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The sphenoid bone is an unpaired
bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, an ...
of the neurocranium. It is situated in the middle of the skull towards the front, in front of the basilar part of the occipital bone. The sphenoid bone is one of the seven bones that articulate to form the orbit. Its shape somewhat resembles that of a
butterfly Butterflies are insects in the macrolepidopteran clade Rhopalocera from the order Lepidoptera, which also includes moths. Adult butterflies have large, often brightly coloured wings, and conspicuous, fluttering flight. The group comprises ...
or bat with its wings extended.


Structure

It is divided into the following parts: * a median portion, known as the body of sphenoid bone, containing the sella turcica, which houses the pituitary gland as well as the paired paranasal sinuses, the
sphenoidal sinuses The sphenoid sinus is a paired paranasal sinus occurring within the within the body of the sphenoid bone. It represents one pair of the four paired paranasal sinuses.Illustrated Anatomy of the Head and Neck, Fehrenbach and Herring, Elsevier, 2012, ...
* two greater wings on the lateral side of the body and two lesser wings from the anterior side. * Pterygoid processes of the sphenoides, directed downwards from the junction of the body and the greater wings. Two sphenoidal conchae are situated at the anterior and inferior part of the body.


Intrinsic ligaments of the sphenoid

The more important of these are: * the pterygospinous, stretching between the spina angularis and the lateral pterygoid plate (see cervical fascia); * the interclinoid, a fibrous process joining the anterior to the posterior clinoid process; * and the caroticoclinoid, connecting the anterior to the middle clinoid process. These ligaments occasionally ossify.


Features

* pterygoid notch * pterygoid fossa * scaphoid fossa * pterygoid hamulus * pterygoid canal * pterygospinous process * sella turcica


Articulations

The sphenoid articulates with the frontal, parietal, ethmoid, temporal, zygomatic, palatine, vomer, and occipital bones and helps to connect the neurocranium to the facial skeleton.


Body of sphenoid


Superior or cerebral surface

Articulates with ethmoid bone anteriorly and basilar part of
occipital bone The occipital bone () is a neurocranium, cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone overlies the occipital lobe ...
posteriorly. It shows: # Jugum sphenoidale #
Sulcus chiasmaticus The superior surface of the body of the sphenoid bone is bounded behind by a ridge, which forms the anterior border of a narrow, transverse groove, the chiasmatic groove (optic groove, prechiasmatic sulcus), above and behind which lies the optic ...
# Tuberculum sellae # Sella turcica # Dorsum sellae # Clivus


Inferior surface

# Rostrum of sphenoid # Sphenoidal conchae # Vaginal processes of medial pterygoid plate


Anterior surface

Sphenoidal crest articulates with the perpendicular plate of ethmoid leading to formation of a part of the septum of nose.


Posterior surface

Basilar part of occipital bone


Lateral surface

Carotid sulcus lodging cavernous sinus and
internal carotid artery The internal carotid artery (Latin: arteria carotis interna) is an artery in the neck which supplies the anterior circulation of the brain. In human anatomy, the internal and external carotids arise from the common carotid arteries, where these b ...


Sphenoidal sinuses

Sphenoidal sinuses are asymmetrical air sinuses in the body of the sphenoid, closed by sphenoidal conchae.


Greater wings


Superior or cerebral surface

This forms the floor of the middle cranial fossa. It presents (starting from the front): * foramen rotundum *
foramen ovale There are multiple structures in the human body with the name foramen ovale (plural: ''foramina ovalia''; Latin for "oval hole"): * Foramen ovale (heart), in the fetal heart, a shunt from the right atrium to left atrium * Foramen ovale (skull), at ...
* Sphenoidal emissary foramen * foramen spinosum


Lateral surface

This is divided into (by
infratemporal crest The lateral surface of the greater wing of the sphenoid is convex, and divided by a transverse ridge, the infratemporal crest, into two portions. The superior or temporal portion, convex from above downward, concave from before backward, forms a ...
): * Upper or temporal surface * Lower or
infratemporal surface The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The tw ...
Foramen In anatomy and osteology, a foramen (;Entry "foramen"
in
pierce it: *
Foramen ovale There are multiple structures in the human body with the name foramen ovale (plural: ''foramina ovalia''; Latin for "oval hole"): * Foramen ovale (heart), in the fetal heart, a shunt from the right atrium to left atrium * Foramen ovale (skull), at ...
* Foramen spinosum


Orbital surface

This forms the posterior wall of the orbit


Lesser wings

These are two triangular wings projecting laterally from anterosuperior part of the body. Each consists of: * A base forming medial end of the wing. * Tip forming the lateral end of the wing. * Superior surface forming floor of anterior cranial fossa. * Inferior surface forming upper boundary of
superior orbital fissure The superior orbital fissure is a foramen or cleft of the skull between the lesser and greater wings of the sphenoid bone. It gives passage to multiple structures, including the oculomotor nerve, trochlear nerve, ophthalmic nerve, abducens ner ...
. * Posterior surface projects into the Sylvian point. * Medially, terminates in the anterior clinoid process.


Development

Until the seventh or eighth month of fetal development, the body of the sphenoid consists of two parts: one in front of the
tuberculum sellæ The tuberculum sellae (or the tubercle of the sella turcica) is a part of the sphenoid bone The sphenoid bone is an unpaired bone of the neurocranium. It is situated in the middle of the skull towards the front, in front of the basilar part of ...
, the presphenoid, with which the small wings are continuous; the other, consisting of the sella turcica and dorsum sellae, the postsphenoid, with which are associated the great wings, and pterygoid processes. The greater part of the bone is ossified in cartilage. There are fourteen centers in all, six for the presphenoid and eight for the postsphenoid.


Presphenoid

By about the ninth week of fetal development an ossific center appears for each of the
small wings The lesser wings of the sphenoid or orbito-sphenoids are two thin triangular plates, which arise from the upper and anterior parts of the body, and, projecting lateralward, end in sharp points ig. 1 In some animals they remain as separate bones c ...
(orbito-sphenoids) just lateral to the optic foramen; this is followed by the appearance of two nuclei in the
presphenoid The sphenoid bone is an unpaired bone of the neurocranium. It is situated in the middle of the skull towards the front, in front of the basilar part of the occipital bone. The sphenoid bone is one of the seven bones that articulate to form the or ...
part of the body. The sphenoidal conchae are each developed from a center that makes its appearance about the fifth month; at birth they consist of small triangular laminae, and it is not until the third year that they become hollowed out and coneshaped; about the fourth year they fuse with the labyrinths of the ethmoid bone, and between the ninth and twelfth years they unite with the sphenoid bone.


Postsphenoid

The first ossific nuclei are those for the great wings ( alisphenoids). One makes its appearance in each wing between the foramen rotundum and
foramen ovale There are multiple structures in the human body with the name foramen ovale (plural: ''foramina ovalia''; Latin for "oval hole"): * Foramen ovale (heart), in the fetal heart, a shunt from the right atrium to left atrium * Foramen ovale (skull), at ...
about the eighth week. The orbital plate and that part of the sphenoid, which is found in the temporal fossa, as well as the lateral pterygoid plate, are ossified in membrane (Fawcett). Soon after, the centers for the postsphenoid part of the body appear, one on either side of the sella turcica, and become blended together about the middle of fetal life. Each medial pterygoid plate (except its hamulus) is ossified in membrane, and its center probably appears about the ninth or tenth week; the hamulus becomes chondrified during the third month, and almost at once ossifies (Fawcett). The medial joins the lateral pterygoid plate about the sixth month. About the fourth month, a center appears for each lingula and speedily joins the rest of the bone. The
presphenoid The sphenoid bone is an unpaired bone of the neurocranium. It is situated in the middle of the skull towards the front, in front of the basilar part of the occipital bone. The sphenoid bone is one of the seven bones that articulate to form the or ...
is united to the postsphenoid about the eighth month, and at birth the sphenoid is in three pieces
ig. 4 IG, Ig, or ig may refer to: Companies * IG Farben, a former German industrial conglomerate * IG Group, a UK financial services company * IG Recordings, a record label formed by the Indigo Girls, an American folk/rock duo * Production I.G, a Japane ...
a central, consisting of the body and small wings, and two lateral, each comprising a great wing and pterygoid process. In the first year after birth the great wings and body unite, and the small wings extend inward above the anterior part of the body, and, meeting with each other in the middle line, form an elevated smooth surface, termed the jugum sphenoidale. By the twenty-fifth year the sphenoid and occipital are completely fused. Between the pre- and postsphenoid there are occasionally seen the remains of a canal, the canalis cranio-pharyngeus, through which, in early fetal life, the hypophyseal diverticulum of the buccal ectoderm is transmitted. The sphenoidal sinuses are present as minute cavities at the time of birth (Onodi), but do not attain their full size until after puberty.


Function

This bone assists with the formation of the base and the sides of the skull, and the floors and walls of the orbits. It is the site of attachment for most of the muscles of
mastication Chewing or mastication is the process by which food is crushed and ground by teeth. It is the first step of digestion, and it increases the surface area of foods to allow a more efficient break down by enzymes. During the mastication process, th ...
. Many foramina and fissures are located in the sphenoid that carry nerves and blood vessels of the head and neck, such as the
superior orbital fissure The superior orbital fissure is a foramen or cleft of the skull between the lesser and greater wings of the sphenoid bone. It gives passage to multiple structures, including the oculomotor nerve, trochlear nerve, ophthalmic nerve, abducens ner ...
(with ophthalmic nerve), foramen rotundum (with maxillary nerve) and
foramen ovale There are multiple structures in the human body with the name foramen ovale (plural: ''foramina ovalia''; Latin for "oval hole"): * Foramen ovale (heart), in the fetal heart, a shunt from the right atrium to left atrium * Foramen ovale (skull), at ...
(with
mandibular nerve In neuroanatomy, the mandibular nerve (V) is the largest of the three divisions of the trigeminal nerve, the fifth cranial nerve (CN V). Unlike the other divisions of the trigeminal nerve (ophthalmic nerve, maxillary nerve) which contain only aff ...
).


Other animals

The sphenoid bone of humans is
homologous Homology may refer to: Sciences Biology *Homology (biology), any characteristic of biological organisms that is derived from a common ancestor *Sequence homology, biological homology between DNA, RNA, or protein sequences * Homologous chrom ...
with a number of bones that are often separate in other animals, and have a somewhat complex arrangement. In the early lobe-finned fishes and tetrapods, the pterygoid bones were flat, wing-like bones forming the major part of the roof of the mouth. Above the pterygoids were the epipterygoid bones, which formed part of a flexible joint between the braincase and the palatal region, as well as extending a vertical bar of bone towards the roof of the skull. Between the pterygoids lay an elongated, narrow parasphenoid bone, which also spread over some of the lower surface of the braincase, and connected, at its forward end, with a sphenethmoid bone helping to protect the
olfactory nerve The olfactory nerve, also known as the first cranial nerve, cranial nerve I, or simply CN I, is a cranial nerve that contains sensory nerve fibers relating to the sense of smell. The afferent nerve fibers of the olfactory receptor neurons tr ...
s. Finally, the basisphenoid bone formed part of the floor of the braincase and lay immediately above the parasphenoid. Aside from the loss of the flexible joint at the rear of the palate, this primitive pattern is broadly retained in
reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephal ...
s, albeit with some individual modifications. In
bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
s, the epipterygoids are absent and the pterygoids considerably reduced. Living amphibians have a relatively simplified skull in this region; a broad parasphenoid forms the floor of the braincase, the pterygoids are relatively small, and all other related bones except the sphenethmoid are absent. In
mammal Mammals () are a group of vertebrate animals constituting the class (biology), class Mammalia (), characterized by the presence of mammary glands which in Female#Mammalian female, females produce milk for feeding (nursing) their young, a ...
s, these various bones are often (though not always) fused into a single structure; the sphenoid. The basisphenoid forms the posterior part of the base, while the pterygoid processes represent the pterygoid bones. The epipterygoids have extended into the wall of the cranium; they are referred to as alisphenoids when separate in mammals, and form the greater wings of the sphenoid when fused into a larger structure. The sphenethmoid bone forms as three bones: the lesser wings and the anterior part of the base. These two parts of the sphenethmoid may be distinguished as orbitosphenoids and
presphenoid The sphenoid bone is an unpaired bone of the neurocranium. It is situated in the middle of the skull towards the front, in front of the basilar part of the occipital bone. The sphenoid bone is one of the seven bones that articulate to form the or ...
, respectively, although there is often some degree of fusion. Only the parasphenoid appears to be entirely absent in mammals. In the dog the sphenoid is represented by 8 bones: basisphenoid, alisphenoids, presphenoid, orbitosphenoids, pterygoids. These bones remain separate and are the: *2 Alisphenoids: each greater wing *2 Orbitosphenoids: each lesser wing * Basisphenoid: back part of body *
Presphenoid The sphenoid bone is an unpaired bone of the neurocranium. It is situated in the middle of the skull towards the front, in front of the basilar part of the occipital bone. The sphenoid bone is one of the seven bones that articulate to form the or ...
: front part of body *2 Pterygoids: medial pterygoid plate


Additional images

File:Sphenoid bone - animation.gif, Position of sphenoid bone (shown in green). Animation. File:Sphenoid bone - inferior view animation.gif, Seen from below ( mandible is removed) File:Sphenoid bone - superior view animation2.gif, Seen from above ( parietal bones are removed) File:Sphenoid bone - close-up - animation.gif, Shape of sphenoid bone. File:Illu facial bones.jpg, Facial bones. File:Gray153.png, Lateral wall of nasal cavity, showing ethmoid bone in position. File:Gray187.png, Base of skull. Inferior surface. File:Gray188.png, Lateral view of the skull. File:Gray191.png, Horizontal section of nasal and orbital cavities. File:Gray193.png, Floor of the skull. File:Gray196.png, Roof, floor, and lateral wall of left nasal cavity. File:Gray190.png, The skull from the front. The sphenoid is labeled with yellow to the left of the picture, both in the orbit and behind the zygomatic process File:Cranium 4.jpg, Sphenoid bone File:Sphenoid bone - superior view.jpg, Sphenoid bone superior view File:Sphenoid and temporal bones.jpg, Sphenoid bone and temporal bones


See also

* Sphenoidal sinus *
Pterygospinal ligament The pterygospinous ligament stretches from the upper part of the posterior border of the lateral pterygoid plate to the spinous process of the sphenoid. Structure Variation It occasionally ossifies, and in such cases, between its upper border ...
* Basilar skull fracture


Notes


References


External links

* - "Lateral view of skull." {{Authority control Bones of the head and neck