The human skeleton of an adult usually consists of around 206 bones, depending on the counting of sternum (which may alternatively be included as the manubrium, body of sternum, and the xiphoid process).[1] It is composed of 270 bones at the time of birth,[2] but later decreases to 206: 80 bones in the axial skeleton and 126 bones in the appendicular skeleton. 172 of 206 bones are part of a pair and the remaining 34 are unpaired.[3] Many small accessory bones, such as sesamoid bones, are not included in this. The precise count of bones can vary among individuals because of natural anatomical variations.
As a person ages, some bones fuse, a process which typically lasts until sometime within the third decade of life. Therefore, the number of bones in an individual may be evaluated differently throughout a lifetime. In addition, the bones of the skull and face are counted as separate bones, despite being fused naturally. Some reliable sesamoid bones such as the pisiform are counted, while others, such as the hallux sesamoids, are not.
Individuals may have more or fewer bones than the average (even accounting for developmental stage) owing to anatomical variations. The most common variations include sutural (wormian) bones, which are located along the sutural lines on the back of the skull, and sesamoid bones which develop within some tendons, mainly in the hands and feet. Some individuals may also have additional (i.e., supernumerary) cervical ribsorlumbar vertebrae. Amputations or other injuries may result in the loss of bones. Complete bone fractures may split one bone into multiple pieces. Other genetic conditions may result in abnormally higher (e.g. polydactylyorconjoined twins) or lower (e.g. oligodactyly) counts of bones.
The axial skeleton, comprising the spine, chest and head, contains 80 bones. The appendicular skeleton, comprising the arms and legs, including the shoulder and pelvic girdles, contains 126 bones, bringing the total for the entire skeleton to 206 bones. Infants are born with about 270 bones[4] with most of it being cartilage, but will later fuse together and decrease over time to 206 bones.
The bony pelvis consists of two parts: anterior and posterior. Note that the posterior bones comprise the lower parts of the spine, where they are primarily counted to.
