Bones are rigid organs that form part of the endoskeleton of vertebrates. They function to move, support, and protect the various organs of the body, produce red and white blood cells and store minerals. Because bones come in a variety of shapes and have a complex internal and external structure, they are lightweight, yet strong and hard, in addition to fulfilling their many other functions. One of the types of tissues that makes up bone is the mineralized osseous tissue, also called bone tissue, that gives it rigidity and honeycomb-like three-dimensional internal structure. Other types of tissue found in bones include marrow, endosteum and periosteum, nerves, blood vessels and cartilage. There are 206 bones in the adult body and about 300 bones in the infant body.
The osseous tissue is a relatively hard and lightweight composite material, formed mostly of calcium phosphate in the chemical arrangement termed calcium hydroxylapatite (this is the osseous tissue that gives bones their rigidity). It has relatively high compressive strength but poor tensile strength, meaning it resists pushing forces well, but not pulling forces. While bone is essentially brittle, it does have a significant degree of elasticity, contributed chiefly by collagen. All bones consist of living cells embedded in the mineralized organic matrix that makes up the osseous tissue.
Bones may be classified according to their shape. There are long bones, short bones, flat bones, irregular bones and pneumatic bones.
骨可根据形状分类，有长骨、短骨、扁骨、不规则骨和含气骨。
Bones have eight main functions:
骨有8大作用：
l Protection — Bones can serve to protect internal organs, such as the skull protecting the brain or the ribs protecting the heart and lungs.
l 保护－骨用于保护体内器官，如颅骨保护大脑，肋骨保护心和肺。
l Shape — Bones provide a frame to keep the body supported.
l 赋形－骨提供一个框架使身体得到支撑。
l 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.
l 造血－骨髓位于长骨髓腔和松质骨间隙内，骨髓通过造血作用产生血细胞。
l Mineral storage — Bones act as reserves of minerals important for the body, most notably calcium and phosphorus.
l 矿物储存－骨担当一些对身体很重要的矿物质的储库，最主要的有钙和磷。
l 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.
l 运动－骨、骨胳肌、腱、韧带和关节一起做功，完成力的生成和转移，从而使全身各部分得以在三维空间内进行操作。
l Acid-base balance — Bone buffers the blood against excessive pH changes by absorbing or releasing alkaline salts.
l 酸碱平衡－骨通过碱盐的吸收和释放调节血液的pH变化。
l 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.
l 解毒作用－骨组织还能够存储重金属和其他多种外来物质，将他们从血液中称除，减少其对其他组织的影响。储存的这些物质将逐渐通过排泄从体内释出。
l Sound transduction — Bones are important in the mechanical aspect of hearing.
