Incus

The incus is the second of three ossicles, very small bones in the middle ear which act to transmit sound. It is shaped like an anvil, and has a long and short crus extending from the body, which articulates with the malleus.^[2] The short crus attaches to the posterior ligament of the incus. The long crus articulates with the stapes at the lenticular process.

The superior ligament of the incus attaches at the body of the incus to the roof of the tympanic cavity.

Vibrations in the middle ear are received via the tympanic membrane. The malleus, resting on the membrane, conveys vibrations to the incus. This in turn conveys vibrations to the stapes.^[2]

"Incus" means "anvil" in Latin. Several sources attribute the discovery of the incus to the anatomist and philosopher Alessandro Achillini.^[3][4] The first brief written description of the incus was by Berengario da Carpi in his Commentaria super anatomia Mundini (1521).^[5] Andreas Vesalius, in his De humani corporis fabrica,^[6] was the first to compare the second element of the ossicles to an anvil, thereby giving it the name incus.^[7] The final part of the long limb was once described as a "fourth ossicle" by Pieter Paaw in 1615.^[8]

The incus is evolutionarily derived from the quadrate, a bone that forms the upper half of the jaw joint in non-mammalian vertebrates. During the evolution of mammals, the suspension of the jaw changed to a dentary-squamosal articulation; the quadrate and the articular took on sound-conduction roles, became smaller, and detached from the jaw, becoming known as the incus and the malleus, respectively.^[9][10]

The quadrate, in turn, evolved as the rear portion of the palatoquadrate, which was the ancestral upper jaw of gnathostomes. In early members of this clade (and in modern sharks and rays), the palatoquadrate articulated with Meckel's cartilage to form the jaw joint. The palatoquadrate is an element of the mandibular arch of the splanchnocranium, meaning that the incus is ultimately an evolutionary descendant of the gill arch.^[11]

1. ^ hednk-023—Embryo Images at University of North Carolina
2. ^ a b Drake, Richard L.; Vogl, Wayne; Tibbitts, Adam W.M. Mitchell; illustrations by Richard; Richardson, Paul (2005). Gray's anatomy for students. Philadelphia: Elsevier/Churchill Livingstone. ISBN 978-0-8089-2306-0.
3. ^ Alidosi, GNP. I dottori Bolognesi di teologia, filosofia, medicina e d'arti liberali dall'anno 1000 per tutto marzo del 1623, Tebaldini, N., Bologna, 1623. http://gallica.bnf.fr/ark:/12148/bpt6k51029z/f35.image#
4. ^ Lind, L. R. Studies in pre-Vesalian anatomy. Biography, translations, documents, American Philosophical Society, Philadelphia, 1975. p.40
5. ^ Jacopo Berengario da Carpi,Commentaria super anatomia Mundini, Bologna, 1521. https://archive.org/details/ita-bnc-mag-00001056-001
6. ^ Andreas Vesalius, De humani corporis fabrica. Johannes Oporinus, Basle, 1543.
7. ^ O'Malley, C.D. Andreas Vesalius of Brussels, 1514-1564. Berkeley: University of California Press, 1964. p. 121
8. ^ Graboyes, Evan M.; Chole, Richard A.; Hullar, Timothy E. (September 2011). "The Ossicle of Paaw". Otology & Neurotology. 32 (7): 1185–1188. doi:10.1097/MAO.0b013e31822a28df. PMC 3158805. PMID 21844785.
9. ^ Ji, Qiang; Luo, Zhe-Xi; Zhang, Xingliao; Yuan, Chong-Xi; Xu, Li (2009-10-09). "Evolutionary Development of the Middle Ear in Mesozoic Therian Mammals". Science. 326 (5950). American Association for the Advancement of Science: 278–281. doi:10.1126/science.1178501.
10. ^ Luo, Zhexi; Crompton, Alfred W. (1994-09-07). "Transformation of the quadrate (incus) through the transition from non-mammalian cynodonts to mammals". Journal of Vertebrate Paleontology. 14 (3): 341–374. Bibcode:1994JVPal..14..341L. doi:10.1080/02724634.1994.10011564. ISSN 0272-4634.
11. ^ Arratia, Gloria; Schultze, Hans-Peter (1991). "Palatoquadrate and its ossifications: Development and homology within osteichthyans". Journal of Morphology. 208 (1): 1–81. doi:10.1002/jmor.1052080102. ISSN 1097-4687.

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