor arising from the base of Meckel's cartilage. The center appears during the fourth fetal month and is usually fully ossified by birth. The lenticular process at the base of the long crus may arise from a second center.

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