The average skeleton contains 206 bones, each one attached and held together by the connective tissue of a ligament, which supports and links different parts of the body. The skeletal system provides a strong framework for the body, whilst the joints hold the bones together and offer flexibility and movement. Bones are required by the human body for structure and movement, and provide sites of attachment for the muscles which pull on the bones to generate movement when they contract.
The skeleton needs to be strong as it bears the weight of all the other tissues of the body. By supporting the weight of the body, it enables us to stand. There are two types of bone, compact and cancellous, which help to give shape to our body, and protect vital organs and tissues.
As well as providing a physical structure for the body, the bones develop platelets, which are blood clotting cells, red and white blood cells in the bone marrow of the cancellous bone tissue, and store important minerals such as calcium and phosphorous. Depending on the levels of calcium in the blood, the bones release or absorb it to maintain stable levels. This process is known as mineral homeostasis and it is controlled by hormones.
When fully developed, the bone consists of water, calcium salts and organic matter. The tissue of the bone is made from cells called osteoblasts. There are two main types of bone tissue, both of which are found in every bone.
Compact bone tissue is the hard section of the bone that forms the main shaft of long bones. It is known as dense bone and provides the firm framework for the body. The osteocytes (mature bone cells derived from osteoblasts) are living bone cells that are located in rings around a central haversian canal. It is through here that nerves, blood and lymphatic vessels pass to nourish the bone and take away waste.
The spongy bone tissue is called cancellous bone, which is lighter, with an open ‘air bubble’ appearance. It is found at the ends of long bones and the centre of other bones. It is made up of a web of thin processes of bone with the spaces filled by red bone marrow. The blood vessels run through every layer of cancellous bone, transporting nutrients and oxygen.
Each bone is made up of two types of marrow: red and yellow. The red marrow creates red blood cells, whilst the yellow marrow is found in the central cavities of long bones. The yellow marrow is a store for fat.
One of the connective tissues is called cartilage. It consists of collagen and elastin fibres and is a flexible and durable tissue. It cushions and absorbs shock and throughout the body there are three types: hyaline, fibrous and elastic. The hyaline cartilage covers the articular bones surfaces, fibrous cartilage is strong and rigid and found between the spinal discs, and elastic cartilage is flexible and found in the auditory canal and pinna of the ear.
Ligaments are a white fibrous connective tissue that links bones together at the joint. It is dense and inelastic but is flexible enough to allow the joint to move freely within a safe range.
Tendons attach muscles to the periosteum of a bone. They enable bones to move when skeletal muscles contract, and are tough fibrous cords of tissue.
Bone Shapes
Long, short, flat, irregular and sesamoid are all classifications of bone.
Long bones are the bones of the limbs, except for the wrist and ankles. They have a long shaft called the diaphysis, with ends called the epiphysis. Smooth cartilage covers the articular surfaces of the shaft endings, and a flat plate of hyaline cartilage called the epiphyseal cartilage grows between the diaphysis and epiphysis. As this cartilage grows, it turns into bone in order for the bone to grow in length.
Short bones are cube-shaped and commonly found in the wrist or ankle. Flat bones tend to resemble plates with broad surfaces such as the sternum and scapulae.
Irregular bones vary in shape and include the vertebrae and some facial bones. Sesamoid bones are small and rounded and are embedded in a tendon. An example of this is the patella (knee cap) which is found in the quadriceps femoris tendon.
| Bone Shape | Examples | Consistency |
| Long Bones | Arms and legs | Mainly made up of compact bone |
| Short Bones | Wrists and ankles | Mainly made up of spongy bone |
| Flat Bones | Rib and skull bones | Made of a layer of spongy bone, sandwiched between two thin layers of compact bone |
| Irregular Bones | The butterfly-shaped vertebrae | Unusual shaped bones, and do not fit into other groups |
The skeleton is divided into two parts: the axial skeleton consisting of the skull, the vertebral column, the ribs and the sternum and the appendicular skeleton consisting of the shoulder girdle, the upper limbs, the pelvic girdle and the lower limbs.
The skull is divided into the face and the cranium, which in total are made up of 22 bones, 14 in the face and 8 cranial bones. The facial bones form our features and support structures such as the eyes.
The mandible, or jawbone, is the only moveable bone and is the largest and strongest bone in our face. It holds the lower teeth in place and it moves to allow for the chewing of food.
All our facial bones are arranged in pairs, apart from the mandible and vomer. This is what makes our face symmetrical. For example, we have two zygomatic bones that form our cheekbones and the outside of our eye sockets on either side of our face.
The septum found below the nasal bones is made of cartilage, not bone. The cranium protects the brain with thin, flat, curved bones. These are joined together by connective tissue which becomes immovable after childhood.