The Axial Skeleton

The axial skeleton acts as the main structural framework of the human body, offering support and protection to vital organs. It serves as the central core to which the appendicular skeleton (limbs and girdles) is connected. Made up of 80 bones, the axial skeleton includes the skull, vertebral column, and thoracic cage (ribs and sternum). These parts are essential for maintaining posture, enabling movement, and safeguarding critical organs like the brain, spinal cord, heart, and lungs.

1.0The Skull

The skull is made up of 22 bones in total:

• 8 cranial bones that surround and protect the brain
• 14 facial bones that shape the face and support facial structures

Cranial Bones

Frontal Bone

• Forms the forehead, roofs of the eye sockets (orbits), and most of the front part of the cranial floor.
• Frontal sinuses lie deep within the bone.

Parietal Bones

• Form the sides and roof of the cranial cavity. 
• Separated on top of the skull by the sagittal suture.

Temporal Bones

• These bones form the lower sides of the cranium and contribute to the cranial floor.
• They also house the external auditory meatus, commonly known as the ear canal.

Occipital Bone 

• Forms the posterior part and most of the cranial base  
• Foramen magnum passes through this bone  
• Occipital condyles are on either side of the foramen magnum that connect with the first vertebrae

Sphenoid Bone

• The “keystone” of the cranial floor because it holds together all of the other cranial bones  
• The hypophyseal fossa is a depression in the pituitary gland.

Facial Bones

Sutures

An immovable joint is found only between skull bones, which hold skull bones together.

2.0Vertebral Column

• Also called the spine or backbone  
• Composed of vertebrae  
• Functions as a strong, flexible rod that can rotate and move in all directions  
• Encloses and protects the spinal cord  
• Supports the skull Point of attachment for ribs, pelvic girdle, and back muscles

Regions of the Vertebrae

• 7 cervical vertebrae in the neck  
• 12 thoracic vertebrae  
• 5 lumbar vertebrae supporting the lower back  
• 1 sacrum (consists of 5 fused sacral vertebrae)  
• 1 coccyx (consists of 4 fused coccygeal vertebrae)

Intervertebral Discs

• Lie in between the vertebrae from the 2nd cervical vertebrae to the sacrum  
• Form strong joints, permit movement, and absorb vertical shock

Sacrum

• The sacrum is a triangular-shaped bone formed by the fusion of five sacral vertebrae, typically between the ages of 16 and 30.
• It acts as a strong foundation for the pelvic girdle.
• There are four sacral foramina on both the anterior and posterior surfaces, which allow the passage of nerves and blood vessels.
• The sacral canal, a continuation of the vertebral canal, runs through the sacrum. Its lower opening is called the sacral hiatus, while the sacral promontory is a prominent projection on the upper border of the sacrum.

Coccyx

• The triangular shape formed by the fusion of 4 coccygeal vertebrae  
• The top articulates with the sacrum

3.0Thoracic Cage

• The thoracic vertebrae's sternum, costal cartilage, ribs, and bodies form the bony cage. 
• It is enclosed and protects the organs of the thoracic cavity and upper abdominal cavity. Provides support for the bones of the shoulder and upper limbs.

Sternum

• Flat, narrow bone located in the centre of the anterior thoracic wall, Also known as the breastbone,  Consists of three parts.
• The manubrium is the upper part, articulating with the clavicles and first and second ribs.  
• The body is the largest and middle part, articulating directly or indirectly with the 2nd-10th ribs.  
• The xiphoid process is the smallest part, with some abdominal muscles attached to it.

4.0Ribs

• The rib cage consists of twelve pairs of ribs that form the sides of the thoracic cavity.
• Each rib connects posteriorly to its matching thoracic vertebra.
• The first seven pairs, known as true ribs, have a direct anterior attachment to the sternum through costal cartilage.

False Ribs

• The 8th to 12th pairs of ribs are known as false ribs. Their costal cartilages either connect indirectly to the sternum or do not connect at all.
• Specifically, the cartilages of ribs 8 to 10 are connected to each other and eventually join the cartilage of the 7th rib to reach the sternum.

Floating Ribs

• The 11th and 12th pairs of ribs are also called floating ribs  
• The costal cartilage at their anterior ends does not attach to the sternum at all  
• Attach only posteriorly to the thoracic vertebrae