The Skull and Axial Skeleton Review Flashcards

Composition and Divisions of the Skull

• The skull, or cranium, serves as the head's skeletal framework, providing essential support for facial structures and protecting the brain.
• It is divided into two primary segments: the facial bones and the brain case (also known as the cranial vault).
  • Facial Bones: These bones form the foundation for facial features, the nasal cavity, and the eye orbits, while also supporting the upper and lower teeth.
  • Brain Case: This rounded structure encapsulates and protects the brain and contains the internal structures of the middle and inner ears.
• Standard Adult Anatomy: The adult skull contains $22$ individual bones.
  • $21$ of these bones are immobile, fused together as a single unit.
  • The $22^{nd}$ bone is the mandible (lower jaw), which is the only mobile bone of the skull.

Anatomy of the Anterior and Lateral Skull

• Anterior View: This perspective is dominated by the eye orbits and the nasal cavity.
  • Orbit: The bony socket housing the eyeball and its associated muscles. Its upper boundary is the supraorbital margin.
  • Supraorbital Foramen: A small opening near the midpoint of the supraorbital margin that acts as a conduit for sensory nerves to the forehead.
  • Infraorbital Foramen: A point of emergence for sensory nerves located below the orbit, supplying the anterior face.
• Nasal Cavity and Septum: The cavity is split into two halves by the nasal septum.
  • The upper portion of the septum is the perpendicular plate of the ethmoid bone.
  • The lower portion is the vomer bone.
  • Projecting from the lateral walls are three bony plates called conchae: the inferior nasal concha (an independent bone), the middle nasal concha (part of the ethmoid), and the superior nasal concha (part of the ethmoid).
• Lateral View: This view highlights the brain case and the bridge known as the zygomatic arch.
  • Zygomatic Arch: Formed by the meeting of the temporal process of the zygomatic bone (anteriorly) and the zygomatic process of the temporal bone (posteriorly).
  • Temporal Fossa: A shallow space located above the zygomatic arch.
  • Infratemporal Fossa: A deeper space located below the zygomatic arch and deep to the mandible.

Detailed Bones of the Brain Case

• The brain case consists of $8$ bones: paired parietal and temporal bones, and unpaired frontal, occipital, sphenoid, and ethmoid bones.
• Parietal Bone: Forms the majority of the upper lateral side of the skull.
• Temporal Bone: Located on the lower lateral side. Key regions and landmarks include:
  • Squamous Portion: The upper flat region.
  • Mastoid Process: A prominent muscle attachment site felt behind the earlobe.
  • Petrous Portion: An internal ridge in the cranial floor housing middle and inner ear cavities.
  • External Acoustic Meatus: The external ear canal.
  • Mandibular Fossa: The site where the mandible articulates at the temporomandibular joint.
  • Styloid Process: A sharp projection serving as an attachment site for neck muscles and the hyoid bone ligament.
  • Carotid Canal: A zig-zag tunnel for the major artery supplying the brain.
• Frontal Bone: Forms the forehead and the roof of the orbit. It contains the glabella (depression between eyebrows) and supraorbital margins.
• Occipital Bone: Forms the posterior skull base.
  • Foramen Magnum: The massive opening for the spinal cord.
  • Occipital Condyles: Joints that connect the skull to the first cervical vertebra ($C1$).
  • External Occipital Protuberance: A midline bump for neck ligament attachment.
• Sphenoid Bone: Often called the "keystone" bone because it connects with nearly every other skull bone. Its features include the sella turcica ("Turkish saddle"), which contains the hypophyseal fossa for the pituitary gland.
• Ethmoid Bone: Forms the upper nasal cavity and part of the orbit. It features the crista galli (anterior attachment for brain coverings) and cribriform plates containing olfactory foramina for sense of smell.

Cranial Sutures and Clinical Disorders

• Sutures are immobile joints filled with dense fibrous tissue.
  • Coronal Suture: Joins the frontal bone to the parietal bones.
  • Sagittal Suture: Runs along the midline, joining the two parietal bones.
  • Lambdoid Suture: Joins the occipital bone to the parietal and temporal bones.
  • Squamous Suture: Connects the temporal and parietal bones.
  • Pterion: An H-shaped junction of the frontal, parietal, temporal, and sphenoid bones; it is the weakest point of the skull.
• Clinical Implications: Head injuries account for roughly $30\,\%$ of injury-related deaths in the United States ($2010$ data).
  • Fracture Types: Linear (radiating lines), comminuted (multiple pieces), depressed (bone pushed inward), and contrecoup (fracture on side opposite of impact).
  • Trauma to the Pterion: Can damage an underlying artery, leading to a life-threatening hematoma that exerts pressure on the brain.

Facial Bones and Developmental Anomalies

• The skull contains $14$ facial bones: $6$ paired (maxilla, palatine, zygomatic, nasal, lacrimal, inferior nasal concha) and $2$ unpaired (vomer, mandible).
• Maxilla: Forms the upper jaw and anterior $75\,\%$ of the hard palate. Each tooth is held in an alveolus within the alveolar process.
• Palatine Bone: Forms the posterior $25\,\%$ of the hard palate.
• Cleft Lip and Palate: Developmental defects where facial or palatal bones fail to fuse at the midline. Cleft lip occurs in approximately $1:1000$ births (more in males). Cleft palate occurs in $1:2500$ births (more in females) and can complicate infant nursing due to communication between oral and nasal cavities.
• Mandible: The only movable bone. Features include the mental protuberance (chin), coronoid process, and condylar process. The mandibular foramen provides access for dental anesthesia to the lower teeth.

Internal Structures and Sinuses

• Cranial Fossae: The internal floor is divided into the shallow anterior cranial fossa, the butterfly-shaped middle cranial fossa, and the deep posterior cranial fossa.
  • Openings in the Middle Fossa: Optic canal (optic nerve), superior orbital fissure, foramen rotundum, foramen ovale, foramen spinosum, and the foramen lacerum.
• Paranasal Sinuses: Air-filled spaces in the frontal, maxillary, sphenoid, and ethmoid bones. They lighten the skull and provide voice resonance. The maxillary sinus is the largest and most prone to infection due to difficult drainage.
• Hyoid Bone: A U-shaped bone in the upper neck that does not touch any other bone. It supports the tongue and moves during swallowing.

Anatomy and Curvatures of the Vertebral Column

• The vertebral column consists of vertebrae and intervertebral discs. It provides support, flexibility, and protection for the spinal cord.
• Divisions: The adult column includes $24$ vertebrae plus the sacrum and coccyx.
  • Cervical ($C1-C7$): The neck region.
  • Thoracic ($T1-T12$): Mid-back, associated with ribs.
  • Lumbar ($L1-L5$): Lower back.
  • Sacrum: Formed by the fusion of $5$ sacral vertebrae (starting at age $20$).
  • Coccyx: Formed by the fusion of $4$ small coccygeal vertebrae.
• Curvatures:
  • Primary Curves: The thoracic and sacrococcygeal curves, retained from fetal development.
  • Secondary Curves: The cervical (neck) and lumbar (lower back) curves, which develop after birth as infants gain motor control.
• Abnormal Curvatures:
  • Kyphosis: Excessive posterior curve of the thoracic region (humpback).
  • Lordosis: Excessive anterior curve of the lumbar region (swayback).
  • Scoliosis: Lateral curvature and twisting.

Characteristics and Modifications of Vertebrae

• Typical Vertebra Structure: Consists of a body (weight-bearing), a vertebral arch (pedicles and laminae), and seven processes (one spinous, two transverse, two superior articular, two inferior articular).
• Regional Modifications:
  • Cervical: Small bodies, bifid spinous processes, and transverse foramina for arteries.
  • Atlas ($C1$): Ring-shaped, lacks a body, supports the head.
  • Axis ($C2$): Features the dens (odontoid process) for head rotation.
  • Thoracic: Heart-shaped bodies with facets for rib articulation and long downward-pointing spinous processes.
  • Lumbar: Largest bodies to support maximum body weight, with short blunt spinous processes.
• Intervertebral Discs: Composed of an anulus fibrosus (outer ring) and nucleus pulposus (gel-like center). They provide padding ($25\,\%$ of total spinal height). Herniation occurs when the nucleus protrudes, potentially causing sciatica (radiating pain).

Ligaments and the Thoracic Cage

• Spinal Ligaments:
  • Anterior Longitudinal Ligament: Resists hyperextension (excessive backward bending).
  • Nuchal Ligament: Supports the head in the cervical region.
  • Ligamentum Flavum: Elastic ligament connecting laminae.
• Thoracic Cage: Composed of the sternum and $12$ pairs of ribs.
• Sternum: Features the manubrium, body, xiphoid process, and the sternal angle (junction of manubrium and body).
• Rib Classifications:
  • True Ribs ($1-7$): Attach directly to the sternum via costal cartilage.
  • False Ribs ($8-12$): Attach indirectly or not at all.
  • Floating Ribs ($11-12$): No anterior attachment.

Embryonic Development of the Skeleton

• Origin: Derived from mesoderm tissue that forms somites. Sclerotomes within somites produce mesenchyme, which develops into bone and cartilage.
• Ossification Methods:
  • Intramembranous Ossification: Formed from mesenchymal cells differentiating directly into bone (e.g., flat bones of the skull case).
  • Endochondral Ossification: Bone replaces a hyaline cartilage model (e.g., vertebrae, ribs, and floor of skull).
• Fontanelles: Large membrane-filled spaces ("soft spots") in newborn skulls that allow for birth and brain growth, closing by age $2$.
• Craniosynostosis: The premature fusion of skull sutures. Scaphocephaly (early fusion of sagittal suture) creates a long, narrow head shape.

Questions & Discussion

• Identify which of the following is part of the axial skeleton: shoulder bones, thigh bone, foot bones, or vertebral column? (Answer: vertebral column).
• What is a primary function of the axial skeleton: movement of the wrist, protection of nerves at the elbow, support of the body trunk, or movement of the ankle? (Answer: supports trunk of body).
• The axial skeleton forms the vertical axis of the body and consists of how many bones? (Answer: $80$ bones total; the skeleton described here includes the skull, vertebrae, and thoracic cage).
• Which bone belongs to the brain case: parietal, zygomatic, maxillary, or lacrimal? (Answer: parietal).
• The lambdoid suture joins the parietal bone to which other bone? (Answer: occipital).
• What constitutes a primary curvature of the vertebral column? (Answer: Remnants of original fetal curvature).
• Describe the structure of a typical lumbar vertebra. (Answer: Short, rounded spinous process and large, thick body).
• Which regions of the vertebral column have primary and secondary curves respectively? (Answer: Thoracic/Sacrococcygeal are primary; Cervical/Lumbar are secondary).