The sphenoid bone (figs. 348 to 350) is situated at the base of the skull, in front of the temporal bones and the basilar part of the occipital bone. It somewhat resembles a bat with wings extended, and consists of a central portion or body, two greater and two lesser wings extending laterally from the sides of the body, and two pterygoid processes, which are directed downwards from the adjoining parts of the body and greater wings.

Figure 348
Sphenoid bone superior view - Figure 348
The body of the sphenoid bone, more or less cubical in shape, contains two large air -sinuses, which are separated from each, other by a septum.

The cerebral or superior surface the body (fig. 348) articulates in front with the cribriform plate of the ethmoid bone. Anteriorly the surface is smooth and is termed the jugum sphenoidale it supports the posterior ends of the gyri recti and the olfactory tracts. It is bounded behind by a ridge, which forms the anterior border of a transverse groove, termed the optic groove (sulcus chiasmatis) ; this sulcus leads laterally to the optic foramen on each side. Posterior to the groove there is a more or less oval elevation, termed the tuberculum sella, and behind this a deep depression, termed the sella turcica, the deepest part of which lodges the hypophysis cerebri and is known as the hypophyseal fossa. The anterior boundary of the sella turcica is completed laterally by two small eminences, called the middle clinoid processes whilst the posterior boundary is formed by a square plate of bone, termed the dorsum sellae ; the superior angles of this plate end in two tubercles, termed the posterior clinoid processes, which vary considerably in form and size and give attachment to the lateral borders of the tentorium cerebella. On each side of the body below the dorsum sellae a small projection articulates with the apex of the petrous portion of the temporal bone and is termed the petrosal process. The sloping area behind the dorsum sellae is termed the clivus, and is uninterruptedly continuous with the superior surface of the basilar portion of the occipital bone in the adult skull ; it supports the upper part of the pons.

The lateral surfaces of the body are united with the greater wings and with the medial pterygoid plates. Above the attachment of each wing a broad groove, termed the carotid sulcus, forms a curve somewhat like the italic letter f; it lodges the internal carotid artery and the cavernous sinus. The carotid sulcus is deepest at its posterior end, where it is overhung medially by the petrosal process. and is limited laterally by a sharp margin called the lingual ; the latter is continued backwards to overlie the posterior opening of the pterygoid canal.

Figure 349
Sphenoid bone posterior view - Figure 349
The posterior surface of the body, quadrilateral in form (fig. 349), is joined, during infancy and adolescence to the front of the basilar part of the occipital bone by a plate of cartilage, which ossifies between the eighteenth and twenty-fifth years.

The anterior surface of the body (fig. 350) presents, in the median plane, a triangular crest which forms a small part of the septum of the nose and is termed the sphenoidal crest. The anterior border of this crest articulates with the perpendicular plate of the ethmoid bone. On either side of the crest a rounded opening leads into the corresponding sphenoidal sinus. The sphenoidal sinuses are two large, irregular cavities in the body of the bone, separated from each other by a bony septum which is commonly bent to one or the other side. They vary considerably in form and size, * are seldom symmetrical, and are often partially subdivided by bony laminae. A lateral recess may extend from one or other sinus into the greater wing and lingula; the sinuses occasionally reach into the basilar part of the occipital bone nearly as far as the foramen magnum. In the articulated skull they are closed in front and below by the sphenoidal conchae, but a round opening is left in the anterior wall of each sinus, by which it communicates with the spheno-ethmoidal recess at the upper and posterior part of the nasal cavity, and occasionally with the posterior ethmoidal sinuses. Each half of the anterior surface of the body of the sphenoid bone consists of two parts : (a) an upper and lateral depressed area which completes, with the labyrinth of the ethmoid bone, the posterior ethmoidal sinuses ; its lateral margin articulates with the orbital plate (lamina papyracea) of the ethmoid bone above; and with the orbital process of the palatine bone below; (b) a lower and medial, smooth, triangular area, which forms the posterior part. of the roof of the nose ; near its superior angle is the orifice of the sphenoidal sinus.

The inferior surface of the body presents in the median plane a. triangular spine, termed the sphenoidal rostrum (fig. 349), which, in the articulated skull, projects into a deep fissure between the anterior parts of the ala, of the vomer. The posterior, triangular parts of the sphenoidal concha extend backwards on the sides of the rostrum and articulate with the ala; of the vomer. On each side of the posterior part, of the rostrum, and immediately behind the apex; of the sphenoidal concha, a thin lamina named the vaginal process; projects medially from the base of the medial pterygoid plate with which it will be described.

The greater wings of the sphenoid bone are two strong processes which curve upwards and laterally from the sides of the body. The posterior part of each is triangular and fits into the angle between the petrous portion and the squamous part of the temporal bone; projecting downwards from the apex of this triangular portion is a pointed process named the spine (spina angularis), on the medial side of which there is usually a small groove, directed downwards and forwards, for the chorda tympani nerve. The tip of the spine gives attachment to the sphenomandibular ligament, and a part of the tensor palati (tensor veli palatini) muscle. The medial aspect of the spine forms part of the, lateral wall of the groove which lodges the pharyngotympanic (auditory) tube on the external surface of the base of the skull.

The cerebral or superior surface of the greater wing (fig. 348) forms part of the floor of the middle fossa of the skull ; it is deeply concave and presents depressions corresponding with the convolutions of the anterior part of the temporal lobe of the brain. At its anteromedial part the foramen rotundum gives passage to the maxillary nerve. Behind and lateral to this foramen the foramen ovale transmits the mandibular nerve, the accessory meningeal artery, and sometimes the lesser superficial petrosal nerve. Medial to the foramen ovate, a small aperture, termed the emissary sphenoidal foramen (foramen Vesalii) is present on one or both sides in nearly 40 per cent of skulls ; it opens below at the lateral side of the scaphoid fossa and transmits a. small vein from the cavernous sinus. In the posterior angle, near to but in front. of the spine, there is a short canal, termed the foramen spinosum, which transmits the middle meningeal artery and the nervus spinosus.

The lateral surface of the greater wing (fig. 320) is convex from above downwards, and is divided by a transverse ridge, termed the infratemporal crest; into an upper or temporal and a lower or infratemporal surface. The temporal surface, concave from before backwards, forms a portion of the temporal fossa and gives origin to a part of the temporalis muscle. The infratemporal surface is concave and directed downwards ; it forms a part of the infratemporal fossa and, together with the infratemporal crest, gives origin to the upper head of the lateral pterygoid muscle. It is pierced by the foramen ovale and foramen spinosum, and its posterior part is marked by the spine of the sphenoid bone (spina angularis). Medial to the anterior extremity of the infratemporal crest a triangular process serves to increase the attachment of the lateral pterygoid. A ridge runs downwards and medially from this triangular process to the front of the lateral pterygoid plate ; it forms the anterior limit of the infratemporal surface and, in the articulated skull, the posterior boundary of the pterygomaxillary fissure.

The orbital surface of the greater wing (fig. 350), quadrilateral in shape, is directed forwards and medially and forms the posterior part of the lateral wall of the orbit. Its upper serrated edge articulates with the orbital plate of the frontal bone ; its lateral serrated margin with the zygomatic bone. Its inferior smooth border forms the posterolateral boundary of the inferior orbital fissure. Its medial sharp margin constitutes the lower boundary of the superior orbital fissure; projecting from near the centre of this margin there is a small tubercle. It gives an attachment to part of the annulus tendinous com munis, from which the rectus muscles of the eyeball take origin. Below the medial end of the superior orbital fissure there is a grooved area which forms the posterior wall of the pterygopalatine fossa and is pierced by the foramen rotundum.

The margin of the greater wing (fig. 348) The portion of the margin of the greater wing which extends frown. the body to the spine of the sphenoid is irregular. Its medial half forms the anterior boundary of the foramen, lacerum and presents the posterior aperture of the pterygoid canal for the passage of the corresponding nerve and artery. Its lateral half articulates, by means of a cartilaginous joint, with the petrous portion of the temporal bone. Between the two bones, on the under surface of the skull, there is a farrow, named the sulcus tubae, which lodges the cartilaginous part of the pharyngotympanic (auditory) tube. Extending forwards from the sphenoidal spine the squamosal margin forms a concave, serrated edge, bevelled at the expense of the inner surface below and of the outer surface above, for articulation with the squamous part of the temporal. bone. The tip of the greater wing is bevelled at the expense of the inner surface and articulates with the anterior inferior angle of the parietal bone at the pterion. Medial to this there is a triangular rough area for articulation with the frontal bone ; the medial angle of this area, is continuous with the sharp edge which forms the lower boundary of the superior orbital fissure, and the anterior angle with the serrated margin for articulation with the zygomatic bone.

The lesser wings of the sphenoid bone are two triangular plates which project laterally from the upper and anterior parts of the body and end in sharp points (figs. 348, 349). The cerebral surface of each is smooth, and supports a small part of the frontal lobe of the brain. The inferior surface forms the posterior part of the roof of the orbit and the upper boundary of the superior orbital fissure ; it overhangs the anterior part of the middle fossa of the skull. The superior orbital fissure is triangular in shape and leads from the cranial cavity into that of the orbit ; it is bounded medially by the body of the sphenoid bone; above, by the lesser wing ; below by the medial margin of the orbital surface of the greater wing ; and is completed laterally, between the greater and lesser wings, by the frontal bone. It transmits to the orbital cavity the oculornotor, trochlear and abducent nerves, the three branches of the ophthalmic division of the trigerninal nerve, the orbital branch of the middle meningeal artery, and some filaments from the cavernous plexus of the sympathetic; and from the orbital cavity the recurrent meningeal branch of the lacrimal artery, and the ophthalmic veins. The anterior border of the lesser wing is serrated for articulation with the posterior edge of the orbital plate of the frontal bone. The posterior border is smooth and projects into the lateral cerebral fissure; the medial end of this border forms the anterior clinoid .process, which gives attachment to the anterior end of the free border of the tentorium cerebelli. The anterior and middle clinoid processes are sometimes united by a spicule of bone, and when this occurs the end of the groove for the internal carotid artery is converted into a foramen (caroticoclinoid foramen). The small wing is connected to the body by two roots, the anterior thin and flat, the posterior thick and triangular; the optic foramen, which lies between them, transmits the optic nerve and ophthalmic artery to the orbital cavity.

Figure 350
Sphenoid bone of 8 year old anterior view - Figure 350
The pterygoid processes of the sphenoid (figs. 349, 350), one on each side, descend perpendicularly from the regions where the greater wings unite with the body. Each process consists of a medial and a lateral plate, the upper parts of which are fused anteriorly. The plates are separated below by an angular cleft, termed the pterygoid fissure, the rough margins of which articulate with the tubercle (pyramidal process) of the palatine bone. The two plates diverge behind, and the wedge-shaped pterygoid fossa between them contains the medial pterygoid and tensor palati (tensor veli palatini) muscles. Above this fossa there is a. small, oval, shallow depression, named the scaphoid fossa, which is placed on the upper part of the posterior border of the medial pterygoid plate and gives origin to part of the tensor palati (tensor veli palatini) muscle. The anterior surface of the pterygoid process is broad and triangular near its root, where it forms the posterior wall of the pterygopalatine fossa ; it is pierced by the anterior orifice of the pterygoid canal.

The lateral plate of the pterygoid process is broad, thin, and everted ; its lateral surface forms part of the medial wall of the infratemporal fossa and gives origin to the lower head of the lateral pterygoid ; its medial surface forms the lateral wall of the pterygoid fossa and gives origin to the greater part of the medial pterygoid muscle. The upper part of the anterior border forms the posterior boundary of the pterygomaxillary fissure ; the lower part articulates with the palatine bone ; its posterior border is free.

The medial plate of the pterygoid process is narrower and longer than the lateral; its lover extremity curves laterally into a book-like process, termed the pterygoid hamulus, around which the tendon of the tensor pala.ti (tensor veli palatini) glides, and to which the pterygomandibular ligament is attached. This plate forms the medial wall of the pterygoid fossa; and the tensor palati lies against it; the medial surface constitutes the lateral boundary of the corresponding posterior nasal aperture. Superiorly the medial plate is prolonged on to the under surface of the body as a thin lamina, named the vaginal process, which articulates anteriorly with the sphenoidal process of the palatine bone and medially with the ala of the vomer. On its under surface there is a furrow, the anterior part of which is converted into a canal by the sphenoidal process of the palatine bone ; this canal is named the palatinovaginal canal, and transmits the pharyngeal branch of the maxillary (internal maxillary) artery and the pharyngeal nerve from the sphenopalatine ganglion. The posterior margin of the medial plate gives attachment in its entire length to the pharyngobasilar fascia, and from the lower end of this margin the superior constrictor muscle of the pharynx takes origin. The upper end of this margin is marked by a small projection named the pterygoid tubercle, immediately above which is the posterior opening of the, pterygoid canal. Projecting backwards from near the middle of the margin is an angular process, sometimes termed the processus tubarius, which supports the pharyngeal end of the pharyngotympanic (auditory) tube. The anterior margin of the medial plate articulates with the posterior border of the perpendicular plate of the palatine bone.

The sphenoidal concave (fig. 350) are two thin, curved plates, situated at the anterior and lower parts of the body of the sphenoid bone; the upper, concave surface of each forms the anterior wall and a, part of the floor of the corresponding sphenoidal sinus. The spbenoidal conches are usually more or less destroyed in the process of disarticulating the skull, but, when seen in situ, each consists of an anterior, vertical, quadrilateral part and a posterior, horizontal, triangular part, The anterior, vertical portion consists of (a) an upper and lateral depressed area, which completes the posterior ethmoidal sinuses and articulates below with the orbital process of the palatine bone; and (b) a lower and medial area, smooth and triangular, which forms part of the roof of the nasal cavity, and is perforated near its superior angle by a round opening through which the sphenoidal sinus communicates with the spheno-ethmoidal recess of the nasal cavity. The anterior vertical portions of the two bones meet in the median plane and are protruded forwards as the sphenoidal crest. The horizontal triangular portion of the concha forms a part of the roof of the nasal cavity and completes the sphenopalatine foramen; its medial margin articulates with the rostrum of the sphenoid and with the ala of the vomer ; its apex, directed backwards, lies medial to and above the vaginal process of the medial pterygoid plate, and articulates with the posterior part of the ala of the vomer. A small piece of the sphenoidal concha sometimes appears in the medial wall of the orbit, between the orbital plate of the ethmoid in .front, the orbital plate of the palatine bone below, and the frontal bone above.

Ossification.-Until the seventh or eighth month of fetal life the body of the sphenoid consists of two parts-viz. one in front of the tuberculum sellae forming the presphenoidal part, with which the lesser wings are continuous ; the other comprising the sella turcica and dorsum sellee, forming the postsphenoidal part, with which the greater wings and pterygoid processes are associated. A considerable part of the bone is preformed in cartilage. There are six centres for the presphenoidal and eight for the postsphenoidal part.

Presphenoidal part-About the ninth week of fetal life a centre of ossification appears for each of the lesser wings, just lateral to the optic foramen; shortly afterwards two centres appear in the presphenoidal part of the body. The sphenoidal concave are each developed from a centre which makes its appearance about the fifth month * ; at birth, they consist of small triangular laminae; about the third year they become hollowed out and cone-shaped; about the fourth year they fuse with the labyrinths of the ethmoid, and between the ninth and twelfth years with the sphenoid bone.

Postsphenoidal part.-The first centres of ossification are those for the greater wings. About the eighth week one appears below the foramen rotundum in the cartilage which forms the base of each wing. This centre forms only the root of the greater wing in the neighborhood of the foramen rotundum and the pterygoid canal. The whole of the rest of the greater wing is ossified in membrane and the process extends downwards into the lateral pterygoid plate (Fawcett). About the fourth month two centres appear in the postsphenoidal part of the body, one on each side of the sella turcica, and fuse about the middle of fetal life. Each medial pterygoid plate (with the exception of its hamulus) is ossified in membrane, and its centre probably appears about the ninth or tenth week; the hamulus is chondrified during the third month and almost at once begins to ossify.* The medial and lateral pterygoid plates join about the sixth month. About the fourth month a centre appears for each lingula and speedily joins the rest of the bone.

The presphenoidal and the postsphenoidal parts of the body fuse about the eighth month of fetal life, but a wedge-shaped piece of cartilage persists for some time after birth in the lower part of the line of fusion. At birth the bone is in three pieces (fig. 351) : a central, consisting of the body and lesser wings, and two lateral, each comprising a greater wing and pterygoid process. In the first year after birth the greater wings and body unite around the margins of the pterygoid canal, and the lesser timings extend medially above the anterior part of the body and meet to form an elevated smooth surface, termed the jugum sphenoidale. By the twenty-fifth year the sphenoid and occipital bones are completely fused. In the anterior part of the hypophyseal fossa there are occasionally seen the remains of the craniopharyngeal canal, through which, in early fetal life, the hypophyseal diverticulum of the buccal ectoderm is transmitted.

Figure 351
Sphenoid bone at birth posterior view - Figure 351
Traces of the sphenoidal sinuses are seen as early as the third month of fetal life, but they do not attain their full size until after puberty. Although they are usually restricted to the presphenoidal and postsphenoidal parts of the body, the sinuses may invade any part of the bone which is preformed in cartilage.

Certain parts of the sphenoid bone are connected by ligaments which occasionally ossify. The more important of these ligaments are; the pterygospinous, stretching between the spine of the sphenoid and the upper part of the lateral pterygoid plate (see cervical fascia) ; the interclinoid, joining the anterior to the posterior clinoid process ; and the caroticoclinoid, connecting the anterior to the middle clinoid process.

Applied Anatomy-Premature ossification or synostosis of the suture between the pre and postsphenoidal parts (which normally begins to join at the eighth month) and of the sphenobasilar suture produces a characteristic physiognomy. This is best seen in profile, and consists in an abnormal depression of the bridge of the nose; it is a feature often observed in dwarfs.

 


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