Quadrate bone
Encyclopedia
The quadrate bone is part of a skull in most tetrapods, including amphibians, sauropsids (reptiles, birds), and early synapsids. In these animals it connects to the quadratojugal
and squamosal
in the skull, and forms part of the jaw joint (the other part is the articular
bone at the rear end of the lower jaw).
In mammals the articular
and quadrate bones have migrated to the middle ear
and are known as the malleus
and incus
. In fact, paleontologists regard this modification as the defining characteristic of mammalian hearing structures.
This migration was first described by Reichert
in 1837. In pig embryos he discovered that the mandible ossifies on the side of Meckel's cartilage
, while the posterior part of that cartilage is ossified and then detaches from the rest of the cartilage to enter the middle ear where it becomes the incus.
Quadratojugal
The quadratojugal is a small jaw bone that is present in most amphibians, reptiles, and birds, but has been lost in mammals. It is connected to the jugal as well as other bones, though these may vary with species....
and squamosal
Squamosal
The squamosal is a bone of the head of higher vertebrates. It is the principal component of the cheek region in the skull, lying below the temporal series and otic notch and bounded anteriorly by postorbital. Posteriorly, the squamosal articulates with the posterior elements of the palatal complex,...
in the skull, and forms part of the jaw joint (the other part is the articular
Articular
The articular bone is part of the lower jaw of most tetrapods, including amphibians, sauropsids and early synapsids. In these animals it is connected to two other lower jaw bones, the suprangular and the angular...
bone at the rear end of the lower jaw).
Evolutionary variation
In snakes, the quadrate bone has become elongated and very mobile, and contributes greatly to their ability to swallow very large prey items.In mammals the articular
Articular
The articular bone is part of the lower jaw of most tetrapods, including amphibians, sauropsids and early synapsids. In these animals it is connected to two other lower jaw bones, the suprangular and the angular...
and quadrate bones have migrated to the middle ear
Middle ear
The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The mammalian middle ear contains three ossicles, which couple vibration of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear has...
and are known as the malleus
Malleus
The malleus or hammer is a hammer-shaped small bone or ossicle of the middle ear which connects with the incus and is attached to the inner surface of the eardrum...
and incus
Incus
The incus or anvil is the anvil-shaped small bone or ossicle in themiddle ear. It connects the malleus to the stapes. It was first described by Alessandro Achillini of Bologna.The incus transmits sound vibrations from the malleus to the stapes....
. In fact, paleontologists regard this modification as the defining characteristic of mammalian hearing structures.
This migration was first described by Reichert
Karl Bogislaus Reichert
Karl Bogislaus Reichert was a German anatomist.Reichert was born in Rastenburg , East Prussia. He studied etiology and histology in Königsberg. He was a student of Friedrich Schlemm and Johannes Peter Müller at the Friedrich-Wilhelm-Institute and at the Charité in Berlin, receiving his doctorate...
in 1837. In pig embryos he discovered that the mandible ossifies on the side of Meckel's cartilage
Meckel's cartilage
The cartilaginous bar of the mandibular arch is formed by what are known as Meckel’s cartilages also known as Meckelian cartilages; above this the incus and malleus are developed....
, while the posterior part of that cartilage is ossified and then detaches from the rest of the cartilage to enter the middle ear where it becomes the incus.