f. Ethmoid. The ethmoid bone (figures 2-27, 2-28, 2-29, and 2-30) is between
the two orbits. The bone is light and spongy in appearance and cubical in shape. It
consists of four parts: a horizontal or cribriform plate, a perpendicular plate, and two
(1) The cribriform plate (figure 2-31) is a perforated plate that is received
into the ethmoidal notch of the frontal bone and the roof of the nasal cavity. The
olfactory nerves pass from the brain through the perforations and into the nasal cavity.
The crista galli projects superiorly from the midline of the cribriform, or horizontal plate.
The fold of the dura mater dividing the hemispheres of the brain is attached to the crista
galli (figure 2-31).
(2) The perpendicular plate (figure 2-28) is a thin plate that projects inferiorly
from the cribriform plate. The perpendicular plate is joined inferiorly with the vomer to
help form the nasal septum.
(3) The lateral masses, or labyrinths, consist of a large number of
thin-walled cellular cavities. The spaces within these cells constitute the ethmoid
sinuses (figure 2-35). The superior and middle nasal conchae are spiral convoluted
plates projecting downward from the inner walls of the masses.
g. Special Joints (Sutures) of the Cranial Bones. The cranial bones are held
rigidly together by means of special interlocking, immovable joints known as sutures
(figures 1-14 and 2-36). The most important of these sutures are the sagittal suture
(between the medial adjacent border of the two parietal bones), the coronal suture
(between the posterior border of the frontal bone and the anterior borders of the two
parietal bones), the lambdoidal suture (between the posterior-inferior border of the two
parietal bones and the posterior-superior border of the occipital bone), and the
squamous suture (between the lateral inferior border of the parietal and the upper
squamous part of the temporal bone). The point of junction of the coronal and sagittal
sutures is known as the bregma and indicates the position of the anterior fontanelle in
the fetal skull, which represents an unossified membranous interval between these
bones before ossification is complete (normally at about 18 months of age). The point
of junction of the sagittal and lambdoidal sutures is the lambda (figure 1-14), which
indicates the position of the posterior fontanelle.
2-28. INDIVIDUAL FACIAL BONES
a. Nasal Bones. The nasal bones (figures 2-27, 2-28, and 2-29) are two small,
oblong, flat bones that constitute the upper portion, or bridge, of the nose. The point at
which they articulate with the frontal bone is called the nasion.
b. Lacrimal Bones. The lacrimal bones (figures 2-27 and 2-28) are two very
thin, fragile bones situated at the front part of the medial walls of the orbits.