These (Ossa turbinata) are delicate, scroll-like bones, four in number,
which are attached to the lateral walls of the nasal cavity. They
project into the cavity and greatly diminish its extent. Each is
composed of a very thin lamina, cribriform in many places, and covered
on both sides with mucous membrane in the fresh state. They are arranged
in two pairs, superior and inferior.
The =superior= or =naso-turbinal= (Concha dorsalis)[17] is somewhat
cylindrical in form, small at its anterior part, and flattened
transversely. It is attached to the turbinal crest of the nasal bone and
the nasal plate of the frontal bone. The anterior part is rolled like a
scroll one and a half times, thus inclosing a cavity which communicates
with the middle meatus nasi. The arrangement is best seen on a
cross-section (Fig. 37). The posterior part is not rolled, but its lower
border is attached to the lateral nasal wall, thus helping to inclose a
large space which is part of the frontal sinus. This cavity is separated
from that of the scroll-like part by a transverse septum. The =internal
surface= is flattened, and is separated from the septum nasi by a narrow
interval, the =common meatus= (Meatus nasi communis). Another narrow
passage, the =superior meatus= (Meatus nasi superior), separates the
upper surface from the roof of the nasal cavity. The space between the
lower surface and the inferior turbinal is the =middle meatus= (Meatus
nasi medius). The anterior extremity is prolonged toward the nostril by
two small bars of cartilage.
The =inferior= or =maxillo-turbinal bone= (Concha ventralis) is shorter
and smaller posteriorly than the upper one. It is attached to the
inferior turbinal crest, and consists, like the upper one, of an
anterior coiled and a posterior uncoiled portion.
To express briefly the mode of coiling of the two bones of the same
side we may say that they are rolled toward the septum and each other.
The inferior and posterior borders of the posterior part are attached to
the maxilla, thus helping to inclose a cavity which is part of the
maxillary sinus. The lower surface is separated from the floor of the
nasal cavity by the =inferior meatus= (Meatus nasi inferior), which is
much larger than the other nasal passages. The anterior extremity is
prolonged to the nostril by a curved bar of cartilage.
=Development.=—Each ossifies in cartilage from a single center.