Axial Skeleton: Definition, Components, Function

The axial skeleton is the section of a vertebrate’s skeleton that comprises the head and trunk bones. The human skeleton is made up of 80 bones and is divided into six sections: the skull (22 bones), middle ear ossicles, hyoid bone, rib cage, sternum, and spinal column. The axial and appendicular skeletons combine to produce the entire skeleton. The bones of the vertebrae, sacrum, coccyx, cranium, ribs, and sternum are also known as the axial skeleton.

The brain and other important organs are housed in flat bones. The axial skeletons of humans are the subject of this article; nonetheless, it is vital to understand the axial skeleton’s evolutionary lineage. The human axial skeleton is made up of 81 bones. It is the body’s medial core, connecting the pelvis to the torso and housing the appendix skeleton. With the exception of the skull, the bones of the skeleton weaken as it ages. The brain is protected by the skull, which stays sturdy. On this page, let us learn everything about the axial skeleton in detail. Read further to find more.

What is a Human Skeletal System?

The Human Skeletal System consists of a framework of bones and a few cartilages to form the endoskeleton in humans. This system has a significant role in providing shape to the body and facilitates movement in combination with muscles.

The skeletal system of humans can be basically divided into two parts (i) the axial skeleton and (ii) the appendicular skeleton.

The axial skeleton constructs the central axis of the body, while the appendicular skeleton comprises the limbs and girdles.

What is an Axial Skeleton?

“Axial skeleton comprises 80 bones distributed along the main central axis of the body. The skull, the vertebral column, sternum, and ribs constitute the axial skeleton.” (Source: NCERT).

Definition: The bones of the limbs are attached to the axial skeleton via girdles. The axial skeleton forms the main frame of the body in the longitudinal axis.

Axial Skeleton Diagram

Fig: The Axial Skeleton

Axial Skeleton Components

The main components of axial skeleton are:

1. The Skull
2. The Vertebral Column
3. Thoracic Cage

Fig: Axial Skeleton Components

1. Skull: The skull is a bony framework of the head that protects the brain. It acts as a cover to the brain and supports the structure of the face. The two parts of the skull are the cranium and facial bones. The skull of humans is said to be dicondylic. The skull consists of \(29\) bones, i.e. \(8\) in the cranium, \(14\) in the face, \(6\) ear ossicles and 1 hyoid bone.

(A) Cranium: This includes \(8\) bones: \(1\) Frontal bone, \(2\) Parietal bone, \(2\) Temporal bone, \(1\) Occipital bone, \(1\) Sphenoid, and \(1\) Ethmoid.

1. Frontal Bone: It is a flat bone that forms our forehead and the upper part of our eye sockets.
2. Parietal Bone: These are pairs of flat bones present behind the frontal bones on either side of the head.
3. Temporal Bone: This pair of irregular bones are present under the parietal bones on the lateral sides.
4. Occipital Bone: It is a flat bone present at the back of the skull. A large hole, the Foramen magnum, is at the base of the skull that allows the brain to continue into the spinal cord located at the back.
5. Sphenoid Bone: It is an unpaired and irregular bone that lies below the frontal bone and forms the depression or cavity called sella turcica in which the pituitary gland is present. It forms the base of the skull, forms walls and floors of the orbit, and spans the width of the head to articulate with other bones and provide rigidity to the structure of skull. Many muscles of mastication are also attached to the sphenoid bone.
6. Ethmoid Bone: The ethmoid bone is a small unpaired bone that divides the nasal cavity from the brain and is located in the midline of the anterior cranium.

Fig: Skull

(B) Facial Bones: This includes \(14\) bones: \(2\) Zygomatic bone, \(2\) Lacrimal bone, \(2\) Nasal bone, \(2\) Inferior nasal conchae bone, \(2\) Palatine, \(2\) Maxilla, \(1\) Vomer, and \(1\) Mandible.

1. Zygomatic bone: This pair of bones form the cheek of the face that articulates with sphenoid, temporal, and maxilla bones.
2. Lacrimal bone: These pairs of bones form the part of the medial wall of the orbit. They are the smallest bone on the face.
3. Nasal bone: These bones are two slender bones located at the front of the nose.
4. Inferior nasal conchae: These are two bones located in the nasal cavity; they increase the surface of the nasal cavity and thus increase the amount of air that can meet cavity walls.
5. Palatine: These pairs of bones are located at the back of the oral cavity and make part of the hard palate.
6. Maxilla: These are two bones comprised of the upper jaw and hard palate.
7. Vomer: It makes the rear side of the nasal septum.
8. Mandible: The mandible is the lower jawbone. This articulates with the temporal bones at the temporomandibular joints.

(C) Hyoid Bone: It is a U-shaped bone that is not attached to any other bone. Instead, it is a single bone present in the neck between the lower jaw and the soundbox. It supports the tongue and provides insertion to some tongue muscles.

(D) Ear Ossicles: The ear ossicles are small, flexible chains of three small bones, called Malleus (Hammer), Incus (Anvil), and Stapes (Stirrup), which amplifies the sound waves in middle ear.

2. Vertebral Column: It forms the main framework of the trunk, and it is known as the backbone or spine. It stretches from the base of the skull to the pelvis. The vertebral column in adults consists of \(33\) bones arranged serially called vertebrae. In adults it includes \(7\) Cervical vertebrae, \(12\) Thoracic vertebrae, \(5\) Lumbar vertebrae, \(5\) Fused Sacral vertebrae, and \(4\) Fused Coccygeal vertebrae. The human adult vertebral formula is \({{\rm{C}}_7}\;{{\rm{T}}_{12}}\;{{\rm{L}}_5}\;{{\rm{S}}_{(5)}}{{\rm{C}}_{(4)}}\)_.

1. Cervical: Cervical vertebrae are \(7\) in number. They are present in the neck region. The Atlas is the first cervical vertebrae and the Axis is the second cervical vertebrae. It is located right below the skull, and is attached to the skull with the help of condyles.
2. Thoracic: Thoracic vertebrae are \(12\) in number. It forms the vertebral spine and it supports the rib cage. It is present on the dorsal side of the thorax region and forms the dorsal part of the thoracic cage.
3. Lumbar: Lumbar vertebrae are 5 in number. They form the dorsal part of the abdomen, i.e. lower back. They are the largest and the strongest vertebrae.
4. Sacral: The five Sacral vertebrae are fused in the adult, forming one structure called the Sacrum, and articulates with the innominate bones of the pelvic girdle. In females, the sacrum is shorter and wider than the males.
5. Coccygeal: The four Coccygeal vertebrae are fused to form a curved triangular bone, called the coccyx. It is also known as tailbone. It is the terminal part of the vertebral column, and it ends in a triangle shape.

Fig: Vertebral Column

3. Thoracic Cage: It is also called rib cage. The thoracic cage consists of ribs and the sternum. It is a bony structure that protects the organs in the thoracic cavity. It includes \(25\) bones, i.e., \(1\) sternum and \(24\) ribs.

A. Ribs: They are a cage-like formation of thin, flat and curved bones. Ribs are mainly of three types:

1. True ribs: The first seven pairs are attached directly to the sternum.
2. False ribs: The 8^th, 9^th, and \({{\rm{10}}^{{\rm{th}}}}\) pairs of ribs are fused with \({{\rm{7}}^{{\rm{th}}}}\) rib, which in turn connect to the sternum.
3. Floating ribs: The \({{\rm{11}}^{{\rm{th}}}}\) and the \({{\rm{12}}^{{\rm{th}}}}\) pair of ribs are not attached to the sternum at all and remain.

B. Sternum: The sternum is also known as breastbone, and it is the long flat bone present in the midline of the thorax. It provides the articulation site for true ribs and pectoral girdle bones.

Fig: Thoracic Cage

Axial Skeleton Functions

The functions of Axial Skeleton are:

Did you know?

1. Stapes are the smallest bones in our body. It is located in the middle ear.
2. Hyoid is the only bone which is not connected to any other bone.

Axial Skeleton and Appendicular Skeleton – Comparison

Fig: Comparison of Axial and Appendicular Skeleton

Summary

The axial skeleton is a part of the human skeleton, comprising the skull, vertebral column, and thoracic cage. The axial skeleton protects the brain, spinal cord, heart, lungs, and kidneys. It provides structural support and provides articulation sites to the appendicular skeleton. The axial and appendicular skeleton together make a complete human skeleton. The axial skeleton is a combination of \(80\) bones.

Frequently Asked Questions (FAQs) on Axial Skeleton

Q.1. What bones are in the axial and appendicular skeleton?
Ans: The axial skeleton forms the vertical axis of the body and includes the bones of the head, neck, back, and chest of the body. It consists of \(80\) bones that include the skull, vertebral column, and thoracic cage. The appendicular skeleton consists of \(126\) bones and includes all bones of the pectoral girdle, the upper limbs, the pelvic girdle, and the lower limbs.

Q.2. What are the axial skeleton parts?
Ans: Axial skeleton can be divided into three parts: the skull, vertebral column, & the rib cage or thoracic cage.

Q.3. Where is the axial skeleton located?
Ans: The axial skeleton lies along the central longitudinal axis of the human body.

Q.4. Are ribs axial or appendicular?
Ans: Axial skeleton includes the bones of the skull, vertebrae, thoracic cage, hyoid bone and ear ossicles. Ribs and sternum are part of the thoracic cage. So, ribs are part of the axial skeleton.

Q.5. What are the functions of the axial skeleton?
Ans: The functions of the axial skeleton are:

1. Skull provides protection to the brain, forms the orbit of the eyes and provide structure to the face.
2. The vertebral column provides protection to the spinal cord and supports the head. It allows flexibility of the back and body.
3. The thoracic cage protects and supports the internal organs of the body such as the heart and lungs and some of the abdominal organs like the kidney and liver.

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