Anatomy and Physiology Chapter 5 Axial Skeleton Anatomy and Physiology Chapter 5

The Axial Skeleton The axial skeleton forms the longitudinal axis of the body. It is divided into three parts: The skull The vertebral column The bony thorax

The Skull The skull is formed by two sets of bones. The cranium encloses and protects the fragile brain tissue. The facial bones hold the eyes in an anterior position and allow the facial muscles to express emotions. All the bones of the skull are held together by sutures (interlocking, immovable joints), except the mandible (jawbone) which is attached with a freely moving joint.

The Cranium The boxlike cranium is composed of eight large, flat bones. Frontal Bone Parietal Bones Temporal Bones Occipital Bone Sphenoid Bone Ethmoid Bone

Frontal Bone 1.) Frontal Bone Forms the forehead, the bony projections under the eyebrows and the superior part of each eye’s orbit.

Parietal Bones 2.) Parietal Bones The paired parietal bones form most of the superior and lateral walls of the cranium They meet at the midline of the skull at the sagittal suture. The coronal suture is formed where the parietal bones meet the frontal bone.

Temporal Bones Temporal bones lie inferior to the parietal bones and join them at the squamous sutures. Several important bone markings appear on the temporal bones: External acoustic meatus: canal that leads to the eardrum and middle ear. Styloid process: sharp, needlelike projection inferior to the external acoustic meatus where several neck muscles attach. Zygomatic process: thin bridge of bone that joins with the cheekbone anteriorly. Mastoid process: rough projection posterior and inferior to the external acoustic meatus where the mastoid sinuses are located. Jugular foramen: junction of the occipital and temporal bones that allows passage of the jugular vein.

Occipital Bone The occipital bone is the most posterior bone of the cranium. It forms the floor and back wall of the skull. The occipital bone joins the parietal bones anteriorly at the lambdoid suture. In the base of the occipital bone is a large opening called the foramen magnum which surrounds the lower part of the brain and allows the spinal cord to connect with it. Lateral to the foramen magnum are the occipital condyles which rest on the first vertebra of the spinal column.

Sphenoid Bone The butterfly shaped sphenoid bone spans the width of the skull and forms part of the floor of the cranial cavity. In the midline of the sphenoid is a depression called sella turcia which holds the pituitary gland. The foramen ovale is a large opening that allows the cranial nerves to connect with the jaw muscles. It also contains the sphenoid sinuses. The part of the sphenoid that forms the eye orbits has two openings: The optic canal that allows the optic nerve to reach the eye. The superior orbital fissure where the cranial nerves controlling eye movements pass through.

Ethmoid Bone The ethmoid bone is very irregularly shaped and lies anterior to the sphenoid. It forms the roof of the nasal cavity and part of the medial walls of the eye orbits. The crista galli projects from it’s surface and attaches the outermost covering of the brain. The cribriform plates are small holes that allow nerve fibers to connect to the olfactory receptors.

Facial Bones The face is composed of fourteen bones. Twelve of those bones are paired, only the mandible and vomer are single.

Maxillae The two maxiallae , or maxillary bones fuse to form the upper jaw. All the facial bones except the mandible are joined to the maxillary bones and thus are called the “keystone” bones of the face. The maxillae carry the upper teeth in the alveolar margin. Extensions of the maxillae called the palatine processes form the anterior hard palate of the mouth. The maxillae also contain the paranasal sinuses which lighten the skull and amplify sounds.

Palatine Bones The paired palatine bones lie posterior to the palatine processes of the maxillae. They form the posterior part of the hard palate. Failure of these or the palatine processes to fuses can result in a cleft palate.

Zygomatic Bones Zygomatic bones are commonly referred to as the cheekbones. They also form a good-sized portion of the lateral walls of the eye orbits.

Lacrimal Bones The lacrimal bones are finger-nail size bones that form part of the medial wall of the eye orbit. Each lacrimal bone has a groove that serves as a passageway for tears.

Nasal Bones The nasal bones are small rectangular bones forming the bridge of the nose. The lower part of the nose is made up of cartilage.

Vomer Bone The single bone in the median line of the nasal cavity is the vomer. The vomer forms most of the nasal septum.

Inferior Nasal Conchae The inferior nasal conchae are thin, curved bones projecting from the lateral walls of the nasal cavity.

Mandible The mandible or lower jaw is the largest and strongest bone of the face. It joins the temporal bones on each side of the face, forming the only freely moving joints in the skull. The horizontal part of the bone forms the chin. The lower teeth lie in the alveoli sockets of the alveolar margin in the superior edge of the mandible.

The Hyoid Bone The hyoid bone is the only bone in the body that does not articulate directly with any other bone. It is suspended in the mid-neck region about 2 cm above the larynx. It has a horseshoe shape and serves as a movable base for the tongue and an attachment point for the neck muscles that raise and lower the larynx for speaking and swallowing.

Vertebral Column The vertebral column extends from the skull which it supports, to the pelvis. The spine is formed of 26 irregular bones connected and reinforced by ligaments into a flexible, curved structure. The delicate spinal cord runs through the central cavity of the vertebral column which supports and protects it.

Divisions of the Vertebral Column At birth, the spine consists of 33 vertebrae. Nine vertebrae fuse to form the sacrum and coccyx. The sacrum and coccyx form the inferior portion of the vertebral column. The other 24 bones make up the three superior sections of the vertebral column: 7 Cervical Vertebrae 12 Thoracic Vertebrae 5 Lumbar Vertebrae

Intervertebral Discs Single vertebrae are separated by intervertebral discs made of fibrocartilage which provide cushion, shock absorption and flexibility. Drying of the discs can lead to a herniated or “slipped” disc. The discs and the S-shape of the spine prevent shock to the head and make the body flexible. The spinal curvatures through the thoracic and sacral regions is called the primary curvatures. Secondary curvatures develop later through maturation in the cervical and lumbar regions.

Structure of Vertebrae All vertebrae have the same basic structures: Body or centrum: weight bearing disc-like part of vertebrae Vertebral Arch: arch formed from the joining of the posterior extensions from the vertebral body Vertebral foramen: canal through which spinal cord passes Transverse processes: two lateral projections from vertebral arch Spinous processes: single projection arising from posterior aspect of the vertebral arch Superior and inferior articular processes: paired projections lateral to vertebral foramen that form joint with adjacent vertebrae.

Cervical Vertebrae There are seven cervical vertebrae that form the neck region of the spine. The first two vertebrae perform unique functions for the spine. The atlas has no body and joins the occipital condyles of the skull and allows you to nod your head “yes”. The axis acts as a pivot rotation for the atlas and contains the joint that allows you to shake your head no.

Cervical Vertebrae Typical cervical vertebrae (C3-C7) are the lightest, and smallest vertebrae with short processes divided into two branches. Cervical vertebrae can be identified by the foramina openings which allow the vertebral arteries to pass through.

Thoracic Vertebrae There are twelve thoracic vertebrae. The thoracic vertebrae are larger and have a heart-shaped body and two costal faces where the ribs attach. The spinous process is long and hooks downward, resembling a giraffe’s head from the side.

Lumbar Vertebrae There are five lumbar vertebrae. The lumbar vertebrae have massive, block-like bodies and short hatchet-shaped spinous processes. The lumbar vertebrae carry the most stress in the vertebral column and are the sturdiest.

Sacrum The sacrum is formed from the fusion of five vertebrae and is located between L5 and the coccyx. It connects to the hips and forms the posterior wall of the pelvis. The coccyx is inferior to the sacrum and is formed by the fusion of 3 to 5 small vertebrae. The coccyx is the human tailbone.

The bony thorax is the last structure in the axial skeleton. It consists of the sternum, ribs and thoracic vertebrae. It is also called the thoracic cage because it forms a protective, cone-shaped cage of slender bones around the organs (heart, lungs, and major blood vessels) of the thoracic cavity. Bony Thorax

Sternum The sternum or breastbone is a flat bone made from the fusion of the manubrium body and the xiphoid process. It attaches to the first seven pairs of ribs. The sternum has three important bony landmarks: The jugular notch (concave upper border of manubrium) Sternal angle (where manubrium and body meets at a slight angle and form a ridge) Xiphisternal joint ( the point where the sternal body and xiphoid process fuse)

Ribs Twelve pairs of ribs form the walls of the bony thorax . All of the ribs articulate with the vertebral column posteriorly. The true ribs (first seven pairs) attach directly to the sternum by costal cartilages. The false ribs (next five pairs) attach indirectly or not at all to the sternum. The last two pairs do not attach and are also called the floating ribs. The intercostal (between ribs) space is filled with muscles that aid in breathing.