Editing Semicircular canals (section)

==Structure==
{{Anchor|Osseous ampulla|Osseous ampullae}}
The semicircular canals are a component of the [[bony labyrinth]] that are at right angles from each other and contain their respective semicircular duct. At one end of each of the semicircular ducts is a dilated sac called a '''membranous ampulla''', which is more than twice the diameter of the ducts. Each ampulla contains an ampullary crest, the [[crista ampullaris]] which consists of a thick gelatinous cap called a [[ampullary cupula|cupula]] and many [[hair cell]]s. The superior and posterior semicircular ducts are oriented vertically at right angles to each other. The lateral semicircular duct is about a 30-degree angle from the horizontal plane. The orientations of the ducts cause a different duct to be stimulated by movement of the head in different planes, and more than one duct is stimulated at once if the movement is off those planes. The lateral semicircular duct detects angular acceleration of the head when the head is turned and the anterior and posterior semicircular ducts detect vertical head movements when the head is moved up or down.<ref name=Saladin2012p607-8>{{cite book |last=Saladin |first=Kenneth S. |title=Anatomy and Physiology: The Unity of Form and Function |year=2012 |publisher=McGraw Hill |location=New York |isbn=978-0-07-337825-1 |pages=607–8}}</ref> When the head changes position, the [[endolymph]] in the ducts lags behind due to inertia and this acts on the cupula which bends the cilia of the hair cells. The stimulation of the hair cells sends the message to the brain that acceleration is taking place. The semicircular canals open into the vestibule by five orifices, one of the apertures being common to two of them.

Among species of mammals, the size of the semicircular canals is correlated with their type of locomotion. Specifically, species that are agile and have fast, jerky locomotion have larger canals relative to their body size than those that move more cautiously.<ref name='SpoorEA2007'>{{cite journal |doi=10.1073/pnas.0704250104 |title=The primate semicircular canal system and locomotion |year=2007 |last1=Spoor |first1=Fred |last2=Garland |first2=Theodore |last3=Krovitz |first3=Gail |last4=Ryan |first4=Timothy M. |last5=Silcox |first5=Mary T. |last6=Walker |first6=Alan |journal=Proceedings of the National Academy of Sciences |volume=104 |issue=26 |pmid=17576932 |jstor=25436020 |bibcode=2007PNAS..10410808S |pages=10808–12 |pmc=1892787|doi-access=free }}</ref>

===Lateral semicircular canal===
The '''lateral semicircular canal''' (also known as '''horizontal''' or '''external semicircular canal''') is the shortest of the three canals. Movement of fluid within its duct corresponds to rotation of the head around a vertical axis (i.e. the neck), or in other words, rotation in the [[transverse plane]]. This occurs, for example, when one turns the head from side to side (yaw axis).

It measures from {{cvt|12|to|15|mm}}, and its arch is directed horizontally backward and laterally; thus each semicircular canal stands at right angles to the other two. Its ampullated end corresponds to the upper and lateral angle of the [[Vestibule of the ear|vestibule]], just above the [[oval window]], where it opens close to the ampullated end of the anterior semicircular canal; its opposite end opens at the upper and back part of the vestibule. The lateral canal of one ear is very nearly in the same plane as that of the other.

===Anterior semicircular canal===
The '''anterior semicircular canal''' (also known as '''superior semicircular canal''') contains the part of the [[vestibular system]] that detects rotations of the head in around the lateral axis, that is, rotation in the [[sagittal plane]]. This occurs, for example, when nodding one's head (pitch axis).

It is {{cvt|15|to|20|mm}} in length, is vertical in direction, and is placed transversely to the long axis of the [[Petrous part of the temporal bone|petrous part]] of the [[temporal bone]], on the anterior surface of which its arch forms a round projection. It describes about two-thirds of a circle. Its lateral extremity is ampullated, and opens into the upper part of the vestibule; the opposite end joins with the upper part of the posterior semicircular canal to form the ''crus osseum commune'', which opens into the upper and medial part of the vestibule.

===Posterior semicircular canal===
The '''posterior semicircular canal''' contains the part of the vestibular system that detects rotation of the head around the antero-posterior (sagittal) axis, or in other words, rotation in the [[coronal plane]]. This occurs, for example, when one moves the head to touch the shoulders, or when doing a [[Cartwheel (gymnastics)|cartwheel]] (roll axis).

It is directed superiorly and posteriorly, as per its nomenclature, nearly parallel to the posterior surface of the petrous part of the temporal bone. The [[vestibular aqueduct]] is immediately medial to it. The posterior semicircular canal is part of the bony labyrinth and its duct is used by the vestibular system to detect rotations of the head in the coronal plane. It is the longest of the three semicircular canals, measuring from {{cvt|18|to|22|mm}}. Its lower or ampullated end opens into the lower and back part of the vestibule, its upper into the ''crus osseum commune''.

===Development===
Findings from a 2009 study demonstrated a critical late role for [[bone morphogenetic protein 2]] (BMP-2) in the morphogenesis of semicircular canals in the [[zebrafish]] inner ear. It is suspected that the role of BMP-2 in semicircular canal duct outgrowth is likely to be conserved between different [[vertebrate]] species.<ref>{{cite journal |doi=10.1371/journal.pone.0004368 |title=A Late Role for bmp2b in the Morphogenesis of Semicircular Canal Ducts in the Zebrafish Inner Ear |year=2009 |editor1-last=Hendricks |editor1-first=Michael |last1=Hammond |first1=Katherine L. |last2=Loynes |first2=Helen E. |last3=Mowbray |first3=Catriona |last4=Runke |first4=Greg |last5=Hammerschmidt |first5=Matthias |last6=Mullins |first6=Mary C. |last7=Hildreth |first7=Victoria |last8=Chaudhry |first8=Bill |last9=Whitfield |first9=Tanya T. |journal=PLOS ONE |volume=4 |issue=2 |pages=e4368 |pmid=19190757 |pmc=2629815|bibcode = 2009PLoSO...4.4368H  |doi-access=free }}</ref>

Additionally, it has been found that the two semicircular canals found in the [[lamprey]] inner ear are developmentally similar to the superior and posterior canals found in humans, as the canals of both organisms arise from two depressions in the [[otic vesicle]] during early development. These depressions first form in lampreys between the 11 and 42 millimeter larval stages and form in zebrafish 57 hours post-fertilization  <ref>{{cite journal | last1 = Higuchi | first1 = S. | last2 = Sugahara | first2 = F. | last3 = Pascual-Anaya | first3 = J. | display-authors = etal | year = 2019 | title = Inner ear development in cyclostomes and evolution of the vertebrate semicircular canals | journal = Nature | volume = 565 | issue = 7739| pages = 347–350 | doi = 10.1038/s41586-018-0782-y | pmid = 30518864 | s2cid = 54458839 | hdl = 10630/33079 | hdl-access = free }}</ref>