Where to start with this amazing bone of the skull?! I love it so much! The sphenoid is bat-shaped, it has four wings total, it has a ton of holes, a few spiny projections, a snot-containing sinus, a huge fissure, it supports a few chewing muscles, it closes in and protects the back of the orbit, it forms part of the side of our head – it does SO MANY THINGS!
Let’s get at the sphenoid bone!!
Where is the sphenoid bone?
The sphenoid is viewable from nearly any angle of the skull – it is that spread out and centrally located! Lateral view of the skull? Greater wing is totally in view. Inferior view of the skull? Pterygoid process poking right at you. Superior view of an open skull (cap removed)? Get a load of those wings and foramina, not to mention a cute saddle-looking thing! Looking into the orbits of a skull? Front sides of those wings are seen plus a large fissure! Even from a posterior view of the skull, it is possible to see sphenoid because those pterygoid plates hang down low enough.
I told you – it’s an awesome bone!! Let’s start with features on the body of the sphenoid bone:
Body of the sphenoid
The body of the sphenoid is found along the midline, is about the size of a large dice, and features many cool things!
First, note that the sphenoid is a pneumatized bone – the body of the sphenoid is hollow, lined by respiratory mucosa, and drains via an opening into the nasal cavity. Snot fills the body of the sphenoid, ha! It drains into the sphenoethmoidal recess of the nasal cavity.
The body of the sphenoid also features the sella turcica – a saddle-shaped bony housing for the pituitary gland. A Turkish saddle has many components, and so does the sella turcica. A prechiasmatic sulcus (also known as the optic groove or the chiasmatic groove) allows the optic chiasm to nestle in real close to this bone. Right above that is a low elevation called the tuberculum sellae. The hypophyseal or pituitary fossa houses the gland of that name. The posterior bar of the “saddle” is the dorsum sellae. Then, anterior and posterior clinoid processes form four “points” surrounding the hypophyseal fossa. “Clinoid” means “bedpost” so picture the pituitary gland asleep in it’s four-poster bony bed and you’ll be fine!
Running nearly vertically on the sides of the body of the sphenoid (in the middle cranial fossa, not inside the sinus) is a large groove for the internal carotid artery, one on each side. This represents the cavernous portion of this important vessel, as it is within the cavernous (dural venous) sinus at this location. The cavernous sinus is on either side of the body of the sphenoid.
The clivus portion of the basilar occiput articulates with the sphenoid bone at the spheno-occipital synchondrosis. This site is a primary cartilaginous joint with hyaline cartilage that is important for proper maxillofacial growth and development. It unites/fuses between 20-25 years of age, making it a good location to use when determining the age at death of an unknown skeletal individual. Because we’ve all done that, right? (I have.)
Wings and processes of the sphenoid bone
Now for the wings and processes of the sphenoid bone:
Extending laterally from the body of the sphenoid are two wings of bone called the greater and lesser wings of the sphenoid bone. The greater wings are larger than the lesser, if you were wondering! The greater wings form a good portion of the basicranium and contain many important foramina and fissures through which neurovascular bundles must pass. Foramen rotundum, foramen ovale, and foramen spinosum are three of the most important foramina on the greater wing of the sphenoid.
The greater wings also form the lower walls of the middle cranial fossa inside the skull, and a portion of the floor of the temporal fossa on the outside of the skull. In the gif images below, most of that green is the greater wings of sphenoid (the body is the central portion and the rest is temporal bone). The greater wings articulate with the frontal, parietal, and temporal bones at the craniometric point pterion.
The external side of the greater wing also features the sphenoidal spine and infratemporal crest. These serve as attachment points for ligaments of the mandible and muscles in the deep face. These two structures are difficult for students to find because they range from being quite visible to barely seen, depending on masticatory stress during life. Look on the external, inferior surface of the sphenoid bone, tucked behind and about the level of the zygomatic arch.
The lesser wings of the sphenoid bone contribute to the floor of the anterior cranial fossa. The anterior clinoid processes are projections of the lesser wing. A large optic foramen lies within the lesser wing, transmitting the optic nerve (cranial nerve 2), and the ophthalmic artery.
The space between the lesser and greater wings of the sphenoid bone is called the superior orbital fissure. This is a large opening in the posterior wall of the orbit. Many neurovascular structures move into and out of the orbit through this fissure. An inferior orbital fissure exists between the greater wing of the sphenoid and the infratemporal surface of the maxilla.
Projecting inferiorly from the body of the sphenoid bone is a funky-looking pterygoid process. This process features two more plates of bone – the medial and lateral pterygoid plates, and an opening, the pterygoid canal. The lateral pterygoid plate serves as the attachment for a few muscles of mastication (chewing!). The medial pterygoid plate features a hook – or a hamulus – around which the tensor veli palatini muscle of the soft palate changes it’s fiber direction. The pterygoid canal allows neurovascular bundles to reach the pterygopalatine fossa and the nasopharynx.
I hope this review of sphenoid has been useful! When developing this osteology series I knew I had to start somewhere, and sphenoid bone is just the coolest, is it not?!