Sphenoid bone

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The sphenoid bone or sphenoid bone is located in the middle of the base of the skull in front of the occiput (occipital bone). It is part of the brain skull and is associated with all other skull bones and also with most of the bones of the facial skull. The body of the sphenoid bone is hollowed out (sphenoid sinus) and divided into two chambers over which the pituitary gland lies. Read everything you need to know about the anatomy and function of the sphenoid bone!

What is the sphenoid bone?

The sphenoid bone (Os sphenoidale) is a central skull bone, which has roughly the shape of a flying wasp with spread wings and drooping legs: It consists of the sphenoid bone body (corpus), two large sphenoid bones (alae majores), two small sphenoid bones wings (Alae minores) and the downward wing-like appendages (Processus pterygoidei).

The sphenoid body (corpus)

The sphenoid bone body (corpus) has an approximately cube-like shape. Inside there are two cavities, separated by a septum, called the sphenoid sinuses.

The posterior surface of the body of the sphenoid bone forms a connection (initially cartilaginous, later bony) with the occiput.

The upper surface of the sphenoid body forms the so-called Turkish saddle (Sella turcica) in the rear area, in which the pituitary gland (pituitary gland) lies. In the front upper area there is a connection to the ethmoid bone via a bone spine. In addition, a small, flat piece of bone (jugum sphenoidale) connects the two small wings of the sphenoid bone in front of the sulcus chiasmatis, in which the optic nerve junction (chiasma opticum) is located. The paired optic nerve (optic nerve) runs through a bone opening, together with its artery.

The anterior border of the sphenoid body forms the posterior area of ​​the eye socket (orbit).There is a connection to the ethmoid bone via a bone ridge, as well as a connection to the frontal and palatal bone. Several openings lead into the sphenoid sinuses, which are in communication with the nasal cavity.

The lower surface of the body of the sphenoid bone has a vertically downward-pointing beak-like bone crest, the rostrum sphenoidale, which is encompassed by the wings of the ploughshare bone and merges into the nasal septum.

The Great Sphenoid Wings (Alae majores)

The large sphenoid bones are strong bony processes on the side of the sphenoid body that are curved outward and upward. They have four faces, four edges, and an angle.

The surfaces of the large sphenoid bones are called:

• Facies cerebralis (points upwards to the brain)
• Facies temporalis (on the outer surface of the skull and separated from the underlying infratemporal facies of the upper jaw by a bone ridge)
• Facies orbitalis (delimits the eye socket with a flat, smooth bone surface)
• Facies maxillaris (directly below the facies orbitalis; represents the border to the upper jaw)

The foramen rotundum - a round opening through which the second branch of the trigeminal nerve (a facial nerve) passes - lies in the facies maxillaris.

The edges of the large sphenoid bones are called:

• Margo frontalis (borders the frontal bone)
• Margo zygomaticus (borders the cheekbone)
• Margo parietalis (borders on the parietal bone)
• Margo squamosus (borders the temporal bone)

The rear edge of the large sphenoid bones is drawn out like a thorn. There is a point of passage for an artery and a nerve. The muscle that tightens the soft palate and keeps the eustachian tube open has its origin here.

The small sphenoid bones (Alae minores)

The small sphenoid bones are thin, triangular plates of bone that sit on the upper front of the sphenoid body. They form the optic canal through which the optic nerve enters the eye socket from the cranial cavity. With the lower surface they delimit the eye socket, with the upper surface the cranial cavity. They form short bony processes towards the middle and towards the back.

The wing process of the sphenoid bone

As the pterygoid process, doctors refer to wing-like appendages that extend almost vertically downwards from the attachment of the large wings of the sphenoid bone to the body of the sphenoid bone. They consist of two bone plates, the medial lamina (central plate) and the lateral lamina (lateral plate).

Between these two is a pit, the pterygoid fossa (palatal fossa). The posterior part of this pit is formed by the wing process of the sphenoid bone, the plate of the palatine bone is perpendicular, and the front is formed by the upper jaw.

The attachment of the wing processes is pierced by a vascular nerve canal, this pit is a central distribution point for vessels and nerves.

The medial lamella has a hook-shaped extension at the lower end. This is where the tendon of the muscle that tightens the palate runs.

The sphenoid sinus

The sphenoid sinus is one of the paranasal sinuses. It is very small, borders with the posterior wall on the posterior fossa and with the lower side on the pharynx wall. It belongs to the so-called pneumatization spaces that are connected to the main nasal cavities. The sphenoid sinus opens into the upper nasal passage, and there is also a connection to the eye sockets and the optic nerve. The sphenoid sinus is - like the nasal mucous membrane - lined with a ciliated epithelium.

What is the function of the sphenoid bone?

Like the other cranial bones, the sphenoid bone serves to protect the brain and as an attachment point for various muscles (such as the masseter muscles). It forms the back of the eye socket and, along with other bones, the base of the skull.

The function of the sphenoid sinus and the other paranasal sinuses is not yet fully understood. The air-filled cavities presumably reduce the weight of the skull and serve as a resonance chamber for the voice.

Where is the sphenoid bone?

As the central bone of the skull, the sphenoid bone lies in the shape of a wedge between all the other skull bones, in front of the occiput (os occipitale) in the middle of the skull base. The sphenoid bone is only cartilaginously connected to the occipital bone until adolescence; it is only in adults that there is a bony connection here.

What problems can the sphenoid bone cause?

Inflammation of the sphenoid sinuses is relatively rare. Since they are connected to the upper turbinate, viruses and bacteria that cause sinusitis can also lead to an infection here. A pressure-like pain develops in the back of the head and in the top of the head because secretion accumulates in the cavity and exerts pressure. Then there is a runny nose and fever.

If the symptoms of the disease persist for more than three months, then one speaks of a chronic sinus infection - mostly caused by anatomical constrictions in the nasal area.

Extensive inflammation occasionally leads to an abscess or empyema (collection of pus) in the sphenoid sinus.

A sphenoid wing meningioma is a benign tumor of the temporal brain that spreads into the small wing of the sphenoid bone. It can also expand into the eye socket or the palatal fossa and then lead to visual disturbances and nerve paralysis.

A fracture of the occiput may involve the sphenoid bone.

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