# Lecture 7: Axial Skeleton

## Anatomy and Physiology I

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## Learning Objectives

By the end of this lecture, students will be able to:

1. Distinguish between the axial and appendicular skeletons
2. Identify the bones of the skull and describe their key features
3. Describe the structure and types of vertebrae
4. Identify the normal curvatures of the vertebral column and common abnormalities
5. Describe the anatomy of the thoracic cage (sternum and ribs)
6. Identify the paranasal sinuses and the hyoid bone

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## Lecture Content

### I. Overview of the Skeleton

The adult skeleton typically contains **206 named bones**, divided into two major divisions. The **axial skeleton (80 bones)** forms the longitudinal axis of the body and includes the skull (22 bones), the hyoid bone (1), the auditory ossicles (6), the vertebral column (26 bones), and the thoracic cage (25 bones). The **appendicular skeleton (126 bones)** comprises the bones of the upper and lower limbs along with the pectoral and pelvic girdles that attach them to the axial skeleton.

### II. The Skull

The skull is composed of 22 bones: 8 cranial bones and 14 facial bones. Most skull bones are joined by **sutures**, which are immovable fibrous joints unique to the skull.

#### Cranial Bones (8) — form the cranial vault (calvaria) and cranial floor (base)

The **frontal bone (1)** forms the forehead, the roof of the orbits, and the anterior cranial fossa. It features the supraorbital foramen (or notch), a passage for the supraorbital nerve and artery, and houses the frontal sinuses, which are paranasal sinuses contained within the bone.

The **parietal bones (2)** form most of the superior and lateral walls of the cranium. The two parietal bones are joined to each other at the midline **sagittal suture** and meet the frontal bone at the **coronal suture**.

The **temporal bones (2)** form the inferolateral aspects of the cranium and part of the cranial floor. Each temporal bone has several distinct regions. The **squamous region** is the thin, flat area forming the temple. The **zygomatic process** projects anteriorly to join the zygomatic bone, forming the zygomatic arch (cheekbone). The **mandibular fossa** receives the condyle of the mandible to form the temporomandibular joint (TMJ). The **tympanic region** surrounds the external acoustic meatus (ear canal). The **mastoid process** is a large projection posterior and inferior to the ear that serves as an attachment point for the sternocleidomastoid and other muscles and contains mastoid air cells. The **styloid process** is a needle-like projection providing attachment for muscles and ligaments of the tongue and pharynx. The **petrous region** forms part of the middle cranial fossa floor and houses the middle and inner ear. Important openings in the temporal bone include the **internal acoustic meatus**, which provides passage for cranial nerves VII and VIII; the **jugular foramen**, through which the internal jugular vein and cranial nerves IX, X, and XI pass; and the **carotid canal**, which transmits the internal carotid artery.

The **occipital bone (1)** forms the posterior part and most of the base of the skull. Its most prominent feature is the **foramen magnum**, the large opening through which the spinal cord connects to the brainstem. The **occipital condyles** flank the foramen magnum and articulate with the atlas (C1), forming the site of the "yes" nodding movement. The **external occipital protuberance** is a midline bump serving as the attachment for the ligamentum nuchae. The occipital bone meets the parietal bones at the **lambdoid suture**.

The **sphenoid bone (1)** is often called the "keystone" of the cranium because it contacts all other cranial bones. Bat or butterfly-shaped, it spans the width of the middle cranial fossa. Its central **body** contains the sphenoid sinuses, and on its superior surface sits the **sella turcica**, a saddle-shaped depression that houses the pituitary gland (hypophysis). The **greater wings** form part of the lateral skull and the floor of the middle cranial fossa, while the **lesser wings** contribute to the floor of the anterior cranial fossa and the posterior wall of the orbits. The **pterygoid processes** project inferiorly and provide attachment for the pterygoid muscles of mastication. The **optic canals** transmit the optic nerves (CN II) and ophthalmic arteries, and the **superior orbital fissure** provides passage for cranial nerves III, IV, V1, and VI to reach the orbit.

The **ethmoid bone (1)** is located between the orbits and contributes to the anterior cranial fossa, the nasal septum, and the lateral walls of the nasal cavity. Its **cribriform plate** is a horizontal plate perforated with tiny olfactory foramina through which olfactory nerve fibers (CN I) pass. The **crista galli** is a superior midline projection that serves as the attachment for the falx cerebri, a meningeal fold. The **perpendicular plate** forms the superior portion of the nasal septum. The **superior and middle nasal conchae (turbinates)** are scroll-shaped projections on the lateral nasal walls that increase surface area to warm, humidify, and filter incoming air. The bone also contains the ethmoidal sinuses, consisting of many small air cells.

#### Facial Bones (14)

The **maxillae (2)** form the upper jaw, the anterior hard palate, the floor of the orbits, and the lateral walls of the nasal cavity. They contain the maxillary sinuses, the largest of the paranasal sinuses. Their alveolar processes bear the upper teeth, and the infraorbital foramen provides passage for the infraorbital nerve and artery.

The **palatine bones (2)** are L-shaped bones that form the posterior portion of the hard palate and part of the nasal cavity floor. The **zygomatic bones (2)**, commonly known as the "cheekbones," form the lateral walls of the orbits and the prominence of the cheeks, articulating with the temporal process to form the zygomatic arch. The **lacrimal bones (2)** are the smallest facial bones and form part of the medial wall of the orbit, containing the lacrimal fossa that houses the lacrimal sac as part of the tear drainage system. The **nasal bones (2)** are small rectangular bones forming the bridge of the nose.

The **inferior nasal conchae (2)** are separate bones, unlike the superior and middle conchae, which are part of the ethmoid. They are the largest of the three conchae and project into the nasal cavity. The **vomer (1)** forms the inferior portion of the nasal septum, complementing the perpendicular plate of the ethmoid, which forms the superior part.

The **mandible (1)** is the lower jaw, the largest and strongest facial bone, and the only freely movable skull bone. Its horizontal **body** bears the lower teeth along the alveolar process. The **rami** are vertical portions extending posteriorly, each terminating in two processes: the **mandibular condyle**, which articulates with the mandibular fossa of the temporal bone at the TMJ, and the **coronoid process**, an anterior projection serving as the attachment for the temporalis muscle. The **mandibular foramen** on the medial surface of the ramus transmits the inferior alveolar nerve and is the dentist's injection site. The **mental foramen** on the anterior body provides passage for the mental nerve and vessels.

<image>Four views of the skull. Panel A: Anterior view showing the frontal bone, orbits, nasal bones, zygomatic bones, maxillae, mandible, supraorbital foramen, infraorbital foramen, and mental foramen. Panel B: Lateral view showing the temporal bone with zygomatic process, mandibular fossa, mastoid process, styloid process, external acoustic meatus, the coronal suture, squamous suture, and lambdoid suture. The zygomatic arch and the pterion are labeled. Panel C: Superior view of the cranial base (internal) showing the anterior, middle, and posterior cranial fossae, cribriform plate with crista galli, sella turcica, foramen magnum, optic canals, and major foramina. Panel D: Inferior view showing occipital condyles, foramen magnum, jugular foramina, carotid canals, styloid and mastoid processes, and the hard palate.</image>

### III. Special Features of the Skull

#### Sutures

The four major sutures of the skull are the **coronal suture**, where the frontal bone meets the parietal bones; the **sagittal suture**, running between the two parietal bones at the midline; the **lambdoid suture**, where the parietal bones meet the occipital bone; and the **squamous suture**, where the parietal bone meets the temporal bone on each side.

#### Fontanelles (in infants)

Fontanelles are fibrous membranes connecting the cranial bones before the sutures fully close. They allow the skull to compress during birth and accommodate the rapid growth of the brain. The **anterior fontanelle** is the largest, located at the junction of the sagittal, coronal, and frontal sutures, and closes by 18 to 24 months. The **posterior fontanelle** lies at the junction of the sagittal and lambdoid sutures and closes by 2 to 3 months. The **anterolateral (sphenoid) fontanelle** and **posterolateral (mastoid) fontanelle** are smaller and close within a few months of birth.

#### Paranasal Sinuses

The paranasal sinuses are air-filled spaces within the frontal, sphenoid, ethmoid, and maxillary bones. They are lined with mucous membrane continuous with the nasal cavity. Their functions include lightening the skull, warming and humidifying inhaled air, providing vocal resonance, and producing mucus. The sinuses drain into the nasal cavity, and blockage of drainage can lead to sinusitis.

#### Hyoid Bone

The hyoid bone is a U-shaped bone situated in the anterior neck between the mandible and the larynx. It is unique in that it does not articulate with any other bone, being suspended entirely by ligaments and muscles. It supports the tongue and provides attachment points for muscles involved in swallowing and speech. The hyoid bone can be fractured during strangulation, giving it forensic significance.

### IV. The Vertebral Column

The adult vertebral column consists of **26 bones** (33 initially, as the sacral and coccygeal vertebrae fuse during development). It supports the head, encloses and protects the spinal cord, provides attachment for the ribs and back muscles, and allows movement. Between adjacent vertebral bodies are **intervertebral discs**, pads of fibrocartilage composed of an outer ring called the **annulus fibrosus** and an inner gel-like core called the **nucleus pulposus**, which acts as a shock absorber. A herniated (slipped) disc occurs when the nucleus pulposus protrudes through the annulus fibrosus and may compress adjacent spinal nerves.

#### Normal Curvatures

The vertebral column has four curvatures that increase its strength and flexibility. The **cervical curvature** is concave posteriorly (lordosis) and develops when the infant begins to hold up its head. The **thoracic curvature** is convex posteriorly (kyphosis) and is a primary curvature present at birth. The **lumbar curvature** is concave posteriorly (lordosis) and develops when the child begins to walk. The **sacral curvature** is convex posteriorly (kyphosis) and is also a primary curvature.

#### Abnormal Curvatures

Three abnormal curvatures are clinically significant. **Scoliosis** is a lateral curvature, most common in the thoracic region. **Kyphosis** is an exaggerated thoracic curvature (sometimes called "hunchback") and is common in patients with osteoporosis. **Lordosis** is an exaggerated lumbar curvature (sometimes called "swayback") and is common during pregnancy and in obesity.

#### Regions and Vertebrae

The **cervical vertebrae (C1-C7)** are the seven smallest and lightest vertebrae. **C1 (the Atlas)** is ring-shaped, lacking a body and spinous process, and supports the skull, allowing the "yes" nodding motion at the atlanto-occipital joint. **C2 (the Axis)** features the **dens (odontoid process)**, which projects superiorly from the body and acts as a pivot for C1 rotation, permitting the "no" head-shaking motion at the atlanto-axial joint. C3 through C7 are typical cervical vertebrae. Distinguishing features of cervical vertebrae include a small body, a bifid spinous process (C2-C6), **transverse foramina** for the passage of vertebral arteries, and a large triangular vertebral foramen. **C7 (the vertebra prominens)** has a long, non-bifid spinous process that is easily palpated at the base of the neck.

The **thoracic vertebrae (T1-T12)** are twelve vertebrae that articulate with the ribs. Their distinguishing features include a heart-shaped body, a long inferiorly-pointing spinous process, **costal facets** on the body and transverse processes for rib articulation, and a circular vertebral foramen.

The **lumbar vertebrae (L1-L5)** are the five largest and strongest vertebrae, bearing the most weight. They are distinguished by a massive kidney-shaped body, a short thick spinous process that is hatchet-shaped, and the absence of costal facets and transverse foramina.

The **sacrum** consists of five fused vertebrae forming a triangular bone. It articulates with L5 superiorly and with the coxal (hip) bones laterally at the sacroiliac joints. Key features include the **sacral promontory**, the anterior lip of the S1 body that serves as an obstetric landmark; the **median sacral crest**, formed by the fused spinous processes; the **sacral foramina** on both the anterior and posterior surfaces, which provide passage for spinal nerves; the **sacral canal**, a continuation of the vertebral canal; and the **sacral hiatus**, the opening at the inferior end of the sacral canal used as the site for caudal epidural injections.

The **coccyx** consists of three to five fused vertebrae and is commonly known as the "tailbone." It provides attachment for ligaments and muscles of the pelvic floor.

#### General Structure of a Typical Vertebra

A typical vertebra has several identifiable features. The **body (centrum)** is the disc-shaped, weight-bearing anterior portion. The **vertebral (neural) arch** is the posterior portion formed by the **pedicles**, short thick processes projecting posteriorly from the body, and the **laminae**, flat plates connecting the pedicles to the spinous process. The **vertebral foramen** is the opening enclosed by the body and arch; collectively, the series of foramina form the vertebral canal that houses the spinal cord. The **intervertebral foramina** are lateral openings between adjacent vertebrae that provide passage for spinal nerves. Several processes project from the arch: the **spinous process** projects posteriorly from the junction of the laminae and serves as an attachment for muscles and ligaments; the **transverse processes** project laterally from the junction of the pedicles and laminae for muscle attachment; and the **superior and inferior articular processes** form zygapophyseal (facet) joints with adjacent vertebrae, restricting and guiding vertebral movement.

<image>Three panels comparing representative vertebrae from each region. Panel A: A superior view of a cervical vertebra (C4) with small body, bifid spinous process, transverse foramina, and large triangular vertebral foramen labeled. Panel B: A superior view of a thoracic vertebra (T6) with heart-shaped body, long pointed spinous process, costal facets on the body and transverse processes, and circular vertebral foramen. Panel C: A superior view of a lumbar vertebra (L3) with massive kidney-shaped body, short hatchet-shaped spinous process, no transverse foramina or costal facets. A lateral view of the assembled vertebral column is shown alongside, with the four curvatures (cervical lordosis, thoracic kyphosis, lumbar lordosis, sacral kyphosis) indicated by arrows, and the intervertebral discs visible between vertebral bodies.</image>

### V. The Thoracic Cage (Bony Thorax)

The thoracic cage is composed of the **sternum**, **12 pairs of ribs**, and the **12 thoracic vertebrae**. It protects the thoracic organs (heart, lungs, and great vessels), supports the shoulder girdles and upper limbs, provides attachment for respiratory and trunk muscles, and plays a key role in breathing mechanics.

#### Sternum (Breastbone)

The sternum is a flat bone in the anterior midline of the thorax, composed of three fused bones. The **manubrium** is the superior portion, articulating with the clavicles at the sternoclavicular joint and with ribs 1 and 2. The **jugular (suprasternal) notch** is the central indentation on its superior border and serves as a palpable landmark. The **body** is the middle and largest portion, articulating with ribs 2 through 7 via costal cartilages. The **xiphoid process** is a small inferior projection that is cartilaginous in youth and ossifies by about age 40; it serves as a landmark for CPR hand placement, with compressions applied above the xiphoid. The **sternal angle (angle of Louis)** is the junction of the manubrium and body, forming a palpable ridge that serves as the landmark for locating the second rib.

#### Ribs

There are 12 pairs of ribs, all of which articulate posteriorly with the thoracic vertebrae. **True ribs (ribs 1-7)** attach directly to the sternum via their own costal cartilage and are also called vertebrosternal ribs. **False ribs (ribs 8-12)** do not attach directly to the sternum. Ribs 8 through 10 attach to the sternum indirectly via the costal cartilage of rib 7 (vertebrochondral ribs), while ribs 11 and 12 are **floating ribs** with no anterior attachment (vertebral ribs). The anatomy of a typical rib (ribs 3-9) includes a wedge-shaped **head** that articulates with the vertebral bodies at two facets, a **neck** that is a short constricted portion, a **tubercle** that articulates with the transverse process of the corresponding vertebra, and a curved, flat **shaft (body)** with a sharp angle where it changes direction. The **costal groove** on the inferior internal surface of the shaft protects the intercostal blood vessels and nerve.