MAMMALIAN SKELETON – AXIAL SKELETON, SKULL, RIBS, VERTEBRAL COLUMN & BONES

The mammalian skeleton is divided into two:

Axial and appendicular.

Axial skeleton is made up of the skull and the vertebral column. Appendicular skeleton is made up of the pelvic and pectoral girdles and limbs (hind limb and forelimbs).

The Axial Skeleton

This consists of the;

1. Skull,
2. The sternum,
3. Ribs,
4. The vertebral column.

The Skull

1. The skull is made up of cranium and facial bones.
2. The cranium; encloses and protects the brain.
3. It is made up of many bones joined together by immovable joints.
4. The facial bones consist of the upper and lower jaws.
5. At the posterior end of the cranium are two smooth rounded protuberances, the occipital condyles.
6. These condyles articulate with the atlas vertebra to form a hinge joint, which permits the nodding of the head.

Sternum and ribs

1. They form the rib-cage.
2. The rib-cage encloses the thoracic cavity protecting delicate organs such as the heart and lungs.
3. The ribs articulate with the vertebral column at the back and the sternum at the front.

The Vertebral Column

1. The vertebral column is made up of bones called vertebrae placed end to end.
2. The vertebrae articulate with one another at the articulating facets.
3. In between one vertebra and another is the cartilaginous material called intervertebral disc.
4. The discs act as shock absorbers and allow for slight movement.
5. Each vertebra consists of a centrum and a neural arch which projects into a neural spine.
6. The neural canal is the cavity enclosed by the centrum and the neural arch.
7. The spinal cord is located inside the canal.
8. The neural spine and other projections e.g. transverse processes serve as points of attachment of muscles.

Cervical Vertebrae

1. These are found in the neck region of a mammal.
2. The distinguishing feature is a pair of verte-braterial canals in the neural arch, through which the blood vessels of the neck pass.
3. Another feature is the structure of the transverse processes.
4. They are flattened out and are known as cervical ribs.
5. The fIrst cervical vertebra is known as the Atlas.
6. It has a large neural canal and no centrum.
7. The second cervical vertebra, is called axis.
8. The other five cervical vertebrae have no specific names.
9. They have the same structure.
10. The cervical vertebrae possess numerous processes for muscle attachment.

Thoracic Vertebrae

Each thoracic vertebra has a large centrum, a large neural canal, neural arch and a long neural spine that projects upwards and backward. There is a pair of prezygapophyses and postzygapophyses for articulation with other vertebra.

They have a pair of short transverse process. The thoracic vertebra also articulates with pair of ribs at tubercular and capitular facets.

Lumbar Vertebrae

1. Each lumbar vertebra has a large, thick centrum for support of the body.
2. It has a neural spine that projects upwards and forwards.
3. There is a pair of large transverse process that is directed forwards.
4. Above the prezygapophyses lies a pair of processes called metapophyses,
5. Below postzygapophyses lies the anapophyses.
6. Metapophyses and anapophysis serve for attachment of muscles of the abdomen.
7. In some mammals, there may be another process on lower side of centrum called hypapophysis also for muscle attachment.

Sacral Vertebrae

1. The sacral vertebrae are fused together to form a rigid bony structure, the sacrum.
2. The centrum of each vertebra is large, but the neural canal is narrow.
3. The neural spine is reduced to a small notch.
4. The transverse processes of the first sacral vertebra are large and wing-like
5. They are firmly attached to the upper part of the pelvic girdle.

Caudal Vertebrae

1. Human beings have only four of these vertebrae which are fused together to form coccyx.
2. Animals with long tails have many caudal vertebrae.
3. A typical caudal vertebra appears as a solid rectangular mass of bone.
4. The entire bone consists of the centrum only.

Appendicular Skeleton

The appendicular skeleton consists of the limbs and their girdles.

Bones of Fore-limbs

Pectoral girdle

1. Pectoral girdle is made of scapula, coracoid and clavicle.
2. A cavity known as glenoid cavity occurs at the apex of the scapula.
3. The humerus of the fore limb fits into this cavity.
4. The clavice is a curved bone connecting the scapular to the sternum.

Humerus

1. Humerus is found in the upper arm.
2. It articulates with the scapula at the glenoid cavity of the pectoral girdle and forms a ball and socket joint.

Ulna and radius

1. These are two bones found in the forearm.
2. The ulna has a projection called olecranon process and a sigmoid notch which articulates with the humerus.

Bones of hind limb

Pelvic Girdle

1. The pelvic girdle consists of two halves fused at the pubic symphysis.
2. Each half is made up of three fused bones:
3. the ilium,
4. ischium
5. Each half has cup-shaped cavity for the acetabulum for articulation with the head of the femur.
6. Between the ischium and pubis is an opening obturator foramen where spinal nerves, blood vessels and tough inflexible connective tissues pass.
7. The ilium, ischium and pubis are fused to form the innominate bone.

The Femur

1. The femur is the long bone joining the pelvic girdle and the knee.
2. The head of the femur articulates with acetabulum forming the ball and socket joint at the hip.
3. The femur has a long shaft.
4. At the distal end it has condyles that articulate with the tibia to form a hinge joint at the knee.
5. The patella covers the knee joint and prevents the upward movement of the lower leg.

Tibia and Fibula

1. The tibia is a large bone, and the fibula a smaller bone is fused to it on the distal part.
2. In humans the tibia and fibula are clearly distinguishable.