ostium

1. A small opening or orifice, as in a body organ or passage.
2. Any of the small openings or pores in a sponge.

[Latin ōstium, door, opening, from ōs, mouth.]

A small opening or orifice, as in a body organ.

Pl. ostia [L.] a mouth or orifice; a general term to designate an opening into a tubular organ, or between two distinct body cavities. The names of most ostia are self-defining.

• o. abdominale — the fimbriated end of the uterine tube.
• o. cardiacum — the opening of the esophagus into the stomach.
• coronary o. — either of the two openings in the aortic sinuses which mark the origin of the (left and right) coronary arteries.
• o. ejaculatorium — the common orifice of the ductus deferens and the excretory duct of the seminal vesicle into the urethra.
• o. internum — ostium uterinum tubae.
• o. pharyngeum — the nasopharyngeal end of the auditory tube.
• o. primum — an opening in the lower portion of the membrane dividing the embryonic atria into right and left sides. Called also foramen primum. See also atrial septal defect.
• o. pulmonary vein — the opening of the pulmonary vein into the left atrium.
• o. ruminoreticulare — the opening between the rumen and the reticulum.
• o. secundum — an opening in the upper portion of the membrane dividing the embryonic atria into right and left sides, appearing later than the ostium primum. Called also foramen secundum. See also atrial septal defect.
• tympanic o., o. tympanicum — the opening of the auditory tube on the carotid wall of the tympanic cavity.
• o. uteri — the external opening of the cervix of the uterus into the vagina.
• o. uterinum tubae — the point where the cavity of the uterine tube becomes continuous with that of the uterus.
• o. vaginae — the external orifice of the vagina.