Axial Skeleton: Bones, Divisions, and Development

Chapter 7: Axial Skeleton Introduction and Divisions

Chapter Objectives

• Describe the functions of the skeletal system and define its two major subdivisions (axial and appendicular).
• Identify the bones and bony structures of the skull, cranial suture lines, cranial fossae, and the openings in the skull.
• Discuss the vertebral column, including its regional variations, bony components, and curvatures.
• Describe the components of the thoracic cage.
• Discuss the embryonic development of the axial skeleton.

The Skeletal System: An Overview

• Framework: Forms the rigid internal framework of the body.

• Components: Consists of bones, cartilages, and ligaments.

• Bone Functions:

  • Supports the weight of the body.
  • Allows for body movements by providing attachment for muscles.
  • Protects internal organs.

• Cartilage Functions:

  • Provides flexible strength and support to structures like the thoracic cage, external ear, trachea, and larynx.
  • Unites adjacent bones or provides cushioning between them at joints.

• Ligament Functions:

  • Strong connective tissue bands that hold bones together at movable joints.
  • Serve to prevent excessive movements of a joint that could result in injury.

• Muscle and Tendon Relationship:

  • Muscles of the body are firmly attached to the skeleton via connective tissue structures called tendons.
  • As muscles contract, they pull on bones to produce body movements.
  • The skeleton is essential for fundamental actions such as standing, running, or feeding oneself.

• Bone Adaptations to Function:

  • Each bone has a particular function, leading to variations in size, shape, and strength.
  • For example, lower back and lower limb bones are thick and strong to support body weight.
  • The size of a bony landmark for muscle attachment is proportional to the strength of the attached muscle; enlarged landmarks resist strong pulling forces.
  • This highlights that both size and shape of bones are directly related to their function, making identification of bony landmarks crucial.

• Dynamic Nature of Bones:

  • Bones are dynamic organs that can modify their strength and thickness in response to changes.
  • Increased Strength/Thickness: Occurs with increased muscle strength (e.g., workout programs) or body weight (e.g., weight gain, running regimen, causing weight-bearing bones to thicken).
  • Reduced Strength/Thickness (Thinning): Occurs with decreased muscle strength or body weight (e.g., prolonged hospital stay, limb immobilization in a cast, weightlessness in outer space).
  • Even dietary changes, like eating only soft food, can lead to a noticeable decrease in the size and thickness of jaw bones.

7.1 Divisions of the Skeletal System

• Definition: The skeletal system includes all bones, cartilages, and ligaments that support and give shape to the body and its structures.
• Number of Bones: Adults typically have $206$ bones. Younger individuals have more bones because some fuse during childhood and adolescence.
• Primary Functions of the Skeleton:
  • Provides a rigid, internal structure to support body weight against gravity.
  • Provides a structure upon which muscles can act to produce body movements.
  • The lower skeleton is specialized for stability (e.g., walking, running).
  • The upper skeleton has greater mobility and range of motion (e.g., lifting, carrying objects, turning head/trunk).
  • Protective Function: Protects internal organs, including the brain, spinal cord, heart, lungs, and pelvic organs.
  • Storage Function: Serves as the primary storage site for important minerals like calcium and phosphate. Bone marrow stores fat and houses blood cell-producing tissue.

Major Divisions of the Skeletal System

• The skeleton is subdivided into two major divisions: the axial skeleton and the appendicular skeleton.

The Axial Skeleton

• Definition: Forms the vertical, central axis of the body.
• Components: Includes all bones of the head, neck, chest, and back (Figure 7.2).
• Functions:
  • Protects the brain, spinal cord, heart, and lungs.
  • Serves as an attachment site for muscles that move the head, neck, and back.
  • Provides attachment for muscles that act across the shoulder and hip joints to move their corresponding limbs.
• Adult Bone Count: Consists of $80$ bones.
  • Skull: Formed by $22$ bones.
  • Associated with the Head: An additional $7$ bones, including the hyoid bone and the $6$ ear ossicles (three small bones in each middle ear).
  • Vertebral Column: Consists of $24$ bones (vertebrae), plus the sacrum and coccyx.
  • Thoracic Cage: Includes the $12$ pairs of ribs and the sternum (flattened bone of the anterior chest).

The Appendicular Skeleton

• Definition: Includes all bones of the upper and lower limbs, plus the bones that attach each limb to the axial skeleton.
• Adult Bone Count: Contains $126$ bones.
• Note: Bones of the appendicular skeleton are covered in a separate chapter.

7.2 The Skull

• Cranium (Skull): The skeletal structure of the head that supports the face and protects the brain.
• Subdivisions:
  • Facial bones: Underliefacial structures, form the nasal cavity, enclose the eyeballs, and support the teeth of the upper and lower jaws.
  • Brain case (cranial vault): Surrounds and protects the brain, and houses the middle and inner ear structures.
• Adult Skull Characteristics: Consists of $22$ individual bones.
  • $21$ bones are immobile and united into a single unit.
  • The $22^{\text{nd}}$ bone, the mandible (lower jaw), is the only movable bone of the skull.

Anterior View of Skull (Figure 7.4)

• Dominated by the openings of the orbits and the nasal cavity, along with the upper and lower jaws and their teeth.
• Orbit: The bony socket that houses the eyeball and muscles for eye movement or upper eyelid opening.
  • Supraorbital margin: The upper margin of the anterior orbit.
  • Supraorbital foramen: A small opening near the midpoint of the supraorbital margin for passage of a sensory nerve to the skin of the forehead.
  • Infraorbital foramen: Located below the orbit, serving as the emergence point for a sensory nerve that supplies the anterior face below the orbit.
• Nasal Cavity: Divided into halves by the nasal septum.
  • Nasal Septum Formation:
    • Upper portion: Formed by the perpendicular plate of the ethmoid bone.
    • Lower portion: Formed by the vomer bone.
  • Nasal Conchae: Bony plates projecting from each lateral wall of the nasal cavity.
    • Inferior nasal concha: The largest, an independent bone of the skull.
    • Middle nasal concha: Part of the ethmoid bone, located just above the inferior concha.
    • Superior nasal concha: A third, much smaller bony plate, also part of the ethmoid bone, located above and behind the middle concha, lateral to the perpendicular plate.

Lateral View of Skull (Figure 7.5)

• Dominated by the large, rounded brain case above and the upper and lower jaws (with teeth) below.
• Zygomatic Arch: A bridge of bone separating the brain case and jaws, spanning from the cheek area to just above the ear canal.
  • Formation: Formed by the junction of two bony processes:
    • A short anterior component: the temporal process of the zygomatic bone (cheekbone).
    • A longer posterior portion: the zygomatic process of the temporal bone, extending forward from the temporal bone.
  • Serves as an origin for one of the major muscles that pulls the mandible upward during biting and chewing.
• Temporal Fossa: A shallow space on the lateral side of the brain case, above the level of the zygomatic arch. Contains muscles that act on the mandible during chewing.
• Infratemporal Fossa: Another space located below the level of the zygomatic arch and deep to the vertical portion of the mandible. Contains muscles that act on the mandible during chewing.

Bones of the Brain Case

• Function: Contains and protects the brain.
• Cranial Cavity: The interior space almost completely occupied by the brain.
• Calvaria (Skullcap): The rounded top of the skull, forming the superior boundary, along with the lateral and posterior sides.
• "Flat" Bones: Bones forming the top and sides of the brain case.
• Base of the Skull: The complex floor of the brain case, varying in depth and featuring numerous openings for cranial nerves, blood vessels, and the spinal cord.
  • Cranial Fossae: The base is subdivided into three large spaces, increasing in depth from anterior to posterior:
    • Anterior cranial fossa
    • Middle cranial fossa
    • Posterior cranial fossa
  • The shape and depth of each fossa correspond to the brain region it houses.
• Eight Bones of the Brain Case: Includes paired parietal and temporal bones, and unpaired frontal, occipital, sphenoid, and ethmoid bones.

1. Parietal Bone (Paired):

   • Forms most of the upper lateral side of the skull.
   • The right and left parietal bones join at the top of the skull.
   • Bounded anteriorly by the frontal bone, inferiorly by the temporal bone, and posteriorly by the occipital bone.

2. Temporal Bone (Paired) (Figure 7.7, 7.8):

   • Forms the lower lateral side of the skull.
   • Named "temporal" (time) as this area (temple) is where hair typically first turns gray.
   • Subdivisions:
     • Squamous portion: The flattened, upper portion.
     • Zygomatic process of the temporal bone: Projects anteriorly, forming the posterior portion of the zygomatic arch.
     • Mastoid portion: The posterior region.
     • Mastoid process: A large prominence projecting inferiorly from the mastoid portion, serving as a muscle attachment site (easily felt behind the earlobe).
     • Petrous portion: On the interior of the skull, forms the prominent, diagonally oriented petrous ridge in the floor of the cranial cavity. Houses the structures of the middle and inner ears.
   • **Important Landmarks (Figure 7.8):
     • External acoustic meatus (ear canal): Large opening on the lateral side of the skull associated with the ear.
     • Internal acoustic meatus: Opening inside the cranial cavity (medial side of the petrous ridge), connecting to the middle and inner ear cavities.
     • Mandibular fossa: Deep, oval-shaped depression on the external base of the skull, just anterior to the external acoustic meatus. Site where the mandible (lower jaw) joins the skull, forming part of the temporomandibular joint (TMJ).
     • Articular tubercle: Smooth ridge immediately anterior to the mandibular fossa. Both articulate to form the TMJ.
     • Styloid process: An elongated, downward bony projection posterior to the mandibular fossa, resembling a stylus. Attachment site for several small muscles and a ligament supporting the hyoid bone.
     • Stylomastoid foramen: Small opening between the styloid and mastoid processes, serving as the exit point for the cranial nerve supplying facial muscles.
     • Carotid canal: A zig-zag shaped tunnel providing passage for one of the major arteries supplying the brain. Its entrance is on the external base of the skull, anteromedial to the styloid process, eventually opening into the middle cranial cavity floor above the foramen lacerum.

3. Frontal Bone (Single) (Figure 7.5, 7.8b):

   • Forms the forehead.
   • Glabella: A slight depression at its anterior midline, between the eyebrows.
   • Forms the supraorbital margin of the orbit.
   • Supraorbital foramen: An opening near the middle of the supraorbital margin, providing passage for a sensory nerve to the forehead.
   • Brow ridges: Thickened, rounded ridges just above each supraorbital margin, behind the eyebrows (vary in size, generally larger in males).
   • Extends posteriorly inside the cranial cavity, forming both the roof of the orbit below and the floor of the anterior cranial cavity above.

4. Occipital Bone (Single) (Figure 7.9, 7.8):

   • Forms the posterior skull and posterior base of the cranial cavity.
   • External occipital protuberance: A small protrusion on its outside posterior midline, serving as an attachment site for a ligament of the posterior neck.
   • Superior nuchal line: Lateral to the protuberance; represents the most superior point for neck muscle attachment, with only the scalp covering the skull above these lines.
   • Foramen magnum: A large opening at the base of the skull, allowing passage for the spinal cord as it exits the skull.
   • Occipital condyle: Oval-shaped structures on either side of the foramen magnum. They form joints with the first cervical vertebra, supporting the skull on the vertebral column.

5. Sphenoid Bone (Single) (Figure 7.10, 7.5, 7.8):

   • A complex, single bone of the central skull, known as a "keystone" bone because it joins with almost every other bone of the skull.
   • Forms much of the base of the central skull and extends laterally to contribute to the sides of the skull.
   • Inside the Cranial Cavity:
     • Lesser wings: Right and left, resemble wings of a flying bird, form the prominent ridge marking the boundary between the anterior and middle cranial fossae.
     • Sella turcica ("Turkish saddle"): Located at the midline of the middle cranial fossa, named for its resemblance to Ottoman Turkish horse saddles.
       • Hypophyseal (pituitary) fossa: The rounded depression in the floor of the sella turcica, housing the pea-sized pituitary (hypophyseal) gland.
     • Greater wings: Extend laterally from the sella turcica, forming the anterior floor of the middle cranial fossa. Externally, they form a rectangular area anterior to the squamous portion of the temporal bone.
   • Inferior Aspect of the Skull:
     • Medial pterygoid plate: Two thin, vertically oriented bony plates, responsible for forming the posterior, lateral walls of the nasal cavity.
     • Lateral pterygoid plate: Somewhat larger, serving as attachment sites for chewing muscles that fill the infratemporal space and act on the mandible.

6. Ethmoid Bone (Single) (Figure 7.11, 7.12, 7.8b, 7.13):

   • A single, midline bone.
   • Forms the roof and lateral walls of the upper nasal cavity, the upper portion of the nasal septum, and contributes to the medial wall of the orbit.
   • Also forms a portion of the floor of the anterior cranial cavity inside the skull.
   • Within the Nasal Cavity:
     • Perpendicular plate: Forms the upper portion of the nasal septum.
     • Superior nasal concha and Middle nasal concha: Thin, curved projections extending into the nasal cavity from the lateral walls.
   • In the Cranial Cavity (Floor of Anterior Cranial Fossa):
     • Crista galli ("rooster's comb or crest"): A small upward bony projection at the midline, functioning as an anterior attachment point for one of the brain's covering layers.
     • Cribriform plate (cribrum = "sieve"): A small, flattened area to either side of the crista galli containing numerous small openings called olfactory foramina. These allow small nerve branches from the olfactory areas of the nasal cavity to pass to the brain.
   • Lateral portions: Located between the orbit and upper nasal cavity, forming the lateral nasal cavity wall and a portion of the medial orbit wall. Contain small, air-filled spaces known as ethmoid air cells, part of the paranasal sinus system.

Sutures of the Skull

• Definition: An immobile joint between adjacent bones of the skull, where the narrow gap is filled with dense, fibrous connective tissue.
• Structure: Long sutures of the brain case follow irregular, tightly twisting paths, serving to interlock bones and add strength for brain protection.
• **Major Sutures:
  • Coronal suture (Figure 7.5): Runs side-to-side across the skull, joining the frontal bone to the right and left parietal bones.
  • Sagittal suture (Figure 7.9): Extends posteriorly from the coronal suture, running along the midline at the top of the skull, uniting the right and left parietal bones.
  • Lambdoid suture (Figure 7.9): On the posterior skull, joins the occipital bone to the right and left parietal and temporal bones. Named for its upside-down "V" shape, resembling the Greek letter lambda ($ext{\Lambda}$).
  • Squamous suture (Figure 7.5): Located on the lateral skull, uniting the squamous portion of the temporal bone with the parietal bone.
• Pterion:
  • A small, capital-H-shaped suture line region at the intersection of four bones: the frontal bone, parietal bone, squamous portion of the temporal bone, and greater wing of the sphenoid bone.
  • It is the weakest part of the skull.
  • Located approximately $2$ finger widths above the zygomatic arch and a thumb’s width posterior to the upward portion of the zygomatic bone.

Disorders of the Skeletal System: Head and Traumatic Brain Injuries

• Impact: Major causes of immediate death and disability, with potential complications like bleeding and infections.
• Statistics (CDC 2010): Approximatley $30$ of all injury-related deaths in the United States are caused by head injuries.
• Common Causes: Mainly falls (most common in young children ($0-4$ years), adolescents ($15-19$ years), and the elderly (over $65$ years)). Also, automobile and motorcycle accidents.
• **Types of Skull Fractures from Blows:
  • Linear skull fracture: Fracture lines radiate from the point of impact (most common).
  • Comminuted fracture: The bone is broken into several pieces at the point of impact.
  • Depressed fracture: The fractured bone is pushed inward.
  • Contrecoup (counterblow) fracture: The bone at the point of impact is not broken, but a fracture occurs on the opposite side of the skull. (e.g., basilar fracture of the occipital bone damaging the carotid artery).
• Pterion Injury:
  • A strong blow to the lateral side of the head can fracture the bones around the pterion.
  • Clinically important because a major branch of an artery supplying the skull and brain coverings lies immediately deep to it.
  • Damage to this artery can cause an epidural hematoma (collection of blood) between the brain and the interior skull.
  • Accumulated blood puts pressure on the brain; symptoms may be delayed but untreated accumulation can lead to death within hours.

Facial Bones of the Skull

• Function: Form the upper and lower jaws, the nose, nasal cavity and nasal septum, and the orbit.
• Number of Bones: Includes $14$ bones: $6$ paired and $2$ unpaired.
  • Paired Bones: Maxilla, palatine, zygomatic, nasal, lacrimal, and inferior nasal conchae.
  • Unpaired Bones: Vomer and mandible.
• Ethmoid bone: Though classified with brain-case bones, it also contributes to the nasal septum and the walls of the nasal cavity and orbit.

1. Maxillary Bone (Maxilla) (Paired) (Figure 7.4, 7.14, 7.8a):

   • One of a pair, forming the upper jaw, much of the hard palate, the medial floor of the orbit, and the lateral base of the nose.
   • Alveolar process of the maxilla: The curved, inferior margin forming the upper jaw and containing the upper teeth (each anchored in a deep socket called an alveolus).
   • Infraorbital foramen: On the anterior maxilla, just below the orbit; exit for a sensory nerve supplying the nose, upper lip, and anterior cheek.
   • Palatine process: From each maxillary bone, joins at the midline to form the anterior three-quarters of the hard palate.
     • Hard palate: The bony plate forming the roof of the mouth and floor of the nasal cavity, separating the oral and nasal cavities.

2. Palatine Bone (Paired) (Figure 7.8a):

   • Irregularly shaped bones contributing small areas to the lateral walls of the nasal cavity and the medial wall of each orbit.

   • Horizontal plate: The largest region; plates from right and left palatine bones join at the midline to form the posterior quarter of the hard palate.

   • Best seen in an inferior view of the skull and hard palate.

   • Homeostatic Imbalances: Cleft Lip and Cleft Palate

     • Cause: Errors in embryonic development due to incomplete fusion of facial structures.
     • Cleft Lip:
       • A common developmental defect affecting approximately $1:1000$ births, mostly males.
       • Involves partial or complete failure of the right and left portions of the upper lip to fuse together, leaving a cleft (gap).
     • Cleft Palate:
       • A more severe defect affecting the hard palate, the bony structure separating the nasal and oral cavities.
       • Formed during embryonic development by the midline fusion of the horizontal plates of the palatine bones and the palatine processes of the maxilla bones.
       • Affects approximately $1:2500$ births, more common in females.
       • Results from a failure of the two halves of the hard palate to completely fuse at the midline, creating a gap between them.
       • This gap allows for communication between the nasal and oral cavities.
       • In severe cases, the bony gap extends into the anterior upper jaw where the alveolar processes of the maxilla bones also fail to join properly, often resulting in a concomitant cleft lip.
       • Complications: Makes suckling for nursing very difficult, leading to a risk of malnutrition for infants.
       • Treatment: Requires surgical repair to correct.

3. Zygomatic Bone (Cheekbone) (Paired) (Figure 7.4, 7.5):

   • Forms much of the lateral wall of the orbit and the lateral-inferior margins of the anterior orbital opening.
   • Temporal process of the zygomatic bone: A short process projecting posteriorly, forming the anterior portion of the zygomatic arch.

4. Nasal Bone (Paired) (Figure 7.11):

   • Two small bones that articulate with each other to form the bony base (bridge) of the nose.
   • Also support the cartilages that form the lateral walls of the nose.
   • These bones are susceptible to damage in "broken nose" injuries.

5. Lacrimal Bone (Paired) (Figure 7.4, 7.5):

   • Small, rectangular bones forming the anterior, medial wall of the orbit.
   • Lacrimal fossa: A shallow depression on the anterior portion of the lacrimal bone.
   • Nasolacrimal canal: Extends inferiorly from the lacrimal fossa. Lacrimal fluid (tears) drains from the medial corner of the eye into this canal, which then opens into the nasal cavity behind the inferior nasal concha.
   • Normal tear drainage is posterior in the nasal cavity; increased tear flow (crying, irritation) can cause some fluid to drain anteriorly, resulting in a runny nose.

6. Inferior Nasal Conchae (Paired) (Figure 7.13):

   • Form a curved bony plate projecting into the nasal cavity space from the lower lateral wall.
   • The largest of the nasal conchae, easily visible when looking into the anterior opening of the nasal cavity.

7. Vomer Bone (Single) (Figure 7.11, 7.8a):

   • An unpaired, triangular-shaped bone forming the posterior-inferior part of the nasal septum.
   • Best seen when looking into the posterior openings of the nasal cavity, where it forms the entire height of the nasal septum. A smaller portion is visible from the anterior nasal opening.

8. Mandible (Single, but Paired at Birth) (Figure 7.15, 7.5):

   • The lower jaw and the only movable bone of the skull.
   • Consists of paired right and left bones at birth, which fuse during the first year to form a single U-shaped bone.
   • **Structure:
     • Body: Horizontal portion.
     • Ramus of the mandible ("branch"): Vertically oriented posterior portion on each side.
     • Angle of the mandible: The outside margin where the body and ramus meet.
   • **Projections from the Ramus:
     • Coronoid process of the mandible: The more anterior, flattened upward projection, providing attachment for a biting muscle.
     • Condylar process of the mandible: The posterior upward projection, topped by an oval-shaped condyle.
       • Condyle of the mandible: Articulates with the mandibular fossa and articular tubercle of the temporal bone, forming the temporomandibular joint (TMJ) for mouth movements.
     • Mandibular notch: The broad U-shaped curve between the coronoid and condylar processes.
   • **Important Landmarks:
     • Alveolar process of the mandible: The upper border of the mandibular body, serving to anchor the lower teeth.
     • Mental protuberance ("chin"): The forward projection from the inferior margin of the anterior mandible.
     • Mental foramen: An opening on each side of the anterior-lateral mandible; the exit site for a sensory nerve supplying the chin.
     • Mylohyoid line: A bony ridge extending along the inner aspect of the mandibular body, where the muscle forming the floor of the oral cavity attaches.
     • Mandibular foramen: An opening on the medial side of the ramus, leading into a tunnel that runs the length of the mandibular body. Sensory nerves and blood vessels to the lower teeth enter here (target for dental anesthesia).
     • Lingula ("little tongue"): A small flap of bone located immediately next to the mandibular foramen, on the medial side of the ramus; attachment site for a ligament anchoring the mandible during mouth movements.

The Orbit (Figure 7.16)

• Definition: The bony socket that houses the eyeball and contains the muscles that move the eyeball or open the upper eyelid.
• Shape: Cone-shaped, with a narrow posterior region that widens toward a large anterior opening.
• Protection: The bony margins of the anterior opening are thickened and somewhat constricted to protect the eye.
• Orientation: The medial walls of the two orbits are parallel, but each lateral wall diverges from the midline at a $45$° angle, providing greater lateral peripheral vision.
• **Contributing Skull Bones (Seven):
  • Frontal bone: Forms the roof.
  • Zygomatic bone: Forms the lateral wall and lateral floor.
  • Maxilla: Primarily forms the medial floor, with a small contribution from the palatine bone.
  • Ethmoid bone and Lacrimal bone: Make up much of the medial wall.
  • Sphenoid bone: Forms the posterior orbit.
• **Openings at the Posterior Apex:
  • Optic canal: Allows passage of the optic nerve from the retina to the brain.
  • Superior orbital fissure: Lateral to the optic canal, elongated and irregularly shaped. Provides passage for the artery that supplies the eyeball, sensory nerves, and nerves supplying the eye movement muscles.

The Nasal Septum and Nasal Conchae (Figure 7.17, 7.11, 7.13)

• **Nasal Septum Components:
  • Bone: Upper portion formed by the perpendicular plate of the ethmoid bone; lower and posterior parts formed by the triangular-shaped vomer bone.
  • Cartilage: The anterior nasal septum is formed by the septal cartilage, a flexible plate filling the gap between the ethmoid's perpendicular plate and the vomer. It also extends outward to separate the nostrils (not present in dry skulls).
• Nasal Conchae: Superior, middle, and inferior nasal conchae are curved bony plates attached to the lateral wall on each side of the nasal cavity.
  • Function: They swirl incoming air, which helps warm and moisturize it before it reaches the lungs. This action, along with mucus from the nasal lining, traps dust, pollen, bacteria, and viruses.
  • **Types and Origin:
    • Inferior nasal concha: The largest, an independent bone of the skull.
    • Middle concha and Superior conchae: Both are formed by the ethmoid bone; the superior concha is the smallest and hidden above and behind the middle concha.

Cranial Fossae (Figure 7.6, 7.8b, 7.11)

• Definition: Three spaces on the floor of the cranial cavity, which increase in depth from anterior to posterior.
• Conformity: Their shape conforms to the shape of the brain regions they contain.
• Divisions: Each is divided at the midline by a significant bony structure or opening.

1. **Anterior Cranial Fossa:

   • Location/Depth: Most anterior and shallowest of the three.
   • Brain Region: Overlies the orbits and contains the frontal lobes of the brain.
   • Boundaries: Bounded anteriorly by the frontal bone (forming most of the floor). The lesser wings of the sphenoid bone form the prominent ledge marking the posterior boundary with the middle cranial fossa.
   • Midline Structures: Contains a portion of the ethmoid bone at the midline floor, consisting of the upward projecting crista galli and the cribriform plates to either side.

2. **Middle Cranial Fossa:

   • Location/Depth: Deeper than and situated posterior to the anterior fossa.
   • Boundaries: Extends from the lesser wings of the sphenoid bone anteriorly to the petrous ridges (petrous portion of the temporal bones) posteriorly.
   • Shape: The large, diagonally positioned petrous ridges give it a butterfly shape—narrow at the midline and broad laterally.
   • Brain Region: Occupied by the temporal lobes of the brain.
   • Midline Structure: Divided at the midline by the upward bony prominence of the sella turcica (part of the sphenoid bone).
   • **Openings for Blood Vessels and Cranial Nerves (Figure 7.8):
     • Optic canal: At the anterior lateral corner of the sella turcica, for passage of the optic nerve into the orbit.
     • Superior orbital fissure: A large, irregular opening into the posterior orbit, on the anterior wall of the middle cranial fossa, lateral to the optic canal. For nerves to the eyeball and associated muscles, and sensory nerves to the forehead.
     • Foramen rotundum ("round"): In the floor of the middle cranial fossa, inferior to the superior orbital fissure. Exit point for a major sensory nerve supplying the cheek, nose, and upper teeth.
     • Foramen ovale: Large, oval-shaped opening in the floor of the middle cranial fossa, for a major sensory nerve to the lateral head, cheek, chin, and lower teeth.
     • Foramen spinosum: Small opening, posterior-lateral to the foramen ovale. Entry point for an important artery supplying the brain's covering layers (identifiable by internal skull grooves).
     • Carotid canal: Zig-zag passageway for a major artery to the brain. Its entrance is on the inferior skull, anteromedial to the styloid process; it runs anteromedially and exits into the middle cranial cavity (near the posterior-lateral base of the sella turcica, above the foramen lacerum).
     • Foramen lacerum ("ragged" or "torn"): Irregular opening in the base of the skull, inferior to the carotid canal exit. It is an artifact in dry skulls as it is completely filled with cartilage in life; nothing passes through it.

3. **Posterior Cranial Fossa:

   • Location/Depth: Most posterior and deepest portion of the cranial cavity.
   • Brain Region: Contains the cerebellum of the brain.
   • Boundaries: Bounded anteriorly by the petrous ridges. The occipital bone forms the floor and posterior wall.
   • Midline Structure: Divided at the midline by the large foramen magnum ("great aperture"), which allows passage of the spinal cord.
   • **Openings:
     • Internal acoustic meatus (Figure 7.11): On the medial wall of the petrous ridge. Provides passage for the nerve from the inner ear's hearing and equilibrium organs, and the nerve supplying facial muscles.
     • Hypoglossal canal: At the anterior-lateral margin of the foramen magnum. Emerge on the inferior skull at the base of the occipital condyle, providing passage for an important nerve to the tongue.
     • Jugular foramen (Figure 7.8a): Large, irregularly shaped, immediately inferior to the internal acoustic meatus. Several cranial nerves exit the skull here, and it is the exit point for all venous return blood leaving the brain (venous structures form large curved grooves on inner walls of this fossa, terminating here).

Paranasal Sinuses (Figure 7.18)

• Definition: Hollow, air-filled spaces located within certain bones of the skull.
• Communication: All sinuses communicate with the nasal cavity (hence "paranasal" = next to nasal cavity).
• Lining: Lined with nasal mucosa.
• **Functions:
  • Reduce bone mass, thereby lightening the skull.
  • Add resonance to the voice. (This effect is noticeable during colds or sinus congestion, as swelling mucosa and excess mucus obstruct passageways, causing a different voice sound).
  • Blockage can lead to sinuses filling with fluid, causing pressure, pain, and discomfort.
• **Types (Named for the Skull Bone Each Occupies):
  • Frontal sinus (Figure 7.17): Located just above the eyebrows, within the frontal bone. Most anterior of the paranasal sinuses. May be divided bilaterally or fused into a single space.
  • Maxillary sinus: The largest sinus. Paired, located within the right and left maxillary bones, just below the orbits. Most commonly involved in sinus infections; difficult to drain due to its connection to the nasal cavity being high on its medial wall.
  • Sphenoid sinus: A single, midline sinus within the body of the sphenoid bone, anterior and inferior to the sella turcica. It is the most posterior of the paranasal sinuses.
  • Ethmoid air cell: Multiple small spaces separated by thin bony walls, located within the lateral aspects of the ethmoid bone. Situated on both sides of the ethmoid bone, between the upper nasal cavity and medial orbit, just behind the superior nasal conchae.

Hyoid Bone (Figure 7.19)

• Uniqueness: An independent bone that does not contact any other bone and is therefore not part of the skull.
• Structure/Location: A small, U-shaped bone located in the upper neck near the level of the inferior mandible, with the tips of the "U" pointing posteriorly.
• **Functions:
  • Serves as the base for the tongue above.
  • Attached to the larynx below and the pharynx posteriorly.
  • Held in position by a series of small muscles that attach to it from above or below.
  • Its movements (up/down or forward/back) are coordinated with movements of the tongue, larynx, and pharynx during swallowing and speaking.

7.3 The Vertebral Column

• Also Known As: Spinal column or spine (Figure 7.20).
• Structure: Consists of a sequence of vertebrae (singular = vertebra), each separated and united by an intervertebral disc. Together, these form the vertebral column.
• **Functions:
  • A flexible column that supports the head, neck, and body.
  • Allows for movements of the head, neck, and body.
  • Protects the spinal cord, which passes down the back through openings in the vertebrae.

Regions of the Vertebral Column

• Development: Originally develops as $33$ vertebrae, but this number is reduced to $24$ vertebrae plus the sacrum and coccyx in adults.
• Naming: Subdivided into five regions, with vertebrae named for their region and numbered in descending order.
  • Cervical Vertebrae (C1-C7): Seven vertebrae in the neck. C1 articulates with the occipital condyles of the skull. Each C vertebra articulates with the one below it.
  • Thoracic Vertebrae (T1-T12): Twelve vertebrae below the cervical region.
  • Lumbar Vertebrae (L1-L5): Five vertebrae in the lower back.
  • Sacrum: A single bone formed by the fusion of five sacral vertebrae. Also part of the pelvis. Fusion typically begins after age $20$ and is not completed until middle age.
  • Coccyx (Tailbone): Results from the fusion of four small coccygeal vertebrae. Fusion also begins after age $20$ and is not completed until middle age.
• Mammalian Anatomy Fact: Almost all mammals, regardless of body size (from shrew to giraffe), have $7$ cervical vertebrae. Giraffe cervical vertebrae can be $11$ inches tall.

Curvatures of the Vertebral Column (Figure 7.20)

• Adult Column: Does not form a straight line but has four curvatures along its length.
• Functions of Curves: Increase strength, flexibility, and ability to absorb shock.
  • When the load on the spine increases (e.g., carrying a heavy backpack), the curvatures deepen to accommodate the extra weight and then return when the weight is removed.
• **Types of Curvatures:
  • Primary Curves: Retained from the original fetal curvature, where the body is flexed anteriorly (concave anteriorly).
    • Thoracic curve: Involves the thoracic vertebrae.
    • Sacrococcygeal curve: Formed by the sacrum and coccyx.
  • Secondary Curves: Develop gradually after birth and are concave posteriorly (opposite to the fetal curvature).
    • Cervical curve: Develops as the infant learns to hold their head upright when sitting.
    • Lumbar curve: Develops later as the child begins to stand and walk. Generally deeper in females in adults.

Disorders of the Vertebral Column (Figure 7.21, 7.22)

• Causes: Developmental anomalies, pathological changes, or obesity can enhance normal vertebral column curves, leading to abnormal or excessive curvatures.
• Kyphosis (Humpback or Hunchback):
  • Excessive posterior curvature of the thoracic region.
  • Can develop due to osteoporosis, which weakens and erodes the anterior portions of the upper thoracic vertebrae, causing gradual collapse.
• Lordosis (Swayback):
  • Excessive anterior curvature of the lumbar region.
  • Most commonly associated with obesity or late pregnancy, where the accumulation of body weight in the abdominal region shifts the line of gravity anteriorly, causing an anterior tilt of the pelvis and a pronounced enhancement of the lumbar curve.
• Scoliosis:
  • An abnormal, lateral curvature, accompanied by twisting of the vertebral column.
  • Compensatory curves may also develop to maintain head position over the feet.
  • Most common vertebral abnormality among girls.
  • Cause: Usually unknown, but may result from weakness of back muscles, defects like differential growth rates in the vertebral column, or differences in lower limb length.
  • Tends to worsen during adolescent growth spurts.
  • Treatment: While most individuals do not require treatment, a back brace may be recommended for growing children. In extreme cases, surgery may be required.
• **Identification of Excessive Curves in Anatomical Position:
  • Kyphosis or Lordosis: Observe the vertebral profile from the side.
  • Scoliosis: Observe from behind. Have the person bend forward. If scoliosis is present, they will have difficulty bending directly forward, and the right and left sides of the back will not be level.

General Structure of a Vertebra (Figure 7.23, 7.24)

• Vertebrae vary by region, but all follow a similar structural pattern.
• A typical vertebra consists of a body, a vertebral arch, and seven processes.
  • Body: The anterior portion, supporting body weight. Progressively increases in size and thickness down the vertebral column. Adjacent vertebral bodies are separated and united by an intervertebral disc.
  • Vertebral Arch: Forms the posterior portion.
    • Pedicles: Right and left, form the lateral sides of the vertebral arch, anchored to the posterior side of the vertebral body.
    • Laminae: Right and left, form part of the posterior roof of the vertebral arch.
    • Vertebral Foramen: The large opening between the vertebral arch and body, containing the spinal cord.
    • Vertebral (Spinal) Canal: Formed by the alignment of the vertebral foramina of all vertebrae, providing bony protection and a passageway for the spinal cord.
    • Intervertebral Foramen: Formed by notches in the pedicles of adjacent vertebrae, providing an opening for a spinal nerve to exit the column.
  • **Seven Processes (Arising from the Vertebral Arch):
    • Transverse process: Paired, projects laterally from the junction point between the pedicle and lamina. Serves as muscle attachment sites.
    • Spinous process (vertebral spine): Single, projects posteriorly at the midline. Easily felt as a series of bumps under the skin down the back. Serves as a muscle attachment site.
    • Superior articular process: Paired, extends or faces upward. Joins with corresponding inferior articular processes from the next higher vertebra.
    • Inferior articular process: Paired, faces or projects downward.
    • Articular Joints: Formed by the junctions of superior and inferior articular processes, these are slightly movable joints. Their shape and orientation vary regionally, determining the type and range of motion.

Regional Modifications of Vertebrae

• Vertebrae exhibit characteristic size and structural features varying between regions.
  • Cervical vertebrae are smaller than lumbar vertebrae due to differences in weight support.
  • Thoracic vertebrae have sites for rib attachment.
  • Sacral and coccygeal vertebrae are fused into single bones.

1. **Cervical Vertebrae (Figures 7.25):

   • **Typical Features (e.g., C4 or C5):
     • Small body: Carries the least amount of body weight.
     • Bifid (Y-shaped) spinous process: Characteristic, except C7's spine is much longer (C3-C6 are short). C7 is prominent and palpable at the base of the neck.
     • Transverse processes: Sharply curved (U-shaped) to allow passage of cervical spinal nerves.
     • Transverse foramen: An opening in each transverse process, for an important artery supplying the brain to ascend the neck.
     • Superior and inferior articular processes: Flattened, largely face upward or downward, respectively.
   • **Modified Cervical Vertebrae:
     • C1 (Atlas): The first cervical vertebra, supporting the skull on top of the vertebral column.
       • Ring-shaped, lacking a body or spinous process.
       • Has an anterior arch and a posterior arch.
       • Transverse processes are longer and extend more laterally than any other cervical vertebra.
       • Superior articular processes are deeply curved for articulation with the occipital condyles of the skull.
       • Inferior articular processes are flat and face downward to join with the superior articular processes of the C2 vertebra.
     • C2 (Axis): The second cervical vertebra, serving as the axis for head rotation.
       • Resembles typical cervical vertebrae but is distinguished by the dens (odontoid process), a bony projection extending upward from the vertebral body.
       • The dens joins the inner aspect of the anterior arch of the atlas, held in place by the transverse ligament.

2. **Thoracic Vertebrae (Figure 7.26):

   • Body: Larger than cervical vertebrae.
   • Spinous process: Long with a pronounced downward angle, causing it to overlap the next inferior vertebra (a characteristic feature).
   • Articular facets: Superior facets face anteriorly, and inferior facets face posteriorly, determining the type and range of movements in the thoracic region.
   • **Rib Articulation Sites (Facets):
     • Costal facet: Most thoracic vertebrae have two on the lateral sides of the body for articulation with the head of a rib.
     • An additional facet is located on the transverse process for articulation with the tubercle of a rib.

3. **Lumbar Vertebrae (Figure 7.28):

   • Body: Carries the greatest amount of body weight, thus characterized by large size and thickness.
   • Transverse processes: Short.
   • Spinous process: Short, blunt, projects posteriorly.
   • Articular processes: Large, with the superior process facing medially and the inferior facing laterally.

4. **Sacrum and Coccyx (Figure 7.29):

   • Sacrum:
     • Triangular-shaped bone, thick and wide across its superior base (weight-bearing), tapering to an inferior, non-weight-bearing apex.
     • Formed by the fusion of five sacral vertebrae, a process typically beginning after age $20$.
     • Anterior surface (older adult): Visible lines of vertebral fusion appear as four transverse ridges.
     • **Posterior surface:
       • Median sacral crest: A bumpy ridge down the midline, remnant of fused spinous processes.
       • Lateral sacral crest: Remnant of fused transverse processes.
     • Sacral promontory: The anterior lip of the superior base.
     • Auricular surface: Roughened surface lateral to the promontory, joining with the ilium portion of the hipbone to form the immobile sacroiliac joints of the pelvis.
     • Sacral canal: A bony tunnel passing inferiorly through the sacrum, terminating at the sacral hiatus near the inferior tip.
     • Sacral foramina (anterior/posterior): Paired openings on the anterior and posterior surfaces connecting to the sacral canal, allowing exit for sacral spinal nerve branches.
     • Superior articular process of the sacrum: One on either side of the superior opening of the sacral canal, articulating with the inferior articular processes from the L5 vertebra.
   • **Coccyx (Tailbone):
     • Derived from the fusion of four very small coccygeal vertebrae.
     • Articulates with the inferior tip of the sacrum.
     • Not weight-bearing in a standing position, but may receive some body weight when sitting.

Intervertebral Discs and Ligaments of the Vertebral Column (Figure 7.24, 7.30, 7.31)

• Intervertebral Discs:
  • Function: Fibrocartilaginous pads that fill the gap between adjacent vertebral bodies, providing padding during weight-bearing and allowing for movement between vertebrae by changing shape.
  • Movement: While individual movement is small, the sum of these movements along the entire column allows for large body movements.
  • Size: Thin in the cervical region, thickest in the lumbar region (carrying the most body weight).
  • Height Contribution: Account for approximately $25$ of total body height between the top of the pelvis and the base of the skull.
  • **Components:
    • Anulus fibrosus: The tough, fibrous outer layer, forming a circle (anulus = "ring"). Firmly anchored to the outer margins of adjacent vertebral bodies.
    • Nucleus pulposus: The softer, more gel-like inner material with a high water content. Resists compression, important for weight-bearing. Its water content declines with age, leading to thinner discs, decreased body height, and reduced flexibility.
  • **Herniated Disc ("Ruptured" or "Slipped" Disc):
    • Occurs when the nucleus pulposus protrudes posteriorly through a weakened posterior anulus fibrosus (often due to injury or age, exacerbated by bending forward and lifting heavy objects).
    • The posterior bulging can compress a spinal nerve exiting through the intervertebral foramen, causing pain and/or muscle weakness in affected body regions.
    • Common Sites: L4/L5 or L5/S1 intervertebral discs often cause sciatica (widespread pain radiating from lower back down thigh and into leg). C5/C6 or C6/C7 injuries (e.g., from hyperflexion like whiplash) can cause pain in the neck, shoulder, and upper limb.
• Ligaments of the Vertebral Column: Unite adjacent vertebrae along both posterior and anterior aspects, resisting excessive bending movements.
  • Anterior longitudinal ligament: Runs down the anterior side of the entire vertebral column, uniting vertebral bodies. Resists excess backward bending. Crucial in the neck to prevent whiplash injuries (stretching/tearing during extreme posterior bending).
  • Supraspinous ligament: On the posterior side of the vertebral column, interconnects the spinous processes of thoracic and lumbar vertebrae. Supports the column during forward bending motions.
    • Nuchal ligament (nuchae = "nape" or "posterior neck"): An expansion of the supraspinous ligament in the posterior neck where cervical spinous processes are short. Attached to cervical spinous processes and extends to the midline base of the skull (external occipital protuberance). Supports the skull and prevents it from falling forward (larger/stronger in quadrupedal animals).
  • Posterior longitudinal ligament: Located inside the vertebral canal, anterior to the spinal cord, attached to the posterior sides of the vertebral bodies.
  • Ligamentum flavum ("yellow ligament"): Posterior to the spinal cord, consisting of a series of short, paired ligaments interconnecting the lamina regions of adjacent vertebrae. Contains many elastic fibers, allowing it to stretch and recoil.
  • Both the posterior longitudinal ligament and ligamentum flavum provide important support during forward bending.

Career Connection: Chiropractor

• Role: Health professionals specializing in musculoskeletal system problems (bones, muscles, ligaments, tendons, nervous system) using nonsurgical techniques.
• Conditions Treated: Neck pain, back pain, joint pain, headaches.
• Holistic Approach: Focus on overall patient health, providing counseling on lifestyle issues (diet, exercise, sleep), and referring to other specialists when needed.
• Diagnosis & Treatment Methods:
  • Drug-free, hands-on approach.
  • Physical exams, posture and spine assessments, X-ray imaging.
  • Primary technique: manual spinal manipulation to adjust spine or other joints.
  • May recommend therapeutic/rehabilitative exercises; some incorporate acupuncture, massage therapy, or ultrasound.
• Specializations: Sport injuries, neurology, orthopaedics, pediatrics, nutrition, internal disorders, diagnostic imaging.
• Education: Typically $3-4$ years of undergraduate education, followed by an accredited four-year Doctor of Chiropractic (D.C.) degree program, and passing a licensure examination.
• Job Outlook: Employment for chiropractors is expected to increase, partly due to the aging baby-boom generation.

7.4 The Thoracic Cage

• Also Known As: Rib cage; forms the thorax (chest) portion of the body (Figure 7.32).
• Components: Consists of $12$ pairs of ribs with their costal cartilages, and the sternum.
• Articulations: Ribs are anchored posteriorly to the $12$ thoracic vertebrae (T1-T12).
• Function: Protects the heart and lungs.

Sternum

• Definition: An elongated bony structure that anchors the anterior thoracic cage.
• **Three Parts:
  • Manubrium: The wider, superior portion.
    • Jugular (suprasternal) notch: A shallow, U-shaped border at the top, easily felt at the anterior base of the neck between the medial ends of the clavicles.
    • Clavicular notch: A shallow depression on either side at the superior-lateral margins, site of the sternoclavicular joint (between the sternum and clavicle).
    • The first ribs also attach to the manubrium.
  • Body: The elongated, central portion of the sternum.
  • Sternal angle: The junction between the manubrium and body, forming a slight bend. The second rib attaches here. It is an important landmark for identifying and counting lower ribs (since the first rib is hidden by the clavicle). Ribs $3-7$ attach to the sternal body.
  • Xiphoid process: The inferior tip of the sternum. Cartilaginous early in life but gradually ossifies starting during middle age.

Ribs

• Structure: Each rib is a curved, flattened bone contributing to the wall of the thorax.

• **Articulations:

  • Posteriorly: Articulate with the T1-T12 thoracic vertebrae.
  • Anteriorly: Most attach via their costal cartilages to the sternum.

• Count: There are $12$ pairs of ribs, numbered $1-12$ corresponding to the thoracic vertebrae.

• **Parts of a Typical Rib (Figure 7.27):

  • Head of the rib: The posterior end. Articulates primarily with the costal facet on the body of the same-numbered thoracic vertebra, and to a lesser degree, with the costal facet on the body of the next higher vertebra.
  • Neck of the rib: The narrowed region lateral to the head.
  • Tubercle of the rib: A small bump on the posterior rib surface, articulating with the facet on the transverse process of the same-numbered vertebra.
  • Body of the rib (shaft): The main remainder of the rib.
  • Angle of the rib: Just lateral to the tubercle, the point of greatest curvature. The angles of the ribs form the most posterior extent of the thoracic cage (align with the medial border of the scapula in anatomical position).
  • Costal groove: A shallow groove along the inferior margin of each rib for the passage of blood vessels and a nerve.

• **Rib Classifications (Figure 7.32):

  • Bony ribs do not extend completely to the sternum; instead, each ends in a costal cartilage (made of hyaline cartilage, extending for several inches).
  • Ribs are classified into three groups based on their relationship to the sternum:
    • True ribs (vertebrosternal ribs): Ribs $1-7$. Their costal cartilage attaches directly to the sternum.
    • False ribs (vertebrochondral ribs): Ribs $8-12$. Their costal cartilages do not attach directly to the sternum.
      • For ribs $8-10$, their costal cartilages attach to the cartilage of the next higher rib (e.g., rib $10$ attaches to rib $9$, which attaches to rib $8$, which attaches to rib $7$).
      • Floating ribs (vertebral ribs): The last two false ribs ($11-12$). These are short ribs that do not attach to the sternum at all; their small costal cartilages terminate within the musculature of the lateral abdominal wall.

7.5 Embryonic Development of the Axial Skeleton

• Timing: The axial skeleton begins to form during early embryonic development.
• Completion: Growth, remodeling, and ossification (bone formation) continue for several decades after birth until the adult skeleton is fully formed.
• Significance: Knowledge of these developmental processes is important for understanding potential abnormalities in skeletal structures.

Development of the Skull

• **Embryonic Tissues:
  • Week 3: Notochord (rod-like structure) develops dorsally. The overlying tissue forms the neural tube (brain and spinal cord).
  • Week 4: Mesoderm tissue on either side of the notochord thickens and separates into repeating block-like structures called somites.
  • Sclerotome: The most medial part of an enlarging somite. Consists of mesenchyme, an embryonic tissue that will give rise to fibrous connective tissues, cartilages, and bones.
• **Two Mechanisms of Skull Bone Development from Mesenchyme:
  • 1. Intramembranous Ossification: Produces the bones forming the top and sides of the brain case.
    • Mesenchymal cells accumulate locally and differentiate directly into bone-producing cells.
    • Fontanelle (Figure 7.33): Large areas of dense connective tissue that separate the growing brain case bones in the fetal skull, also known as "soft spots" on an infant’s head.
      • Functions: Important during birth by allowing the skull to change shape as it squeezes through the birth canal. After birth, they allow for continued growth and expansion of the skull as the brain enlarges.
      • Closure: The largest is on the anterior head (at the junction of frontal and parietal bones). Fontanelles decrease in size and disappear by age $2$.
      • Sutures: After fontanelle closure, skull bones remain separated at sutures (containing dense fibrous connective tissue) which allow for continued growth during childhood.
  • 2. Endochondral Ossification: Produces the facial bones and the floor of the brain case.
    • Localized mesenchymal cells accumulate and differentiate into hyaline cartilage models of future bones.
    • As the cartilage model grows, it is gradually converted into bone.
    • This is a slow process; the cartilage is not completely converted to bone until the skull reaches its full adult size.
• Skull Development from Birth to Childhood:
  • At birth: The brain case and orbits are disproportionally large compared to the jaws and lower face, reflecting the underdevelopment of the maxilla and mandible (which lack teeth) and the small sizes of the paranasal sinuses and nasal cavity.
  • During early childhood: The mastoid process enlarges; the two halves of the mandible and frontal bone fuse; the paranasal sinuses enlarge; the jaws expand as teeth appear. These changes contribute to rapid facial growth.

Development of the Vertebral Column and Thoracic Cage

• **Development of Vertebrae:
  • Mesenchyme cells from each sclerotome accumulate around the notochord.
  • These cells differentiate into a hyaline cartilage model for each vertebra.
  • The cartilage models grow and eventually ossify into bone through endochondral ossification.
  • The notochord largely disappears during vertebral development, but small areas persist as components of each intervertebral disc.
• **Development of Ribs and Sternum:
  • Ribs: Also develop from mesenchyme. Initially, they develop as part of the cartilage model for each vertebra, but in the thorax region, the rib portion separates from the vertebra by the eighth week.
    • The cartilage model of the rib then ossifies, except for the anterior portion, which remains as the costal cartilage.
  • Sternum: Initially forms as paired hyaline cartilage models on either side of the anterior midline, beginning during the fifth week of development.
    • The cartilage models of the ribs become attached to the lateral sides of the developing sternum.
    • Eventually, the two halves of the cartilaginous sternum fuse along the midline and then ossify into bone.
    • The manubrium and body ossify first, with the xiphoid process remaining as cartilage until late in life.

Homeostatic Imbalances: Craniosynostosis

• Definition: The premature closure (fusion) of a suture line in the skull. This error in normal development results in abnormal skull growth and head deformity.
• Causes: Produced by defects in the ossification process of skull bones or failure of the brain to properly enlarge. Genetic factors are involved, but the underlying cause is unknown.
• Prevalence: A relatively common condition, occurring in approximately $1:2000$ births, with males more commonly affected.
• **Types:
  • Primary Craniosynostosis: Involves the early fusion of one cranial suture.
    • Prevents additional enlargement of the cranial bones and skull along that particular suture line.
    • Continued growth of the brain and skull is diverted to other head areas, causing abnormal enlargement in those regions (compensatory growth).
    • Example (Scaphocephaly): Early disappearance of the anterior fontanelle and premature closure of the sagittal suture prevent growth across the top of the head. This is compensated by upward growth of the lateral skull bones, resulting in a long, narrow, wedge-shaped head. Accounts for approximately $50$ of craniosynostosis abnormalities. While the skull is misshapen, the brain usually has adequate room to grow, so there is typically no accompanying abnormal neurological development.
  • Complex Craniosynostosis: Results from the premature fusion of several sutures.
    • The amount and degree of skull deformity depend on the location and extent of the involved sutures.
    • These cases result in more severe constraints on skull growth, which can alter or impede proper brain growth and development.
• Treatment: Craniosynostosis cases are usually treated with surgery.
  • Physicians open the skull along the fused suture, allowing the skull bones to resume growth in that area.
  • In some cases, parts of the skull may be removed and replaced with an artificial plate.
  • Prognosis: The earlier surgery is performed after birth, the better the outcome. Most children continue to grow and develop normally and do not exhibit neurological problems after treatment.