Axial Skeleton: Structure, Bones and Functions
Introduction
The human skeleton provides the structural framework of the body. It supports body weight, protects internal organs, provides attachment sites for muscles and works with joints to produce movement. Based on the location of bones, the skeleton is divided into the axial skeleton and the appendicular skeleton.
The axial skeleton forms the central axis of the body. It includes the bones of the head, neck and trunk, along with the small bones of the middle ear and the hyoid bone. Its major functions are to support the body, protect the brain, spinal cord, heart and lungs, and provide attachment for muscles involved in posture, breathing and movement.
In a typical adult, the axial skeleton consists of 80 bones. These are distributed among the skull, auditory ossicles, hyoid bone, vertebral column and thoracic cage.
1. Skull
The skull forms the bony framework of the head. It protects the brain, supports the structures of the face and provides openings for the passage of nerves and blood vessels. In the standard adult count, the skull contains 22 bones, divided into 8 cranial bones and 14 facial bones.
Cranial Bones
The cranial bones form the cranium, which encloses and protects the brain. They also provide attachment sites for muscles and help form the cavities that contain the brain and sensory structures.
The eight cranial bones are:
Frontal bone: Forms the forehead and the upper part of the eye sockets.
Parietal bones (2): Form much of the upper and lateral walls of the skull.
Temporal bones (2): Form parts of the sides and base of the skull and contain structures associated with hearing and balance.
Occipital bone: Forms the posterior part and base of the skull. It contains the foramen magnum, through which the brainstem continues into the spinal cord.
Sphenoid bone: Lies at the base of the skull and contributes to the floor of the cranium and the walls of the eye sockets.
Ethmoid bone: Contributes to the nasal cavity, the medial walls of the eye sockets and the anterior cranial floor.
The bones of the cranium are largely joined by immovable fibrous joints called sutures. These joints provide a strong protective enclosure for the brain.
Facial Bones
The 14 facial bones form the framework of the face. They support the teeth, contribute to the shape of the nasal and oral cavities and help form the eye sockets.
These include the paired maxillae, zygomatic bones, nasal bones, lacrimal bones, palatine bones and inferior nasal conchae, together with the unpaired mandible and vomer.
The maxillae form the upper jaw and part of the hard palate, while the mandible forms the lower jaw and is important for chewing and speech. Unlike most skull bones, the mandible moves at the temporomandibular joints.
Real-life example: When you chew food, the mandible moves relative to the skull, allowing the teeth to cut and grind food.
2. Auditory Ossicles
The auditory ossicles are six tiny bones located in the middle ear, with three in each ear. They are the smallest bones in the human body and form an important part of the mechanism of hearing.
The three auditory ossicles in each ear are the malleus, incus and stapes.
Malleus: The hammer-shaped bone attached to the tympanic membrane (eardrum). It receives vibrations from the eardrum.
Incus: The anvil-shaped bone positioned between the malleus and stapes. It transmits vibrations from the malleus to the stapes.
Stapes: The stirrup-shaped bone that transmits vibrations to the oval window of the inner ear.
When sound waves strike the eardrum, it vibrates. The auditory ossicles transmit and help amplify these vibrations, allowing them to reach the fluid-filled inner ear, where they contribute to the conversion of sound vibrations into nerve signals.
Important: There are three auditory ossicles in each ear, making a total of six bones in the adult axial skeleton.
3. Hyoid Bone
The hyoid is a small, U-shaped bone located in the anterior part of the neck, approximately between the mandible and the larynx. It is unusual because it does not directly articulate with any other bone.
Instead, muscles and ligaments suspend it in position. It provides attachment for muscles associated with the tongue, floor of the mouth, pharynx and larynx.
The hyoid bone contributes to swallowing, speech and movements of the tongue and larynx.
During swallowing, coordinated muscular activity moves the hyoid and associated structures, helping food pass from the mouth towards the pharynx.
Real-life example: The muscles attached to the hyoid help coordinate the movements needed to swallow water or food.
Exam tip: The hyoid is the only bone in the human body that does not directly articulate with another bone.
4. Vertebral Column
The vertebral column, commonly called the backbone or spine, forms the central supporting axis of the trunk. It extends from the skull to the pelvis and protects the spinal cord.
In adults, the vertebral column is counted as 26 bones because the five sacral vertebrae fuse to form one sacrum, and the coccygeal vertebrae usually fuse to form one coccyx.
Before fusion, the vertebral column typically develops from 33 vertebrae, although the number of coccygeal vertebrae can vary.
Regions of the Vertebral Column
The vertebral column is divided into five regions.
Cervical vertebrae — 7
Located in the neck. The first vertebra, the atlas (C1), supports the skull, while the axis (C2) has the dens, which assists in rotation of the head.
Thoracic vertebrae — 12
Located in the chest region. They articulate with the ribs and contribute to the formation of the thoracic cage.
Lumbar vertebrae — 5
Located in the lower back. Their large bodies are adapted to bear substantial body weight.
Sacrum — 1 bone
Formed by the fusion of five sacral vertebrae. It connects the vertebral column with the pelvic girdle through the sacroiliac joints.
Coccyx — 1 bone
The tailbone, usually formed by the fusion of four small coccygeal vertebrae. It provides attachment for pelvic floor muscles and ligaments.
General Structure of a Vertebra
A typical vertebra has a vertebral body, a vertebral arch and several processes.
The vertebral body is the weight-bearing anterior portion. The vertebral arch lies behind it, and together they enclose the vertebral foramen. The foramina of successive vertebrae form the vertebral canal, which houses and protects the spinal cord.
The spinous and transverse processes provide attachment points for muscles and ligaments. Articular processes help neighbouring vertebrae form joints.
Intervertebral discs made of fibrocartilage lie between most adjacent vertebral bodies. They absorb shock and permit limited movement between vertebrae.
Curvatures and Functions of the Vertebral Column
The normal vertebral column has cervical, thoracic, lumbar and sacral curvatures. These curves help distribute mechanical loads, maintain balance and absorb forces during walking and other activities.
Its main functions are to support the head and trunk, protect the spinal cord, provide attachment for muscles and allow movements such as bending and rotation.
Real-life example: When you bend forward to pick up a bag, the vertebral column allows controlled movement while maintaining support for the trunk.
5. Thoracic Cage
The thoracic cage, also called the rib cage, forms the bony framework of the chest. It protects the heart and lungs, provides attachment for muscles and assists in breathing by allowing changes in the dimensions of the thoracic cavity.
In the standard adult axial skeleton, the thoracic cage contains 25 bones: 24 ribs and one sternum. The 12 thoracic vertebrae are counted separately within the vertebral column.
Sternum
The sternum, or breastbone, is a flat bone located in the midline of the anterior chest. It consists of three main parts:
Manubrium: The upper portion, which articulates with the clavicles and the first pair of ribs.
Body: The longest portion, connected to most of the costal cartilages of the ribs.
Xiphoid process: The small inferior part, which provides attachment for certain muscles and ligaments.
The sternum helps protect the organs within the chest and provides attachment for the ribs through their costal cartilages.
Ribs
Humans typically have 12 pairs of ribs, numbered from the first to the twelfth pair.
True ribs — 7 pairs
The first seven pairs attach directly to the sternum through their own costal cartilages.
False ribs — 3 pairs (8–10)
These attach indirectly to the sternum through the costal cartilage of the rib above.
Floating ribs — 2 pairs (11–12)
These do not attach to the sternum anteriorly. Their anterior ends remain free within the abdominal wall musculature.
Functions of the Thoracic Cage
The thoracic cage protects the heart, lungs and major blood vessels. It also provides attachment for muscles of the chest, back and abdomen.
During inspiration, movements of the ribs and sternum increase the dimensions of the thoracic cavity, contributing to expansion of the lungs. During expiration, the thoracic cage returns towards its resting position as respiratory muscles relax during quiet breathing.
Real-life example: When you take a deep breath, your ribs move upward and outward, helping expand the chest cavity.
Bone Count of the Axial Skeleton
Component | Number of bones |
|---|---|
Cranial bones | 8 |
Facial bones | 14 |
Auditory ossicles | 6 |
Hyoid bone | 1 |
Vertebral column | 26 |
Ribs | 24 |
Sternum | 1 |
Total | 80 |
Important: The standard adult count is 80 axial bones. The sacrum and coccyx are counted as single bones each after fusion in the adult skeleton.
Clinical Significance
Knowledge of the axial skeleton is important for understanding common injuries and clinical conditions.
A skull fracture may damage structures protecting the brain. Vertebral fractures can affect spinal stability and may injure the spinal cord or nearby nerves. Rib fractures can cause severe pain during breathing and, in some cases, damage underlying organs.
Changes in the vertebral column, such as excessive curvature, can also affect posture and the distribution of mechanical forces across the spine.
Key Concepts for Examination
The axial skeleton forms the central axis of the body and contains 80 bones in a typical adult.
The skull has 22 bones: 8 cranial and 14 facial.
The six auditory ossicles transmit sound vibrations in the middle ears.
The hyoid bone does not directly articulate with another bone.
The adult vertebral column is counted as 26 bones.
The thoracic cage contains 24 ribs and one sternum.
True ribs are pairs 1–7, false ribs are pairs 8–12, and floating ribs are specifically pairs 11–12.
In summary, the axial skeleton forms the central supporting framework of the human body. Its bones protect vital organs, support the head and trunk, protect the spinal cord and provide attachment for muscles involved in posture, breathing and movement. Understanding its individual components and standard bone counts is essential for Human Anatomy and Physiology examinations.