Axial Skeleton

The skeletal system in mammals are grouped into two parts: Axial skeleton and Appendicular skeleton.


The axial skeleton is a skeletal system, which consists of the skull, vertebral column, the ribs and sternum.



THE SKULL

The skull is a bony structure, which forms the head of skeleton in most vertebrates. It is made up of several flat bones which are joined together by means of joints called structures. 

The skull is majorly divided into three parts. These are:

1. the cranium: it holds or contains the brain.
2. the facial skeleton: It supports the nose, eyes and the muscles of the cheek.
3. The jaws: They contain the teeth. They are made up of the upper jaw (maxilla) and lower jaw (mandible).

Functions of the skull

The skull performs the following functions:

1. It gives shape to the head.
2. It protects the brain.
3. It protects other vital organs in the head.
4. It bears the teeth.

THE VERTEBRAL COLUMN

The vertebral column is also known spinal column or backbone. It is the central supporting structure of the skeleton. It protects the spinal cord. It consists of five groups of bones, referred to as vertebrae(singular: vertebra) in mammals. The vertebrae are held one to the others with strong ligaments, having compressible cartilage pads called intervertebral discs between consecutive vertebrae. 

In mammals, the five different vertebrae are:

1. Cervical vertebrae - found in the neck region
2. Thoracic vertebrae - found in the chest region
3. Lumber vertebrae - found in the upper abdomen
4. Sacral vertebrae - found in the lower abdomen
5. Caudal vertebrae - found in the tail region

Vertebrates and their number in man, rabbit and rat

Vertebrae	Man	Rabbit	Rat
Cervical	7	7	7
Thoracic	12	12	13
Lumber	5	7	6
Sacral	5	3-4	4
Caudal	4	16	27-30
TOTAL	33	45-46	57-60

Features Common in All Vertebrae

The following are the features all vertebrae have in common:

• Neural canal: It serves as the passage of spinal cord.
• Neural spine: It projects upwards dorsally for the attachment of muscles.
• Centrum: It is a solid piece of bone marrow below the neural canal.
• Facet: It is a small, smooth, flat or slightly depressed area on a bone that is usually a point of contract with another bone.
• Transverse processes: They project from the sides of each vertebra for the attachment of muscles and ligaments.
• Zygaphophysis: These are articular surfaces for the articulation of successive vertebrae.

FEATURES OF EACH VERTEBRA

1. Cervical Vertebrae

 These are the vertebrae found in the neck region. In man, rabbit and rat, they are seven in number. The first cervical vertebra is known as atlas, the second is known as axis. The rest (third to seventh) bones are the normal cervical bones.

The Atlas

Features/Characteristics of the Atlas

The features of atlas vertebra include:

1. It has a large neural canal.
2. The centrum is absent.
3. It has a flat and broad transverse process.
4. The neural spine is very short or absent.
5. It has a vertebrarterial canal for the passage of blood vessels.

Function of the Atlas

The atlas makes the nodding of head possible.

The Axis

Features/Characteristics of the Axis

The features of the axis vertebra include:

1. It has a broad and flat centrum.
2. It articulates with the atlas through the odontoid process.
3. It has a vertebrarterial canal.
4. It has a large, flattened neural spine.
5. The transverse process is reduced to a pin-like structure.

Function of the Axis

The axis makes twisting of the head possible.

2. Thoracic Vertebrae

These are the vertebrae found in the chest region. They are twelve in man.

Features/Characteristics of Thoracic Vertebrae

These include:

1. They have a long and prominent neural spine which projects upwards and backwards.
2. A large cylindrical centrum is present.
3. There is a large neural canal and neural arch.
4. They have a pair of short transverse process.
5. There is presence of demifacets and articular surfaces for attachment of ribs.

Functions of Thoracic Vertebrae

1. They assist in breathing alongside with the ribs.
2. They aid the attachment of the ribs.
3. Muscles of the shoulder and back are attached to neural spines.

3. Lumber Vertebrae

Lumber vertebrae are found in the region of the upper abdomen. In man, they are five in number.

Features/Characteristics of Lumber Vertebrae

These include:

1. They have broad and flat neural spine.
2. They have large and thick centrum.
3. They have large, flattened transverse process.
4. They have well developed pre- and post zygapophyses.
5. They also have two extra-paired projections (anapophysis and metapophysis) for attachment of abdomianl muscles.

Functions of Lumber Vertebrae

1. They bear considerable weight of the body.
2. They provide attachment for abdominal muscles.

4. Sacral Verterbrae

These are found in the region of lower abdomen. In man, they are five in number. They fused together to form a structure called sacrum.

Features/Characteristics of the Sacral Vertebrae

1. They have a large centrum.
2. They have a narrow neural canal.
3. They have a neural spine which is reduced to a small notch.
4. The first sacral vertebra has a small neural canal which gradually becomes narrower in other vertebrae.
5. The first sacral vertebra has a pair of transverse process which is large and wing-like, but the others are attached to the muscles of the back.

Function of the Sacral Vertebrae

They are joined to the pelvic girdles to provide support, rigidity and strength.

5. Caudal Vertebrae

These are found in the tail region. They decrease in size from the sacrum backwards. In man, they are four in numbers. They are fused together to form a structure called coccyx which does not project from the body.

Features/Characteristics of Caudal Vertebrae

1. They have no neural canal.
2. They have no neural spine.
3. They appear as a solid, rectangular mass of bone.
4. They have no transverse process.

Funcctions of the Caudal Vertebrae

1. They provide attachment for tail muscles.
2. They support the tail.

THE RIBS

The ribs are long semi-circular rods connecting the thoracic vertebrae to the sternum. It is found in the chest region of mammals. In man, they are twelve in number. The bony cage formed by ribs protects the lungs and heart. It also assists in breathing.

A typical rib consists of a head, a neck and a shaft. The head fits between successive thoracic vertebrae. The shaft is the main part. Each rib articulates with the thoracic vertebrae by two processes: the capitulum and tuberculum. The capitulum articulates with facets of the center of two nearby vertebrae, while the tuberculum articulates with a transverse process.

The first seven pairs of ribs are called the true ribs because they are connected directly with the sternum in front (ventrally) by costalcartilages. The next five pairs of ribs are called false ribs because the eighth, ninth and tenth pair of ribs have a commonconnection with the sternum, each being attached to the costal cartilage of the rib above. The eleventh and twelfth pair are called floating ribs because they have no connection whatsoever with the sternum.

Categories: Biology, Skeletal System, Tissue and Supporting System