The Choanae are the posterior nasal aperture, separated by the vomer.
It is the opening between the nasal cavity and the nasopharynx.
It is therefore not a structure but a space bounded as follows:
anteriorly and inferiorly by the horizontal plate of palatine bone,
superiorly and posteriorly by the sphenoid bone
laterally by the medial pterygoid plates.
This definition incorporates text from the wikipedia website - Wikipedia: The free encyclopedia. (2004, July 22). FL: Wikimedia Foundation, Inc. Retrieved August 10, 2004, from http://www.wikipedia.org