The Adult human skeleton consists of 206 bones (see diagram below) – at birth there are more bones but as development takes place into adulthood this decreases to the number indicated.
The bones are attached by ligaments (connective tissue) providing support and links in various parts of the body. The skeleton provides a strong framework for the body and movement and flexibility are provided by the joints. 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 2 types of Bone: 1) Compact 2) Cancellous – these provide shape to our body and protect vital organs.
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 type 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 the 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 layers 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 of fat.
One of the connective tissues is called cartilage. It consists of collagen and elastin fibres and is 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 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.