Classifications of Bones

The human adult skeleton is composed of 206 bones. There are two common classifications. Firstly, according to the site of its existence, bones can be divided into the axial and appendicular skeleton. The axial skeleton forms the central core or “axis” of the body’s bony framework, which consists of the skull and trunk bones. The skull bones include 29 bones which can be divided into the cranial bones, facial bones, and auditory ossicles, while the trunk bones include 51 bones which are made up of vertebrae, ribs, and sternum. The appendicular skeleton is attached or “appended” to the axial skeleton, which includes the upper and lower limbs. The upper limbs include 64 bones, while the lower limbs include 62 bones. Secondly, based on the different shapes, bones can be divided into long bones, short bones, flat bones and irregular bones. A long bone is an elongated tubular bone that has a shaft and two ends, which is longer than it is wide. Long bones have a thick outside layer of compact bone and a spongy inner cancellous bone containing bone marrow. In addition, both ends of a long bone contain an epiphyseal line. The epiphyseal line is a remnant of an area that contained hyaline cartilage that grew during childhood to lengthen the bone. Femur, humerus, and tibia are typical long bones. Some of the smallest including the metacarpals, metatarsals, and phalanges are classified as long bones. This is due to the shape of the bones, not their size. On the contrary, short bones are short and cuboidal, which are generally distributed in solid and flexible connecting parts. Examples of short bones are the carpals and tarsals - the wrist and foot bones. Flat bones consist of two compact bone plates separated by spongy bone and the main function of spongy bone is to provide protection to the vital organs and be a base for muscular attachment. The classic examples of flat bones are scapula, cranium, sternum, and ribs. Irregular bones such as vertebra and bones of the face are bones with various shapes. They primarily consist of cancellous bone, with a thin outer layer of compact bone. Some irregular bones which contain cavities are known as pneumatic bones, such as maxilla. Some irregular bones which embedded in some tendons or ligaments and are small and flat round, are called sesamoid bone, such as patella and sesamoid bone at the base of the first metatarsal of the foot.

 
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