Did you know that at birth your skeleton as a new-born had more than 300 bones? As we grow and age, these bones grow together and fuse into larger bones, leaving us with 206 bones when adults. The skeletal system is one of the longest units on our anatomy and physiology course because it is important for therapists such as sports massage therapists to understand what exactly they are working on and what can go wrong in injury.
The bones in our bodies can be broken down into five types:
Long bones are longer than wider and are the major bones of the limbs. Long bones grow more than the other types of bone throughout childhood and are responsible for our height as adults. A hollow medullary cavity is found in the centre of the long bones and serves as a storage area for bone marrow. The femur, tibia, fibula, metatarsals, and phalanges are just some examples of Long bones.
Short bones are roughly as long as they are wide and are often shaped as cube or round. The carpal bones of the wrist and the tarsal bones of the foot are examples of short bones.
Flat bones vary greatly in size and shape; however, they have the common feature of being very thin in one direction. Because they are thin, flat bones do not have a medullary cavity like the long bones. The frontal, parietal, and occipital bones of the cranium along with the ribs and hip bones are all examples of flat bones.
Irregular bones have a shape that does not fit the pattern of the long, short, or flat bones. The vertebrae, sacrum, and coccyx of the spine—as well as the sphenoid, ethmoid, and the zygomatic bones in the cheek of the skull—are all irregular bones.
The sesamoid bones are formed after birth inside of tendons that run across joints. Sesamoid bones grow to protect the tendon from stresses and strains at the joint and can help to give a mechanical advantage to muscles pulling on the tendon. The patella of the knee and the carpals pisiform are the only sesamoid bones counted as part of the 206 bones of the body. Other sesamoid bones can form in the joints of the hands and feet but are not present in all people.
Flat bones follow the process of intramembranous ossification where the young bones grow from a primary ossification centre in fibrous membranes and leave a small region of fibrous tissue in between each other. In the skull these soft spots are known as fontanels which give the skull flexibility and room for the bones to grow. Bone slowly replaces the fontanels until the individual bones of the skull fuse together to form a rigid adult skull.
Long bones follow the process of endochondral ossification where the diaphysis grows inside of cartilage from a primary ossification centre until it forms most of the bone. The epiphyses then grow from secondary ossification centres on the ends of the bone. A small band of hyaline cartilage remains in between the bones as a growth plate. As we grow through childhood, the growth plates grow under the influence of growth and sex hormones, slowly separating the bones. At the same time the bones grow larger by growing back into the growth plates. This process continues until the end of puberty, when the growth plate stops growing and the bones fuse permanently into a single bone. The vast difference in height and limb length between birth and adulthood are mainly the result of endochondral ossification in the long bones.
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