The Shapes & Characteristics of Bones

Bones are responsible for providing the framework of the body, as well as enabling the body to create movements. The skeletal system is responsible for many other functions as well. The skeletal system, comprised of 206 bones, contains four main types of bones (long bones, short bones, flat bones and irregular bones) and all of these bones share some general characteristics with one another.

The Shapes of Bones
Bones fall within one of four categories when it comes to shape; long bones, short bones, flat bones and irregular bones.

Long Bones
Long bones, primarily composed of compact bone, are longer than they are wide with a slightly curved shaft and hyaline cartilage present on both ends of the bone. Long bones are capable of supporting and withstanding great deals of weight and pressure. The femur (found in the thigh) is a good example of a long bone.

Short Bones
Short bones, primarily composed of spongy bone coated with a layer of compact bone, are usually equal or close to equal in length as they are width. The short bones are limited in their range of motion and glide across one another. A good example of short bones includes the bones found within the wrist.

Flat Bones
Flat bones are as the name implies, thin and flat and are composed of a layer of spongy bone in between two layers of compact bone. Flat bones serve to protect soft organs and offer a smooth surface, optimal for muscle attachment. An example of flat bones could include the scapula (shoulder blade) or the sternum (breast bone).

Irregular Bones
Bones that cannot be classified as long bones, short bones, or flat bones are classified as irregular bones. These bones vary greatly in size and shape from one another. Some good examples of irregular bones include the vertebrae and most of the cranial bones.

Characteristics of the Bones
All bones have some general characteristics in common with one another. Long bones, for example, all contain a layer of dense bone along the outermost surface. Dense bone is often referred to as compact bone. The bone is formed in a cylinder surrounding a space known as the medullary cavity or the marrow cavity. This cavity, as it’s second name implies, houses the marrow within the bones. At the ends of the long bones, the innermost space of the bone contains what is known as spongy bone, or cancellous bone. Spongy bone is must more loosely organized and lighter than compact bone. A certain type of spongy bone, known as diploe can be found between two layers of compact bone in the cranium and serves to absorb the force behind a blow to the skull and protect the brain.

Children contain what is known as an epiphyseal plate between the medullary cavities of the epiphysis and diaphysis. The epiphyseal plate is composed of hyaline cartilage and is responsible for allowing the growth of bones. Adults do not contain an epiphyseal plate, as they are still growing, however, a small line can be seen in the bones where the epiphyseal plate used to be. This line is referred to as the epiphyseal line.

The shaft of the long bone is referred to as the diaphysis, while the heads at each end of the bone are referred to as the epiphysis. The epiphysis serves to strengthen the joint while the diaphysis acts as a lever. The epiphysis is commonly found as part of a joint and is coated with a layer of hyaline catrilage in order to help cushion and protect the bones within the joint from damaging one another. This hyaline cartilage, also known as articular cartilage, is combine with a special lubricating fluid between the bones to make the interactions of the bones of joints the least damaging as possible. The articular cartilage not only protects the bones from one another, but it provides a much easier, more fluid range of motion than if the bones were to rub directly against one another.

The bones, although they may appear solid, contain small holes known as nutrient foramina which allow the passage of blood vessels into the bone. The external surface of the bone is covered by a layer known as the periosteum. The periosteum is composed of two layers; a thich fibrous collagen located on the outermost side, and a layer of bone forming cells on the innermost side. Some of the collagen fibers found within the outer layer of the periosteum are continuous with the tendons responsible for adhering bone to muscle. Other fibers continue into the matrix of the bone and are referred to as perforating (Sharpey) fibers. The bones serve as a durable and strong site of attachment for this reason. The inner layer of the bone is composed of endosteum; reticular connective tissue that contains cells that both dissolve and deposit osseous tissue.

References
Saladin, Kenneth S.. Anatomy & physiology: the unity of form and function. 5th ed. Dubuque: McGraw-Hill, 2010. Print.

Bone

Discovery Health “Bone Shapes

What is the Epiphyseal Plate?