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A gap in or entrance to an organ, tube, or cavity. SEE ALSO: aperture, fossa, ostium, orifice, pore.

- access o. SYN: access.

- aortic o. SYN: aortic hiatus.

- o. of aqueduct of midbrain [TA] entrance to the cerebral aqueduct; point at which the caudal part of the third ventricle is continuous with the cerebral aqueduct of the midbrain; located on the midline immediately ventral to the posterior commissure. SYN: apertura aqueductus mesencephali [TA], apertura aqueductus cerebri, o. of cerebral aqueduct, aditus ad aqueductum cerebri, Bartholin anus.

- cardiac o. SYN: cardial orifice.

- openings of carotid canal [TA] the o. at each extremity of the carotid canal in the pyramidal petrous part of the temporal bone; the external o. of the carotid canal is on the inferior surface of the pyramid; the internal o. of the canal is at the apex of the petrous part. SYN: carotid foramen.

- caval o. of diaphragm [TA] an o. in the right lobe of the central tendon of the diaphragm that transmits the inferior vena cava and branches of the right phrenic nerve. SYN: foramen of vena cava, foramen quadratum, foramen venae cavae, vena caval foramen.

- o. of cerebral aqueduct o. of aqueduct of midbrain.

- o. of coronary sinus [TA] orifice by which the coronary sinus enters and drains into the right atrium of the heart. SYN: ostium sinus coronarii [TA].

- esophageal o. SYN: esophageal hiatus.

- external o. SYN: meatus.

- o. of external acoustic meatus SYN: external acoustic pore.

- external o. of cochlear canaliculus [TA] the external o. of the cochlear aqueduct on the temporal bone medial to the jugular fossa. SYN: apertura canaliculi cochleae, external aperture of cochlear canaliculus.

- external o. of urethra SYN: external urethral orifice.

- femoral o. SYN: adductor hiatus.

- o. of frontal sinus [TA] one of a pair of openings in the floor of the frontal sinuses in the nasal part of the frontal bone, through which the frontal sinuses communicate with the ethmoidal infundibulum via the frontonasal duct. SYN: apertura sinus frontalis [TA], frontal sinus aperture.

- ileocecal o. SYN: ileal orifice.

- o. of inferior vena cava [TA] the orifice through which the inferior vena cava opens into the right atrium. SYN: ostium venae cavae inferioris [TA], orifice of inferior vena cava.

- internal acoustic o. [TA] SYN: internal acoustic pore.

- o. of internal acoustic meatus SYN: internal acoustic pore.

- internal urethral o. internal urethral orifice.

- lacrimal o. SYN: lacrimal punctum.

- oral o. oral fissure.

- orbital o. [TA] the somewhat quadrangular anterior entrance to the orbit that forms the base of the pyramid-shaped orbital cavity. It is bounded by the sharp supra-, infra-, and lateral orbital margins and a less obvious medial margin on each side of the upper nose. SYN: aditus orbitae [TA], aperture of orbit.

- openings of papillary ducts [TA] numerous minute openings, the apertures of the papillary ducts converging on the apical pole of each renal papilla. SYN: foramina papillaria renis [TA], papillary foramina of kidney.

- pharyngeal o. of eustachian tube SYN: pharyngeal o. of pharyngotympanic (auditory) tube.

- pharyngeal o. of pharyngotympanic (auditory) tube [TA] an o. in the upper part of the nasopharynx about 1.2 cm behind the posterior extremity of the inferior concha on each side. SYN: ostium pharyngeum tubae auditivae [TA], ostium pharyngeum tubae auditoriae, pharyngeal o. of eustachian tube.

- piriform o. SYN: piriform aperture.

- o. of pulmonary trunk [TA] the o. of the pulmonary trunk from the right ventricle, guarded by the pulmonary valve. SYN: ostium trunci pulmonalis [TA], pulmonary orifice.

- openings of pulmonary veins [TA] the orifices of the pulmonary veins, usually two on each side, in the wall of the left atrium. SYN: ostia venarum pulmonalium [TA].

- saphenous o. [TA] the o. in the fascia lata inferior to the medial part of the inguinal ligament through which the saphenous vein passes to enter the femoral vein. SYN: hiatus saphenus [TA], fossa ovalis (2), saphenous hiatus.

- openings of smallest cardiac veins [TA] a number of fossae in the wall of the right atrium, containing the openings of minute intramural veins. SYN: foramina of the smallest veins of heart, foramina of the venae minimae, foramina venarum minimarum cordis, Lannelongue foramina, thebesian foramina, Vieussens foramina.

- o. of the sphenoidal sinus [TA] one of the pair of openings in the body of the sphenoid bone through which the sphenoid sinuses communicate with the sphenoethmoidal recess of the nasal cavity. SYN: apertura sinus sphenoidalis [TA], sphenoidal sinus aperture.

- o. of superior vena cava [TA] the point of entry of the superior vena cava into the right atrium. SYN: ostium venae cavae superioris [TA], orifice of superior vena cava.

- tendinous o. SYN: adductor hiatus.

- tympanic o. of canaliculus for chorda tympani SYN: tympanic aperture of canaliculus for chorda tympani.

- tympanic o. of eustachian tube SYN: tympanic o. of pharyngotympanic (auditory) tube.

- tympanic o. of pharyngotympanic (auditory) tube [TA] an o. in the anterior part of the tympanic cavity below the canal for the tensor tympani (muscle). SYN: ostium tympanicum tubae auditivae [TA], tympanic o. of eustachian tube.

- ureteral o. SYN: ureteric orifice.

- urethral openings external urethral orifice, internal urethral orifice.

- uterine o. of uterine tubes SYN: uterine ostium of uterine tubes.

- o. of uterus SYN: external os of uterus.

- vaginal o. SYN: vaginal orifice.

- vertical o. SYN: vertical dimension.

- o. of vestibular canaliculus [TA] the external o. of the vestibular aqueduct on the posterior surface of the petrous part of the temporal bone near the groove for the sigmoid sinus. SYN: apertura canaliculi vestibuli, external aperture of vestibular aqueduct.

open·ing 'ōp-(ə-)niŋ n

1) an act or instance of making or becoming open

2) something (as an anatomical aperture) that is open or opens