USMLE Reviewer
(By Subscription)
Bones
Update October 31, 2019
The skull consists of bones that protect the brain.
The cranial bones consist of spongy bone “sandwiched” between two layers of compact bone.
The outer and inner surfaces of the skull, are covered by periosteum, known respectively as the pericranium and the endocranium.
The periosteum is continuous at the sutures of the skull.
Hover on Label for more Information
Click on Label for More Information
- Temporal bone. The paired temporal bones consist of a squamous part that forms the lateral portion of the skull; the petrous part, which encloses the internal ear (cochlea and semicircular canals) and the middle ear (malleus, incus, and stapes); the mastoid part, which contains the mastoid air cells; and the tympanic part, which houses the external auditory meatus and tympanic cavity.
- Occipital bone. The occipital bone encloses the foramen magnum, which transmits the spinal cord and vertebral arteries.
- Sphenoid bone. The sphenoid bone consists of a body, which houses the sphenoid sinus, and the greater and lesser wings and the pterygoid processes.
- Ethmoid bone. The ethmoid bone is located between the orbits and consists of the cribriform plate, the perpendicular plate, and the ethmoid air cells.
Sutures are immovable fibrous joints between the bones of the skull. The principal sutures are:
- Coronal suture. Joins the frontal bone and the two parietal bones. The coronal suture courses in the coronal plane.
- Sagittal suture. Joins the paired parietal bones. The sagittal suture courses in the sagittal plane.
- Squamous suture. Joins the parietal and temporal bones.
- Lambdoid suture. Joins the parietal bones with the occipital bone.
- Pterion. Junction of the frontal, parietal, and temporal bones in the lateral aspect of the skull.
Clinical Correlation
The anterior division of the middle meningeal artery courses deep to the pterion on the inner surface of the skull. A blow to the pterion, therefore, could rupture the middle meningeal artery and result in an epidural (extradural) hematoma, which is a buildup of blood between the dura mater and the skull. On a radiograph, an epidural hematoma appears as a convex shape because sutures at the sites where the periosteal dura is more firmly attached to the skull stop the hematoma from expanding. As a result, epidural hematomas expand inward toward the brain instead of along the side of the skull, which occurs in a subdural hematoma. Unconsciousness and death occur rapidly because of the bleeding that dissects a wide space as it strips the dura from the inner surface of the skull, causing pressure on the brain.