Study Notes: The Axial Skeleton (Skeletal System II)

The Axial Skeleton

Overview of the Skeletal System

The human skeletal system is divided into two major divisions: the axial skeleton and the appendicular skeleton. The axial skeleton forms the central axis of the body and provides support and protection for the brain, spinal cord, and thoracic organs.

• Axial Skeleton: Includes the skull, vertebral column, and thoracic cage (rib cage).

• Appendicular Skeleton: Includes the limbs and girdles (shoulder and pelvic).

The axial skeleton consists of 80 bones and can be broken down into three main parts:

• Skull

• Vertebral Column

• Thoracic Cage (Rib Cage)

Levels of Organization

In anatomy and physiology, biological organization is hierarchical, ranging from atoms and molecules up to the entire organism. The skeletal system is studied at the organ and organ system levels.

• Cell → Tissue → Organ → Organ System → Organism

The Skull

General Features

The skull is a complex structure formed by 22 bones. It is subdivided into the brain case (cranial bones) and the facial bones. Most of these bones are flat bones, and many are joined by immovable joints called sutures.

• Brain Case: Encloses and protects the brain.

• Facial Bones: Form the structure of the face, support teeth, and anchor muscles for facial expression.

Features of the Skull

• Orbits: Bony sockets that house the eyes.

• Nasal Cavity: Divided by the nasal septum, formed by the vomer and ethmoid bones.

• Sutures: Immovable joints between skull bones (e.g., coronal, sagittal, lambdoid, squamous).

The Brain Case (Cranial Bones)

The brain case consists of eight bones that surround and protect the brain. These bones are:

• Frontal

• Parietal (2)

• Temporal (2)

• Occipital

• Sphenoid

• Ethmoid

The cranial cavity, formed by these bones, encloses the brain and has a volume of approximately 1200–1500 cm³ in humans.

Parietal Bones

• Form the upper lateral sides of the skull.

• Joined at the sagittal suture along the midline.

Temporal Bones

• Form the lower lateral sides of the skull.

• Contain structures for hearing (external auditory meatus).

• Include the mastoid and styloid processes.

Frontal Bone

• Forms the forehead and the roof of the orbits.

• Single bone, derived from two ossification centers that fuse during development.

Occipital Bone

• Forms the posterior base of the skull.

• Contains the foramen magnum, through which the spinal cord passes.

• Articulates with the first cervical vertebra (atlas).

Sphenoid Bone

• Located at the base of the skull, central to the cranial floor.

• Called the "keystone" bone because it articulates with all other cranial bones.

• Features include the greater and lesser wings, sella turcica (houses the pituitary gland).

Ethmoid Bone

• Single, midline bone forming the roof and lateral walls of the nasal cavity and part of the orbit.

• Contains the cribriform plate (for olfactory nerves) and perpendicular plate (part of nasal septum).

Facial Bones

The facial bones form the structure of the face and include 14 bones. Major facial bones include:

• Maxilla (2): Paired bones forming the upper jaw and part of the hard palate.

• Nasal (2): Small, paired bones forming the bridge of the nose.

• Zygomatic (2): Cheekbones.

• Mandible: Single bone forming the lower jaw.

• Lacrimal (2), Palatine (2), Inferior Nasal Concha (2), Vomer: Other facial bones.

Maxilla

• Forms the upper jaw, supports the upper teeth.

• Alveolar process contains tooth sockets.

• Forms part of the hard palate.

• In children, the maxilla contains sockets for both deciduous and permanent teeth.

Nasal Bones

• Small, paired bones forming the bridge of the nose.

Summary Table: Cranial and Facial Bones

Bone	Location	Main Features
Frontal	Forehead, roof of orbits	Single bone, forms anterior cranium
Parietal (2)	Upper lateral sides	Paired, joined at sagittal suture
Temporal (2)	Lower lateral sides	Contains ear structures, mastoid process
Occipital	Posterior base	Foramen magnum, occipital condyles
Sphenoid	Central cranial floor	Keystone bone, sella turcica
Ethmoid	Roof of nasal cavity, part of orbit	Cribriform plate, perpendicular plate
Maxilla (2)	Upper jaw	Alveolar process, hard palate
Nasal (2)	Bridge of nose	Small, paired bones

Key Terms and Definitions

• Suture: An immovable joint between skull bones.

• Foramen Magnum: Large opening in the occipital bone for the spinal cord.

• Alveolar Process: Ridge containing tooth sockets in the maxilla and mandible.

• Cribriform Plate: Part of the ethmoid bone, allows passage of olfactory nerves.

• Sella Turcica: Depression in the sphenoid bone housing the pituitary gland.

Example Application

Understanding the structure and features of the axial skeleton is essential for identifying injuries, congenital anomalies, and diseases affecting the skull and vertebral column. For example, fractures of the maxilla can affect dental alignment and facial symmetry, while damage to the occipital bone may impact the spinal cord.

Additional info: The notes focus on the skull and its components; further study would include the vertebral column and thoracic cage for a complete overview of the axial skeleton.