Bony anatomy of skull Flashcards
Describe constituents and general position of sphenoid bone in skull and which part of sphenoid bone cannot be seen on an inferior view
Most of the anterior part of the middle part of the base of the skull consists of the sphenoid bone.
The sphenoid bone is made up of a centrally placed body , paired greater and lesser wings projecting laterally from the body, and two downward projecting pterygoid processes immediately lateral to each choana.
Three parts of the sphenoid bone, the body, greater wings, and pterygoid processes, are seen in the inferior view of the skull ( Fig. 8.23 ). The lesser wing of the sphenoid is not seen in the inferior view.
Describe the body of the sphenoid bone
ody
The body of the sphenoid is a centrally placed cube of bone containing two large air sinuses separated by a septum.
It articulates anteriorly with the vomer, ethmoid, and palatine bones, posterolaterally with the temporal bones, and posteriorly with the occipital bone.
Describe the pterygoid process of sphenoid bone, scaphoid fossa , pterygoid canal and its importance
Extending downward from the junction of the body and the greater wings are the pterygoid processes ( Fig. 8.23 ). Each of these processes consists of a narrow medial plate and broader lateral plate separated by the pterygoid fossa .
Each medial plate of the pterygoid process ends inferiorly with a hook-like projection, the pterygoid hamulus , and divides superiorly to form the small, shallow scaphoid fossa .
The scaphoid fossa is the origin of the tensor veli palatini muscle that is important in tensing the soft palate in order to assist with the levator veli palatini in raising the palate to occlude the nasopharynx to prevent the entry of food during swallowing. Tensor veli palatini also attaches to the lateral cartilaginous lamina of the auditory tube to assist with opening during swallowing or yawning to equalise pressure.
Just superior to the scaphoid fossa, at the root of the medial plate of the pterygoid process is the opening of the pterygoid canal , which passes forward from near the anterior margin of the foramen lacerum.
The pterygoid canal is important due to its connection with foramen lacerum. the greater petrosal nerve (branch of facial nerve) leaves the foramen lacerum and joins the deep petrosal nerve (which arise from internal carotid plexus) to form the nerve of pterygoid canal (vidian nerve) which synapses at the sphenopalatine ganglion
important as tumours commonly pass through foramen lacerum e.g. in nasopharyngeal carcinoma
Describe the greater wing of sphenoid bone
Lateral to the lateral plate of the pterygoid process is the greater wing of the sphenoid ( Fig. 8.23 ), which not only forms a part of the base of the skull but also continues laterally to form part of the lateral wall of the skull. It articulates laterally and posteriorly with parts of the temporal bone.
Important features visible on the surface of the greater wing in an inferior view of the skull are the foramen ovale and the foramen spinosum on the posterolateral border extending outward from the upper end of the lateral plate of the pterygoid process.
Describe the lesser wing of the sphenoid bone
The two lesser wings of the sphenoid project laterally from the body of the sphenoid and form a distinct boundary between the lateral parts of the anterior and middle cranial fossae.
Overhanging the anterior part of the middle cranial fossae, each lesser wing ends laterally as a sharp point at the junction of the frontal bone and the greater wing of the sphenoid near the upper lateral edge of the superior orbital fissure that is formed between the greater and lesser wings.
Medially each lesser wing widens, curves posteriorly, and ends as a rounded anterior clinoid process ( Fig. 8.25 ). These processes serve as the anterior point of attachment for the tentorium cerebelli , which is a sheet of dura that separates the posterior part of the cerebral hemispheres from the cerebellum. Just anterior to each anterior clinoid process is a circular opening in the lesser wing of the sphenoid (the optic canal ), through which the ophthalmic artery and optic nerve [II] pass as they exit the cranial cavity to enter the orbit. The optic canals are usually included in the middle cranial fossa.
Describe the features of the sella turcica, what portion of shphenoid bone it is from, boundaries of hypohyseal fossa and the location of clinoid processes
Just posterior to the chiasmatic sulcus is the uniquely modified remainder of the body of the sphenoid (the sella turcica ), which consists of a deep central area (the hypophyseal fossa ) containing the pituitary gland with anterior and posterior vertical walls of bone ( Fig. 8.26 ).
The anterior wall of the sella is vertical in position with its superior extent visible as a slight elevation (the tuberculum sellae ) at the posterior edge of the chiasmatic sulcus.
Lateral projections from the corners of the tuberculum sellae (the middle clinoid processes ) are sometimes evident.
The posterior wall of the sella turcica is the dorsum sellae , a large ridge of bone projecting upward and forward. At the top of this bony ridge the lateral edges contain rounded projections (the posterior clinoid processes ), which are points of attachment, like the anterior clinoid processes, for the tentorium cerebelli.
