Bones

Long bones

These bones develop via endochondral ossification, a process in which the hyaline cartilage plate is slowly replaced. A shaft, or diaphysis, connects the two ends known as the epiphyses (plural for epiphysis). The marrow cavity is enclosed by the diaphysis which is thick, compact bone. The epiphysis is mainly spongy bone and is covered by a thin layer of compact bone; the articular ends participate in the joints.

The metaphysis is situated on the border of the diaphysis and the epiphysis at the neck of the bone and is the place of growth during development.

Some examples of this type of bones include:

• The humerus
• The fibula
• The tibia
• The metacarpal bones
• The metatarsal bones
• The phalanges
• The radius and ulna. 

Short bones

The short bones are usually as long as they are wide. They are usually found in the carpus of the hand and tarsus of the foot. 

In the short bones, a thin external layer of compact bone covers vast spongy bone and marrow, making a shape that is more or less cuboid. 

The main function of the short bones is to provide stability and some degree of movement.

Some examples of these bones are:

• The scaphoid bone
• The lunate bone
• The calcaneus
• The talus
• The navicular bone

Flat bones

In flat bones, the two layers of compact bone cover both spongy bone and bone marrow space. They grow by replacing connective tissue. Fibrocartilage covers their articular surfaces. This group includes the following bones:

• The skull bones
• The ribs
• The sternum
• The scapulae

The prime function of flat bones is to protect internal organs such as the brain, heart, and pelvic organs. Also, due to their flat shape, these bones provide large areas for muscle attachments. 

Irregular bones

Due to their variable and irregular shape and structure, the irregular bones do not fit into any other category. In irregular bones, the thin layer of compact bone covers a mass of mostly spongy bone.

The complex shape of these bones help them to protect internal structures. For example, the irregular pelvic bones protect the contents of the pelvis. 

Some examples of these types of bones include:

• The bones of the spine (i.e. vertebrae)
• The bones of the pelvis (ilium, ischium and pubis)

Sesamoid bones

Sesamoid bones are embedded within tendons. These bones are usually small and oval-shaped. 

The sesamoid bones are found at the end of long bones in the upper and lower limbs, where the tendons cross.

Some examples of the sesamoid bones are the patella bone in the knee or the pisiform bone of the carpus.

The main function of the sesamoid bone is to protect the tendons from excess stress and wear by reducing friction.

Learn the basics of the skeletal system with this interactive quiz.