The =ethmoid= (Os ethmoidale) is situated in front of the body and
orbital wings of the sphenoid. It projects forward between the orbital
plates of the frontal bones and enters into the formation of the
cranial, nasal, and paranasal cavities.[12] It consists of four
parts—the cribriform plate, two lateral masses, and the perpendicular
plate.
The =cribriform plate= (Lamina cribrosa) is a sieve-like partition
between the cranial and nasal cavities. Its margin joins the orbital
wings of the sphenoid laterally, and the cranial plate of the frontal
bones dorsally. Its =cranial surface= is divided into two parts by a
median ridge, the =crista galli=, which is the intracranial portion of
the perpendicular plate. Each half forms a deep oval cavity, the
=ethmoidal= or =olfactory fossa=, which lodges the olfactory bulb. The
plate is perforated by numerous small foramina for the passage of the
olfactory nerve filaments, and on either side is the much larger
=ethmoidal foramen.= The =nasal surface= is convex, and has the lateral
masses attached to it.
The =lateral masses= or =labyrinth= project forward from the cribriform
plate into the posterior part of the nasal cavity, which they nearly
fill. Each mass is somewhat conical in shape, with the base attached to
half of the cribriform plate. The =inner surface= is separated by a
narrow space from the perpendicular plate. The =outer surface= is convex
and faces chiefly into the frontal and maxillary sinuses, but is
attached behind to the inner wall of the orbital cavity; it is covered
by a very thin layer of bone, the =lamina papyracea.= The mass consists
of a large number of delicate, scroll-like plates of bone, termed
=ethmoturbinals= or =ethmoidal cells.= These are attached to the lamina
papyracea, and are separated by narrow intervals termed =ethmoidal
meatuses=, which communicate with the nasal cavity. In the living animal
the ethmoturbinals are covered with mucous membrane.
[Illustration:
FIG. 29.—CROSS-SECTION OF LATERAL MASS OF ETHMOID BONE OF HORSE.
]
The lateral mass is a very complex structure, the arrangement of which
may be studied on cross-sections of decalcified specimens with the
mucous membrane retained. Each mass consists of six turbinals which
extend almost to the perpendicular plate and are termed
=endoturbinals=. These diminish in size from above downward; the
largest is attached to the nasal bone, and is hence usually called the
naso-turbinal or superior turbinal; the second is much smaller, and is
very commonly termed the great ethmoid cell. Between the endoturbinals
are twenty-one small =ecto=turbinals, and all are beset with secondary
and tertiary coiled lamellæ.
The =perpendicular plate= or =mesethmoid= (Lamina perpendicularis) is
median, and forms the posterior part of the septum nasi. Its =lateral
surfaces= are nearly plane, but are marked below by some grooves and
ridges; they are covered by the nasal mucous membrane. The =anterior
border= is irregular and is continuous with the septal cartilage. The
=posterior border= projects into the cranial cavity as a ridge, the
=crista galli.= The =dorsal border= joins the frontal bones at their
line of junction—the frontal suture. The =ventral border= is received
into the groove of the vomer.
=Development.=—The ethmoid develops in cartilage from five centers, two
for each lateral mass, and one for the perpendicular plate; from the
latter ossification extends into the cribriform plate. At birth the
perpendicular and cribriform plates are entirely cartilaginous. By the
time ossification is complete the ethmoid has united with surrounding
bones to such an extent that it cannot be separated intact for study.
[Illustration: