Thesphenoid bone[note 1] is an unpairedbone of theneurocranium. It is situated in the middle of theskull towards the front, in front of thebasilar part of the occipital bone. The sphenoid bone is one of the seven bones that articulate to form theorbit. Its shape somewhat resembles that of abutterfly,bat orwasp with its wings extended. The name presumably originates from this shape, sincesphekodes (σφηκώδης) means'wasp-like' inAncient Greek.
Seven bones articulate to form theorbit. The sphenoid bone is in red (directly in the middle of theorbital cavity)Wedge-shaped sphenoid bone seen laterally
These ligaments occasionallyossify, though the incidence of ligamentous ossification (both partial and complete) varies according to the ligament type, with the interclinoid ligament being most commonly identified as having ossified and thepterygoalar ligament least commonly identified.[6]
Figure 4 : Sphenoid bone at birth, posterior aspect.
Until the seventh or eighth month offetal development, the body of the sphenoid consists of two parts: one in front of thetuberculum sellae, the presphenoid, with which the small wings are continuous; the other, consisting of thesella turcica anddorsum 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.
By about the ninth week offetal development an ossific center appears for each of thesmall wings (orbito-sphenoids) just lateral to theoptic foramen; this is followed by the appearance of two nuclei in thepresphenoid 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 theethmoid bone, and between the ninth and twelfth years they unite with the sphenoid bone.
The first ossific nuclei are those for the great wings (alisphenoids). One makes its appearance in each wing between theforamen rotundum andforamen ovale 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 thepostsphenoid part of the body appear, one on either side of the sella turcica, and become blended together about the middle of fetal life.
Eachmedial 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.
Thepresphenoid is united to the postsphenoid about the eighth month, and at birth the sphenoid is in three pieces [Fig. 4]: 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 buccalectoderm 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.
The sphenoid bone of humans ishomologous with a number of bones that are often separate in other animals, and have a somewhat complex arrangement.
In the earlylobe-finned fishes andtetrapods, 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 asphenethmoid bone helping to protect theolfactory nerves. Finally, the basisphenoid bone formed part of the floor of the braincase and lay immediately above the parasphenoid.[8]
Aside from the loss of the flexible joint at the rear of the palate, this primitive pattern is broadly retained inreptiles, albeit with some individual modifications. Inbirds, the epipterygoids are absent and the pterygoids considerably reduced. Livingamphibians 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.[8]
Inmammals, 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 asalisphenoids 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 asorbitosphenoids andpresphenoid, respectively, although there is often some degree of fusion. Only the parasphenoid appears to be entirely absent in mammals.[8]
In thedog the sphenoid is represented by eight bones: basisphenoid, alisphenoids, presphenoid, orbitosphenoids, pterygoids. These bones remain separate and are the:
^According to most dictionaries, the wordsphenoid (/ˈsfiːnɔɪd/[1][2]) derives from Greeksphenoeides, "wedgelike". Thieme Atlas of Anatomy[3] disagrees and says that the sphenoid bone was originally calledos sphecoidale, meaning "bone resembling a wasp", and that the word was later written 'sphenoidale' by a transcription error. As an anterior view of the bone resembles more the body of a wasp or a bat[4] with wings than a wedge.
^Touska, P., Hasso, S., Oztek, A. et al. Skull base ligamentous mineralisation: evaluation using computed tomography and a review of the clinical relevance. Insights Imaging 10, 55 (2019).https://doi.org/10.1186/s13244-019-0740-8
^Fehrenbach; Herring (2012).Illustrated Anatomy of the Head and Neck. Elsevier. p. 52.ISBN978-1-4377-2419-6.
^abcRomer, Alfred Sherwood; Parsons, Thomas S. (1977).The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 220–244.ISBN0-03-910284-X.
