Sphenoid bone
Encyclopedia
The sphenoid bone is an unpaired bone
situated at the base of the skull
in front of the temporal bone
and 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
or bat with its wings extended.
Two sphenoidal conchae
are situated at the anterior and posterior part of the body.
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 tetrapod
s, 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
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
s, albeit with some individual modifications. In bird
s, the epipterygoids are absent and the pterygoids considerably reduced. Living amphibian
s 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
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, 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.
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
situated at the base of the skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...
in front of the temporal bone
Temporal bone
The temporal bones are situated at the sides and base of the skull, and lateral to the temporal lobes of the cerebrum.The temporal bone supports that part of the face known as the temple.-Parts:The temporal bone consists of four parts:* Squama temporalis...
and basilar part of the occipital bone
Occipital bone
The occipital bone, a saucer-shaped membrane bone situated at the back and lower part of the cranium, is trapezoidal in shape and curved on itself...
.
The sphenoid bone is one of the seven bones that articulate to form the orbit
Orbit (anatomy)
In anatomy, the orbit is the cavity or socket of the skull in which the eye and its appendages are situated. "Orbit" can refer to the bony socket, or it can also be used to imply the contents...
. Its shape somewhat resembles that of a butterfly
Butterfly
A butterfly is a mainly day-flying insect of the order Lepidoptera, which includes the butterflies and moths. Like other holometabolous insects, the butterfly's life cycle consists of four parts: egg, larva, pupa and adult. Most species are diurnal. Butterflies have large, often brightly coloured...
or bat with its wings extended.
Portions
It is divided into the following parts:- a median portion, known as the body of sphenoid boneBody of sphenoid boneThe body of the sphenoid bone, more or less cubical in shape, is hollowed out in its interior to form two large cavities, the sphenoidal air sinuses, which are separated from each other by a septum.- Superior surface :...
, containing the sella turcicaSella turcica-External links:*...
which houses the pituitary glandPituitary glandIn vertebrate anatomy the pituitary gland, or hypophysis, is an endocrine gland about the size of a pea and weighing 0.5 g , in humans. It is a protrusion off the bottom of the hypothalamus at the base of the brain, and rests in a small, bony cavity covered by a dural fold... - two greater wings and two lesser wings
- Pterygoid processes of the sphenoidPterygoid processes of the sphenoidThe pterygoid processes of the sphenoid, one on either side, descend perpendicularly from the regions where the body and great wings unite.Each process consists of a medial pterygoid plate and a lateral pterygoid plate, the upper parts of which are fused anteriorly; a vertical sulcus, the...
es which project from it posteriorly (below)
Two sphenoidal conchae
Sphenoidal conchae
The sphenoidal conchae are two thin, curved plates, situated at the anterior and lower part of the body of the sphenoid...
are situated at the anterior and posterior part of the body.
Named features
- pterygoid notch
- pterygoid fossaPterygoid fossaThe pterygoid fossa is an anatomical term for the fossa formed by the divergence of the lateral pterygoid plate and the medial pterygoid plate of the sphenoid bone.-Structure:...
- scaphoid fossaScaphoid fossaIn the pterygoid processes of the sphenoid, above the pterygoid fossa is a small, oval, shallow depression, the scaphoid fossa, which gives origin to the Tensor veli palatini.-External links:*...
- pterygoid hamulusPterygoid hamulusThe medial pterygoid plate of the sphenoid curves laterally at its lower extremity into a hook-like process, the pterygoid hamulus, around which the tendon of the Tensor veli palatini glides.-External links: *...
- pterygoid canalPterygoid canalThe pterygoid canal is a passage in the skull leading from just anterior to the foramen lacerum in the middle cranial fossa to the pterygopalatine fossa.-Structure:...
- pterygospinous process
In other animals
The sphenoid bone of humans is homologousHomology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
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 tetrapod
Tetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...
s, 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
Olfactory nerve
The olfactory nerve, or cranial nerve I, is the first of twelve cranial nerves. It is instrumental in the sense of smell. Derived from the embryonic nasal placode, the olfactory nerve is capable of regeneration.-Anatomy:...
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
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s, albeit with some individual modifications. In bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s, the epipterygoids are absent and the pterygoids considerably reduced. Living amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...
s 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
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
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, respectively, although there is often some degree of fusion. Only the parasphenoid appears to be entirely absent in mammals.
In the dog
Dog
The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...
the sphenoid is represented by 8 bones: basisphenoid, alisphenoids, presphenoid, orbitosphenoids, pterygoids.
See also
- Bone terminology
- Ossification of the sphenoidOssification of the sphenoidUntil the seventh or eighth month of fetal life the body of the sphenoid consists of two parts: one in front of the tuberculum sellæ, the presphenoid, with which the small wings are continuous; the other, comprising the sella turcica and dorsum sellæ, the postsphenoid, with which are associated the...
- Pterygospinal ligamentPterygospinal ligamentA fibrous band that spans the sphenoid bone from its spine to the superior border of the lateral pterygoid plate....
- Terms for anatomical location