March 29, 2024

A bone is strong, hard and tough connective tissue mainly made of calcified material.

 

Composition of bone

A fully developed bone is hard and brittle. Bones can break, and a break in a bone is called fracture. The bone is however not the hardest substance in the body of man or other mammal. The dentine of the tooth is the hardest substance in the body of a mammal. A bone consists of organic and inorganic materials. The organic materials in a bone is gelatinous and accounts for the tenacity and elasticity of bones. The inorganic components of bone consist of magnesium, phosphorous calcium compounds and other mineral matter which give rigidity and hardness to bone. The organic matter of the bone is first formed and the laying down of inorganic substances takes place later.

The laying down of mineral substances in the bone is called ossification. When bones turn, it is the organic matter that is burnt off while the mineral salts remain as ash.The bones of young children are soft and flexible. They are not as brittle as those of the adults. The bones can be more brittle in the very old people. When bones are soaked in dilute acid, the mineral salts in them are dissolved out leaving the organic matter. The three bones in the ears are not ossified, they vibrate to transmit sound waves across the middle ear into the inner ear.

They are the smallest, softest and lightest bones in the body of man. They are called malleus, incus and stapes. They are arranged in the inner ear in that order.

 

The structure of a bone

Each bone has two heads and a shaft in between the heads. The head to the body is called proximal head and the other way from the body is called distal head. A long bone such as the femur of the thigh, the proximal head forms a joint which articulates with the hip bones, ilium, ischium and pubis, while the distal head forms a joint with tibia and fibula as the knee joint with the knee cap keeping them in position. The heads are not ossified and so remain soft throughout. They contain red marrow which manufactures red blood cells, white blood cells and blood platelets. The heads are therefore spongy and not compact so that a lot of capillaries enter them to supply nutrients and remove manufactured blood cells and waste products. The shaft is largely ossified, dense, compact and strong. For this reason, it is hollow at the centre.

 

This cavity contains the yellow marrow which has a rich supply of blood vessels and fat. In this yellow marrow some white blood cells are manufactured. The shaft has numerous tiny holes called Haversian canals. These canals have blood vessels and nerves which carry nutrients and impulses to the cells of the bones respectively. The heads are covered with cartilage while the shaft is covered by a tough fibrous tissue called periosteum.

 

Kinds of bones

The bones are classified according to their shape and length into four classes.

1) Long bones

The long bones are found in the limbs such as the humerus of the upper arm, radius and ulna of the lower arm in the fore limb. The femur in the thigh and tibia and fibula in the leg all of the hind limb. The ribs and clavicles are also long bones. The longest of these bones is the thigh bone (femur). All long bones are curved at their shaft in order to give them strength.

 

2) Short bones

Short bones are largely made up of spongy tissue covered externally by a thin coat of compact tissue. The bones of the vertebral column, carpals, metacarpals, tarsals, metatarsals, phallanges and patella are all short bones.

 

3) Flat bones

Flat bones are thin and plate-like. A flat bone is made up of two sheets of compact tissue with a narrow layer of spongy tissue in-between. Examples are the bones of the skull, scapula and ilium.

 

4) Irregular bones

These bones are peculiar in shape, structure and function. They are the small bones of the middle ear.

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