Anatomy and Physiology: Chapter 8 - Axial Skeleton

Learning Objectives

Upon completion of this chapter, you should be able to:

Discuss and identify the cranial and facial bones of the skull.
Identify bony markings of the skull and their functions.
Describe the structure of a typical vertebrae.
Discuss the anatomy of the sternum and ribs.

Divisions of the Skeletal System

The skeletal system comprises all the bones, cartilage, and ligaments of the body.
It consists of 206 bones in an adult skeleton, with more bones present in children.
Primary functions include:
- Providing support for the body.
- Aiding in body movements.
- Assisting in calcium homeostasis.
The skeletal system is divided into two major divisions:
- Axial Skeleton
- Appendicular Skeleton

The Axial Skeleton

Forms the vertical, central axis of the body, providing core support and protection.
Protects vital internal organs.
Allows for movement of the head, neck, back, and respiratory muscles.
Comprises a total of 80 bones.
Key components include:
- Skull
- Vertebral column
- Ribs
- Sternum
Protected Internal Organs:
- The axial skeleton safeguards organs such as the brain, heart, lungs, and spinal cord.
- The brain is considered the best-protected organ due to its almost complete enclosure by bone.

The Appendicular Skeleton

Forms the upper and lower extremities of the body.
Includes the bones that attach the extremities to the axial skeleton.
Consists of 126 bones in total.
Detailed discussion is typically reserved for Chapter 9.

The Skull

Composed of 22 bones, which are categorized into two main groups:
1. Cranial bones: These bones surround and protect the brain.
2. Facial bones: These bones form the face, nasal cavity, mouth, and orbit.
The skull bones form openings, or cavities, which:
- Contain soft organs (e.g., eyeballs, brain, sensory organs).
- Help to decrease the overall weight of the skull.

Bones of the Skull

Cranial Bones (Total: 8)
- Frontal bone (1)
- Parietal bones (2) - Paired
- Occipital bone (1)
- Temporal bones (2) - Paired
- Sphenoid bone (1)
- Ethmoid bone (1)
Facial Bones (Total: 14)
- Mandible (1)
- Maxillae (2) - Paired
- Lacrimal bones (2) - Paired
- Nasal bones (2) - Paired
- Palatine bones (2) - Paired
- Zygomatic bones (2) - Paired
- Inferior nasal conchae (2) - Paired
- Vomer (1)
Some skull bones are paired (found on left and right sides), while others are unpaired and centrally located.

Cavities of the Skull

These cavities house softer structures within the skull.
Key cavities include:
- Cranial cavity (houses the brain)
- Orbits (house the eyeballs)
- Nasal cavity (for breathing and olfaction)
- Oral cavity (for eating and speech)
- Paranasal sinuses (air-filled spaces)

Individual Cranial Bones and Bony Markings

Frontal Bone (1)
- Forms the forehead and a portion of the cranium.
- Bony Markings:
  - Glabella: Smooth area superior to the nasal root, between the superciliary arches.
  - Supraorbital margin: The superior rim of each orbit.
  - Supraorbital foramen: A small opening within the supraorbital margin for nerves and vessels.
Parietal Bones (2)
- Form the superior lateral sides of the skull.
- Articulate with the frontal, temporal, and occipital bones.
Occipital Bone (1)
- Forms the posterior skull and the posterior base of the cranial cavity.
- Bony Markings:
  - External occipital protuberance: A prominent bump on the posterior skull.
  - Superior nuchal line: A ridge extending laterally from the external occipital protuberance.
  - Foramen magnum: The large opening at the base of the skull allowing the spinal cord to connect to the brain.
  - Occipital condyles: Oval-shaped processes on either side of the foramen magnum that articulate with the atlas (C1).
Temporal Bones (2)
- Form the lower lateral sides of the skull.
- Bony Markings:
  - Squamous portion: The flattened, fan-shaped superior part.
  - Zygomatic process: A projection that extends anteriorly to form part of the zygomatic arch.
  - Mastoid process: A prominent projection inferior to the ear, often palpable.
  - External acoustic meatus: The ear canal opening.
  - Mandibular fossa: An indentation that articulates with the condylar process of the mandible.
  - Articular tubercle: A small prominence anterior to the mandibular fossa.
  - Styloid process: A slender, pointed projection extending inferiorly.
Sphenoid Bone (1)
- Forms a significant portion of the base of the central skull and part of the temples.
- Bony Markings:
  - Greater and lesser wings: Wing-like extensions that contribute to the cranial floor and orbit.
  - Sella turcica: A saddle-shaped depression on the superior surface, housing the pituitary gland.
  - Pituitary fossa: The specific depression within the sella turcica that holds the pituitary gland.
  - Medial and lateral pterygoid processes: Inferior projections involved in chewing muscle attachment.
Ethmoid Bone (1)
- Forms part of the orbit and nasal cavity.
- Bony Markings:
  - Perpendicular plate: Forms the superior portion of the nasal septum.
  - Superior and middle nasal conchae: Scroll-like projections into the nasal cavity that increase surface area.
  - Crista galli: A superior projection that serves as an attachment point for the dura mater.
  - Cribriform plate: A perforated plate containing openings for olfactory nerves.
  - Olfactory foramina: Small holes in the cribriform plate through which olfactory nerves pass.

Individual Facial Bones and Bony Markings

Mandible (1)
- Forms the lower jaw, unique as the only movable bone of the skull.
- Bony Markings:
  - Body, ramus, and angle: Main parts of the mandible, with the angle defining the corner.
  - Coronoid process: An anterior projection of the ramus for muscle attachment.
  - Condylar process: A posterior projection of the ramus that articulates with the temporal bone.
  - Mandibular notch: The depression between the coronoid and condylar processes.
  - Mental foramina: Openings on the anterior surface of the body for nerves and blood vessels.
  - Mental protuberance: The prominence of the chin.
Maxillary Bones (Maxillae) (2)
- Also known as the hard palate or maxilla.
- Forms the upper jaw, most of the roof of the mouth, part of the orbit, and the lateral base of the nose.
- Bony Markings:
  - Alveolar processes: Ridges that contain the sockets for the upper teeth.
  - Infraorbital foramen: An opening inferior to the orbit for nerves and blood vessels.
Lacrimal Bones (2)
- Form part of the medial wall of the orbit.
- Contain lacrimal glands that are responsible for secreting tears.
Palatine Bones (2)
- Form the posterior portion of the hard palate, the medial part of the orbit, and contribute to the vertical section of the nasal cavity.
Zygomatic Bones (2)
- Commonly known as the cheekbones.
- Forms a significant portion of the lateral part of the orbit.
- Bony Markings:
  - Temporal process: Unites with the zygomatic process of the temporal bone to form the zygomatic arch.
Vomer (1)
- Forms part of the nasal septum.
Nasal Bones (2)
- Form the bony base (bridge) and lateral walls of the nose.
Inferior Nasal Conchae (2)
- Distinct bones that project into the nasal cavity, increasing its surface area.

Specific Skull Features and Structures

The Articulated Skull: An anterior view allows for the identification of many individual skull bones in their relative positions.
The Orbit:
- Protects the eyeball and its associated muscles.
- Composed of contributions from the frontal, zygomatic, maxilla, ethmoid, lacrimal, palatine, and sphenoid bones.
- Optic canal: Allows for the entry of the optic nerve to the eye.
- Superior orbital fissure: Facilitates the passage of nerves to the eyeball, associated muscles, and sensory nerves to the forehead.
The Nasal Cavity, Septum, and Conchae:
- Nasal cavity: Bordered by the maxillae and nasal bones.
- Nasal septum: Divides the nasal cavity into left and right halves.
  - Formed by the perpendicular plate of the ethmoid bone, the vomer, and septal cartilage.
- Nasal conchae: Covered by mucous membranes.
  - Their primary function is to warm, filter, and moisten inhaled air.
Paranasal Sinuses:
- Hollow, air-filled spaces found within the frontal, maxillary, sphenoid, and ethmoid bones.
- Connect to the nasal cavity.
- Functions:
  - Add volume to the skull without significantly increasing its weight.
  - Provide resonance for the voice.
- Named after the skull bone they are located in:
  - Frontal sinus
  - Maxillary sinuses
  - Sphenoid sinus
  - Ethmoid sinus
Oral Cavity:
- Formed by the maxillae, mandible, and palatine bones.
- Teeth are securely embedded in the alveolar processes.
- Hard palate: Formed by the palatine process of the maxillae and the horizontal plate of the palatine bones.
- Mandibular (mental) foramen: An opening allowing for the entry of nerves and blood vessels to the lower jaw.
Sutures of the Skull:
- Lateral View:
  - Zygomatic arch: Formed by the temporal process of the zygomatic bone articulating with the zygomatic process of the temporal bone.
  - Temporal fossa: The area located superior to the zygomatic arch.
  - Squamous suture: Unites the parietal and temporal bones.
- Posterior View:
  - Sagittal suture: Unites the two parietal bones along the midline.
  - Lambdoid suture: Unites the parietal bones with the occipital bone.
- Midsagittal Section:
  - Coronal suture: Unites the frontal and parietal bones.

Inside the Cranial Cavity

Primarily occupied by the brain.
The base of the skull is internally divided into 3 fossae (depressions):
- Anterior cranial fossa: Contains the crista galli and cribriform plates. Primarily houses the frontal lobes of the brain.
- Middle cranial fossa: Contains numerous openings for blood vessels and nerves. Houses the temporal lobes.
- Posterior cranial fossa: Contains the internal acoustic meatus, hypoglossal canal, and jugular foramen. Houses the cerebellum and brainstem.

Openings of the Middle Cranial Fossa

Optic canal: Provides for the passage of the optic nerve ($ ext{CNII}$) into the orbit.
Superior orbital fissure: Passage for nerves to the eyeball, associated muscles, and sensory nerves to the forehead.
Foramen rotundum: Allows the exit of a major sensory nerve to the face.
Foramen ovale: Allows the exit of another major sensory nerve to the face.
Foramen spinosum: Facilitates the entry of blood supply (middle meningeal artery) into the cranial cavity.
Stylomastoid foramen: Allows the exit of the facial nerve ($ ext{CNVII}$) to the muscles of the face.
Carotid canal: Provides the entry point for blood supply (internal carotid artery) to the brain.
Foramen lacerum: Notably, no structures pass directly through here; it is covered by cartilage in living individuals.

Openings of the Posterior Cranial Fossa

Internal acoustic meatus: Allows for the passage of nerves for hearing and equilibrium ($ ext{CNVIII}$), and the facial nerve ($ ext{CNVII}$) that supplies muscles of the face.
Hypoglossal canal: Allows for the passage of the hypoglossal nerve ($ ext{CNXII}$) to the tongue.
Jugular foramen: Allows for the passage of several nerves ($ ext{CN IX, X, XI}$) and the jugular vein.

Development and Aging of the Skull

At birth, the bones of the skull are not yet completely fused.
Fontanelles:
- Commonly known as “soft spots.”
- Allow the skull to change shape during the birthing process, facilitating passage through the birth canal.
- Accommodate the rapid growth of the brain during infancy.
- The bones of the skull are not fully fused until adulthood.

Application: Fusion Disorders

Cleft lip: Results from a partial or complete failure of the upper lip to fuse together during embryonic development.
Cleft palate: Results from a failure of the hard palate to fuse completely.
Craniosynostosis: The premature fusion of one or more suture lines, leading to abnormal growth of the skull and cranial deformity.

Hyoid Bone

A unique U-shaped bone located in the superior part of the neck.
Distinctively, it does not articulate directly with any other skull bones or other bones of the skeleton.
Its primary function is for muscle attachment, particularly for muscles of the tongue, pharynx, and neck.
Due to its isolated location, its breakage (e.g., in strangulation) can be a crucial indicator in forensic investigations.
- Forensic Application: If a medical examiner finds a crushed hyoid bone, it strongly suggests direct pressure was applied to the neck, as is typical in cases of strangulation, because it does not articulate with other bones that might transmit force indirectly.

The Vertebral Column

Consists of 24 individual vertebrae, plus the sacrum and the coccyx.
Divided into five distinct regions:
- Cervical (7 vertebrae, C1-C7)
- Thoracic (12 vertebrae, T1-T12)
- Lumbar (5 vertebrae, L1-L5)
- Sacral (5 fused vertebrae, forming the sacrum)
- Coccygeal (4 fused vertebrae, forming the coccyx)

Curvatures of the Vertebral Column

The vertebral column exhibits 4 natural curvatures that enhance its strength, flexibility, and shock absorption capabilities.
- Cervical curve: Posteriorly oriented (concave posteriorly).
- Thoracic curve: Anteriorly oriented (convex posteriorly).
- Lumbar curve: Posteriorly oriented (concave posteriorly).
- Sacrococcygeal curve: Anteriorly oriented (convex posteriorly).

Curvature Abnormalities

Kyphosis: An excessive posterior curvature of the thoracic region, often appearing as a