Because they function as an organ.

Because they are full of living cells and actually create red blood cells for your circulatory system in their marrow. Bones are more than structural. http://www.mnsu.edu/emuseum/biology/humananatomy/skeletal/skeletalsystem.html

1) "In biology, an organ (Latin: organum, "instrument, tool") is a group of tissues that perform a specific function or group of functions." Source and further information: http://en.wikipedia.org/wiki/Organ_%28anatomy%29 "Biological tissue is a collection of interconnected cells that perform a similar function within an organism." Source and further information: http://en.wikipedia.org/wiki/Biological_tissue "Bones function to move, support, and protect the body, produce red and white blood cells and store minerals." Source and further information: http://en.wikipedia.org/wiki/Bone So bones are organs because the definition of an organ applies to them. 2) Here some more details "Bones have eight main functions: - Protection — Bones can serve to protect internal organs, such as the skull protecting the brain or the ribs protecting the heart and lungs. - Shape — Bones provide a frame to keep the body supported. - Blood production — The marrow, located within the medullary cavity of long bones and the interstices of cancellous bone, produces blood cells in a process called haematopoiesis. - Mineral storage — Bones act as reserves of minerals important for the body, most notably calcium and phosphorus. - Movement — Bones, skeletal muscles, tendons, ligaments and joints function together to generate and transfer forces so that individual body parts or the whole body can be manipulated in three-dimensional space. The interaction between bone and muscle is studied in biomechanics. - Acid-base balance — Bone buffers the blood against excessive pH changes by absorbing or releasing alkaline salts. - Detoxification — Bone tissues can also store heavy metals and other foreign elements, removing them from the blood and reducing their effects on other tissues. These can later be gradually released for excretion. - Sound transduction — Bones are important in the mechanical aspect of hearing. " Source and further information: http://en.wikipedia.org/wiki/Bone Further information: http://web.jjay.cuny.edu/~acarpi/NSC/14-anatomy.htm 3) Of course, this applies to bones as parts of a living body. Inside a living body, they have also the capacity of regeneration, for instance after a fracture. A short time after the death of the body, the bones can also be used for an organ transplantation. They could also "be cultured in vitro. The main objective is to maintain the architecture of the tissue and direct it towards normal development." Source and further information: http://en.wikipedia.org/wiki/Bone_healing http://en.wikipedia.org/wiki/Bone_transplantation http://en.wikipedia.org/wiki/Organ_culture http://en.wikipedia.org/wiki/Tissue_culture 4) Long time after death, a part of the bones still resist the process of decomposition. But these are neither living tissues nor organs.

An organ is a structure that contains at least two different types of tissue functioning together for a common purpose. Bones are typically thought of as calcified, inert structures, but researchers at Columbia University Medical Center have now identified a surprising and critically important novel function of the skeleton. They’ve shown for the first time that the skeleton is an endocrine organ that helps control our sugar metabolism and weight and, as such, is a major determinant of the development of type 2 diabetes. Bones are composed of tissue that may take one of two forms. Compact, or dense bone, and spongy, or cancellous, bone. Most bones contain both types. Compact bone is dense, hard, and forms the protective exterior portion of all bones. Spongy bone is inside the compact bone and is very porous (full of tiny holes). Spongy bone occurs in most bones. The bone tissue is composed of several types of bone cells embedded in a web of inorganic salts (mostly calcium and phosphorus) to give the bone strength, and collagenous fibers and ground substance to give the bone flexibility. The main misconception about bones is that they are made up of dead tissue. This is not true, they have cells, nerves, blood vessels and pain receptors. http://www.leeds.ac.uk/chb/lectures/anatomy3.html http://www.physorg.com/news105890812.html http://library.thinkquest.org/2935/Natures_Best/Nat_Best_Low_Level/skeletal_page.L.html