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Shapes and types of bones

Even though bones are created at a foetal stage of our life, their development finishes when human being reaches full sexual maturity. The bones themselves cannot move, thanks to their close cooperation with muscles they allow the body to move. They are, in fact, a passive element of the motor system. They are the scaffolding and support for the muscles, they protect the organs, store minerals and participate in the creation of blood.

Bones can have many shapes which depend on the functions they have within the body and the pressure they are put through by the muscles and other bones. When it comes to their shape, we divide bones into: long bones, short bones, flat bones and irregular bones.

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Ilustracja przedstawia cztery elementy szkieletu w kolorze szarym. W ten sposób ukazano różne rodzaje kości. Od lewej pionowo kość długa (tu: ramienna), long bone (humerus). Ma rurkowaty trzon, a z obu stron grubsze nasady. Kolejna kość to kość płaska (tu: łopatka), flat bone (shoulder bone). Jest szeroka i spłaszczona, a z lewej ma grube wyrostki. Obok kość różnokształtna w formie rombu z otworem w środku. To jeden z kręgów, irreagular bone (vertebra). Po prawej krótka kość dłoni (tu: nadgarstka), short bone (bones of wrist) . Ramka wskazuje nadgarstek, a w nim czworokątne kości krótkie, short bone (bones of wrist).

Physical composition of bones

Long bones are composed of diaphysis and epiphysis on both sides. The diaphysis is composed of cortical bone, whereas the epiphysis is composed of cancellous (spongy) bone. Cortical bone is surrounded by the marrow cavity filled with jelly‑like substance – bone marrowbone marrowbone marrow. In the diaphysis the marrow consists mainly of fat, whereas in the epiphysis it is red and participates in the production of blood - it produces certain blood cells.

On the outside the diaphysis is covered with a membrane made of connective tissue – periosteumperiosteumperiosteum. This is where we found blood vessels and neural cells which are responsible for innervation of the bone. On the inside, the periosteum is surrounded by bone cells which participate in the development and the regeneration of the bone. Thanks to them broken bones mend, and the elements of the bone are changed every 10 years. Cylindrical build of the diaphysis allows the bones to be impervious to be crushed or torn apart, whereas the sponge‑like structure allows bones to be sturdy and rigid, despite having a small mass.

Chemical composition of bones

Mechanical features of a bone are the result of not only its physical build, but also of its chemical structure. Stroma of the bone tissue decides on theses features. It includes:

• organic compounds, mainly proteins (collagen fibres), which make bones elastic,

• minerals, mainly calcium carbonate and calcium phosphate, which make bones hard.

In children, the bone matrix is dominated by proteins, which makes the bones more elastic and harder to break. As we grow old, the ratio of inorganic to organic compounds changes. Bones of adults and older people lose minerals (they become demineralized), they also have less collagen, which makes them easier to break.

Types of bone connections

Bones of a human can be connected in a mobile or immobile form. Immobile bone connections are mainly the stitches, which make it impossible for the bones to move. Mobile bone connections are the joints.

Joint is formed by bones that fit with each other because of the shape of their epiphysis. The place where the bones touch are covered with cartilage that absorbs shocks and lessens friction. The outer part of the epiphysis is covered by a joint socket, which prevents the bones from moving and prevents the joint from becoming dislocated. The joint socket is filled with synovial fluid which protects the bones from grinding away and getting damaged.

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Ilustracja przedstawia schematycznie trzy rodzaje stawów. Kości w kolorze szarym, żółte strzałki wskazują możliwość ruchu. Od lewej: staw kulisty, inaczej panewkowy, spherical joint (acetabular), tu: staw biodrowy, hip-joint. W środku staw siodełkowaty, saddle pond, tu: nadgarstkowo – śródręczny, the wrist and the thumb joint. Z prawej staw zawiasowy, Hinge Joint, tu: kolanowy, knee-joint.

Joints can vary depending on the place where they are in the body and their functions.
We divide them in accordance with:

• the number of bones that create the joint: simple joints are composed of 2 bones (shoulder joint), complex – that consist of more than 2 bones (carpal joint);

• the shape of the joint surfaces and the scope of movements: ball and socket joints (shoulder), which allow movements in many directions and hinge joints (elbow), which are only able to flex and extend.

Summary

• Bones can have various shapes, depending on what their function is.

• Physical and chemical composition decides on the mechanical properties of bones.

• Minerals makes bones more hard, whereas proteins make them more elastic.

• Periosteum covers the outside of the bones, nourishes them and helps them regenerate.

Keywords

bones, cranial sutures, shapes of the bones

Glossary