The =sphenoid bone= (Os sphenoidale) is situated in the base of the
cranium, its central part lying in front of the basilar part of the
occipital. It consists of a central part, the body, two pairs of wings,
and two pterygoid processes.
The =body= (Corpus) is a cylindrical bar, flattened dorso-ventrally, and
wider in front than behind. Its =ventral surface= (Facies externa) is
convex in the transverse direction; and its anterior part is concealed
to a large extent by the vomer and pterygoid bones. The =cerebral
surface= (Facies cerebralis) presents the following features: (1) In
front is a raised, flattened part which is subdivided by a median
elevation into two slightly concave lateral areas; this part has a
posterior, thin, free margin (Limbus sphenoidalis), which overlies the
entrance to the optic foramina. (2) Just behind this and at a lower
level is a smooth transverse depression, the =optic groove= (Sulcus
chiasmatis), on which the optic chiasma rests. (3) From each end of this
groove the =optic foramen= passes forward and outward to terminate in
the posterior part of the orbital fossa.[9] (4) Near the posterior end
is a central depression, the =hypophyseal= or =pituitary fossa= (Fossa
hypophysea), which lodges the hypophysis cerebri or pituitary body. On
each side of this is an ill-defined groove for the internal carotid
artery and the cavernous sinus. The =anterior end= is expanded, and is
excavated to form the =sphenoidal sinuses.= These cavities extend back
as far as the optic groove, and are usually continuous in front with the
cavities in the vertical parts of the palate bones; they are separated
by a complete septum which is not always median.[10] The =posterior end=
is flat and is joined to the basilar part of the occipital; at the line
of junction there is dorsally a transverse elevation, the
=spheno-occipital= crest (Crista sphenooccipitalis).
The =orbital wings= (Alæ orbitales) curve upward and somewhat outward
from the sides of the body of the presphenoid. Their =inner= or
=cerebral surfaces= are concave, and are marked by =digital impressions=
(Impressiones digitatæ) for the gyri of the cerebrum. The =external
surface= is convex and is largely concealed by the overlapping temporal
wing and the squamous temporal and frontal bones; a narrow part of it
(Facies orbitalis) is uncovered on the inner wail of the orbital cavity
at the sphenoidal notch of the frontal bone. The =dorsal border= unites
with the frontal bone at the spheno-frontal suture. The =anterior
border= joins the ethmoid at the spheno-ethmoidal suture; at its lower
part it concurs with the frontal in the formation of the =ethmoidal= (or
internal orbital) =foramen=. The =posterior border= is overlapped by the
temporal wing and the squamous temporal. The =root= of the wing is
perforated by the =optic foramen=. Immediately below and behind the
latter (_i. e._, beneath the root) is the =foramen lacerum orbitale= or
=orbital fissure=. Below this, and separated from it usually by a thin
plate, is a larger opening, the =foramen rotundum=, which is bounded
externally by the root of the pterygoid process. Behind these foramina
is the =pterygoid crest= (Crista pterygoidea), which is continued
downward and forward on the pterygoid process; on its upper part may be
found the small and inconstant =trochlear= (or pathetic) =foramen=. Just
behind the crest is the =temporal foramen= (For. alare parvum), through
which the anterior deep temporal artery emerges from the alar canal of
the pterygoid process.
The =temporal wings= (Alæ temporales) extend outward and somewhat upward
from the body of the postsphenoid; they are irregularly quadrilateral in
outline. The =external surface= (Facies temporalis) enters into the
formation of the infratemporal fossa, and bears the pterygoid process on
its anterior part; at the junction with the body there is a small groove
for the pterygoid nerve. The =internal surface= (Facies cerebralis)
presents, at the junction with the body, two longitudinal =grooves=
(Sulci nervorum). The outer groove is the larger, and leads forward to
the foramen rotundum; it contains the maxillary nerve. The inner groove
conducts to the orbital fissure, and contains the third, sixth, and
ophthalmic nerves. The outer groove is bounded externally by a thin
overhanging crest, on which is a small groove for the fourth nerve. The
remainder of the surface is concave and supports the pyriform lobe of
the brain. The =dorsal border= joins the squamous temporal at the
spheno-squamous suture. The =anterior border= joins the orbital wing.
The =posterior border= forms the anterior boundary of the foramen
lacerum; it presents =three notches=, which are (from within outward)
the carotid, oval, and spinous (Incisura carotica, ovalis, spinosa). The
angle of junction of the dorsal and posterior borders articulates with
the parietal bone.
The =pterygoid processes= (Processus pterygoidei) arise from the
temporal wings and the body. They project downward and forward, and
curve outward at the lower part. The root is perforated by the =alar
canal= (Canalis alaris),[11] which transmits the internal maxillary
artery. From this canal a branch leads upward and forward to open at the
temporal foramen. The =external surface= is concave, and is marked by
lines for muscular attachment. The =internal surface= is largely
concealed by the overlapping palate and pterygoid bones, with which it
concurs in the formation of the =pterygoid= or =Vidian canal=.
=Development.=—The sphenoid is ossified in cartilage, and consists in
early life of two distinct parts, the presphenoid and postsphenoid. The
former develops from two centers, one in each wing; the latter has three
centers, one for the body and one for each wing. The pterygoid processes
ossify from the centers of the temporal wings.
=Variation.=—The dorsal border of the orbital wing may come to the
surface through a defect in the frontal bone at the place where the
horn process is situated in animals which have frontal horns.