Skeletal system consists of a framework of bones and a few cartilages. Bone and cartilage are specialised connective tissues. Bone has a very hard matrix due to calcium salts in it and cartilage has slightly pliable matrix due to chondroitin salts.
In human beings, the skeletal system is made up of 206 bones and a few cartilages. It is grouped into two principal divisions: the axial and the appendicular skeleton.
Axial skeleton comprises 80 bones distributed along the main axis of the body. The skull, vertebral column, sternum and ribs constitute axial skeleton.
The skull is composed of two sets of bones: cranial and facial, that totals to 22 bones
Cranial bones are 8 in number. They form the hard protective outer covering, cranium for the brain. The facial region is made up of 14 skeletal elements which form the front part of the skull. A single U-shaped bone called hyoid is present at the base of the buccal cavity and it is also included in the skull.
Each middle ear contains three tiny bones: Malleus, Incus and Stapes, collectively called Ear Ossicles.
The skull region articulates with the superior region of the vertebral column with the help of two occipital condyles (dicondylic skull).
The vertebral column is formed by 26 serially arranged units called vertebrae and is dorsally placed. It extends from the base of the skull and constitutes the main framework of the trunk.
Each vertebra has a central hollow portion (neural canal) through which the spinal cord passes. First vertebra is the atlas and it articulates with the occipital condyles.
The vertebral column is differentiated into following regions starting from the skull:
- Cervical (7),
- Thoracic (12),
- Lumbar (5),
- Sacral (1-fused) and
- Coccygeal (1-fused) regions
The number of cervical vertebrae is seven in almost all mammals including human beings. The vertebral column protects the spinal cord, supports the head and serves as the point of attachment for the ribs and musculature of the back.
Sternum is a flat bone on the ventral midline of thorax.
There are 12 pairs of ribs. Each rib is a thin flat bone connected dorsally to the vertebral column and ventrally to the sternum. It has two articulation surfaces on its dorsal end and is hence called bicephalic.
First seven pairs of ribs are called true ribs. Dorsally, they are attached to the thoracic vertebrae and ventrally connected to the sternum with the help of hyaline cartilage.
The 8th, 9th and 10th pairs of ribs do not articulate directly with the sternum but join the seventh rib with the help of hyaline cartilage. These are called vertebrochondral (false) ribs.
Last 2 pairs (11th and 12th) of ribs are not connected ventrally and are therefore, called floating ribs. Thoracic vertebrae, ribs and sternum together form the rib cage.