8.2. The Skull

Auditory ossicles

2 Malleus (hammer) 2 Incus (anvil) 2 Stapes (stirrup)

Facial bones (14)

2 Maxilla 2 Palatine 2 Zygomatic 2 Lacrimal 2 Nasal 1 Vomer 2 Inferior nasal concha 1 Mandible

Skull bones (22)

8 Cranial bones 14 Facial bones

Sinuses

Frontal Sphenoid Ethmoid Maxillary

Vomer

Inferior half of the nasal septum

Cranial bones (8)

1 Frontal 2 Parietal 1 Occipital 2 Temporal 1 Sphenoid 1 Ethmoid

Cranial fossae

Anterior - frontal lobes Middle - temporal lobes Posterior - cerebellum

Cranium

Calvaria (skullcap) Base (floor) of the cranial cavity with fossae

Skull cavities

Cranial Orbits Nasal Oral (buccal) Middle-ear Inner-ear Paranasal sinuses

Cranial bones

Enclose the brain Compose the cranium (brain case)

Foramen magnum

Large opening at the base floor of the cranial cavity where the spinal cord meets the brain

The Skull

Most complex part of skeleton 22 bones or more Bones connected by immovable joints called sutures Contains several prominent cavities Bones have especially prominent foramina

Sinuses description

Named for the bones in which they occur Connected with nasal cavity Lined by mucuous membrane Filled with air

Nasal cavity conchae

Superior conchae - part of ethmoid bone Middle conchae - part of ethmoid bone Inferior conchae - separate bone

The Hyoid bone

The hyoid bone is a slender U-shaped bone between the chin and larynx. It is one of the few bones that does not articulate with any other. It is suspended from the styloid processes of the skull, somewhat like a hammock, by the small stylohyoid muscles and stylohyoid ligaments. The median body of the hyoid is flanked on either side by hornlike projections called the greater and lesser horns (cornua). The larynx (voice box) is suspended from the hyoid bone by a broad ligament, and the hyoid serves for attachment of several muscles that control the mandible, tongue, and larynx. Forensic pathologists look for a fractured hyoid as evidence of strangulation.

Lacrimal bones

The lacrimal(LACK-rih-mul) bones form part of the medial wall of each orbit. They are the smallest bones of the skull, about the size of the little fingernail. A depression called the lacrimal fossa houses a membranous lacrimal sac in life. Tears from the eye collect in this sac and drain into the nasal cavity.

The Mandible

The mandible is the strongest bone of the skull and the only one that can move significantly. It supports the lower teeth and provides attachment for muscles of mastication and facial expression. The horizontal portion, bearing the teeth, is called the body; the vertical to oblique posterior portion is the ramus (RAY-mus) (plural, rami); and these two portions meet at a corner called the angle. The mandible develops as separate right and left bones in the fetus, joined by a median cartilaginous joint called the mental symphysis (SIM-fih-sis) at the point of the chin. This joint ossifies in early childhood, uniting the two halves into a single bone. The point of the chin itself is called the mental protuberance. The inner (posterior) surface of the mandible in this region has a pair of small points, the mental spines, which serve for attachment of certain chin muscles. On the anterolateral surface of the body, the mental foramen permits the passage of nerves and blood vessels of the chin. The inner surface of the body has a number of shallow depressions and ridges to accommodate muscles and salivary glands. The angle of the mandible has a rough lateral surface for insertion of the masseter, a muscle of mastication. Like the maxilla, the mandible has pointed alveolar processes between the teeth. The ramus is somewhat Y-shaped. Its posterior branch, called the condylar (CON-dih-lur) process, bears the mandibular condyle—an oval knob that articulates with the mandibular fossa of the temporal bone. The meeting of this condyle with the temporal bone forms a hinge, the temporomandibular joint (TMJ). The anterior branch of the ramus is a blade called the coronoid process. It is the point of insertion for the temporalis muscle, which pulls the mandible upward when you bite. The U-shaped arch between the two processes is the mandibular notch. Just below the notch, on the medial surface of the ramus, is the mandibular foramen. The nerve and blood vessels that supply the lower teeth enter this foramen and then travel through the bone of the mandibular body, giving off branches to each tooth along the way. Dentists commonly inject lidocaine near the mandibular foramen to deaden sensation from the lower teeth.

The Maxillae

The maxillae (mac-SILL-ee) are the largest facial bones. They form the upper jaw and meet each other at a median intermaxillary suture. Small points of maxillary bone called alveolar processes grow into the spaces between the bases of the teeth. The root of each tooth is inserted into a deep socket, or alveolus. If a tooth is lost or extracted so that chewing no longer puts stress on the maxilla, the alveolar processes are resorbed and the alveolus fills in with new bone, leaving a smooth area on the maxilla.

Palatine bones

The palatine bones are located in the posterior nasal cavity. Each has an L shape formed by a horizontal plate and a perpendicular plate. The horizontal plates form the posterior one-third of the bony palate. Each is marked by a large greater palatine foramen, a nerve passage to the palate. The perpendicular plate is a thin, delicate, irregularly shaped plate that forms part of the wall between the nasal cavity and the orbit.

Bones associated with the skull (7)

The three auditory ossicles in each middle ear cavity The hyoid bone beneath the chin

Zygomatic bones

The zygomatic bones form the angles of the cheeks at the inferolateral margins of the orbits and part of the lateral wall of each orbit; they extend about halfway to the ear. Each zygomatic bone has an inverted T shape and usually a small zygomaticofacial (ZY-go-MAT-ih-co-FAY-shul) foramen near the intersection of the stem and crossbar of the T. The prominent zygomatic arch that flares from each side of the skull is formed mainly by the union of the zygomatic bone, temporal bone, and maxilla.

Inferior nasal conchae

There are three conchae in the nasal cavity. The superior and middle conchae, as discussed earlier, are parts of the ethmoid bone. The inferior nasal concha—the largest of the three—is a separate bone

Nasal bones

Two small rectangular nasal bones form the bridge of the nose and support cartilages that shape its lower portion. If you palpate the bridge, you can easily feel where the nasal bones end and the cartilages begin. The nasal bones are often fractured by blows to the nose.