2-27. INDIVIDUAL CRANIAL BONES
a. Frontal. The frontal bone consists of two portions: a convex portion, the
squama which constitutes the region of the forehead and an orbital, or horizontal
portion, which enters into the formation of the roofs of the orbits and the nasal cavity
(1) The external surface of the squama is smooth and convex anteriorly.
Above the orbits are two arched elevations, the superciliary and arches (figure 2-29)
which are joined by the glabella (figures 2-33 and 2-34). The frontal sinuses (figure
2-35) are situated internally behind the glabella and the superciliary arches. The
supraorbital margin (figure 2-27) that forms the upper boundary of each orbit is
perforated by a supraorbital notch, or foramen. The supraorbital margins terminate in
the zygomatic process that joins the frontal bone and the zygomatic bone.
(2) The orbital portion consists of two orbital plates that are separated by a
median gap, the ethmoidal notch. The orbital plates contribute to the formation of the
roofs of the orbits and the nasal cavity. The ethmoidal notch is filled by the cribriform
plate (figure 2-28) of the ethmoid bone.
b. Parietal. The parietal bones (figures 2-27 and 2-29) are two flat bones that
unite to form the sides and the roof of the cranium. The four borders are: the sagittal
(medial), squamous (lateral), frontal (anterior), and occipital (posterior). These borders
form the frontal angle, the sphenoidal angle, the mastoid angle, and the occipital angle.
c. Temporal. Each of the two temporal bones (figures 2-27 and 2-28) consists
of four divisions: the squama, mastoid, tympanic, and petrous portions.
(1) The squama forms the anterior and superior portion of the temporal
bone. The zygomatic process projects from the lower part of the squama, which
presents inferiorly a large oval depression, the mandibular fossa (glenoid fossa).
(2) The mastoid portion constitutes the posterior part of the temporal bone.
The mastoid process (figure 2-29) is a cone-shaped projection that provides attachment
for several muscles. A coronal section of the mastoid portion reveals a large number of
mastoid air cells and a tympanic antrum.
(3) The tympanic portion (figure 2-29) is a curved plate lying below the
squama and anterior to the mastoid process. The external acoustic (auditory) meatus
(EAM) is situated in the tympanic portion. The styloid process (figure2-30) is a slender,
pointed projection attached to the inferior surface of the tympanic portion.
(4) The petrous portion (pars petrosa) (figure 2-28) resembles a pyramid
hewn from rock. The base of the pyramid is fused with the internal borders of the
mastoid and squamous portions. The apex, which presents the anterior opening of the
carotid (arterial) canal, is directed medially and anteriorly. In most skulls, the petrous
portions form approximately a 45 angle with the side of the skull. The anterior and
posterior surfaces of the pyramid meet superiorly to form a dense ridge that is referred