Between bones, there is a special device called joint, which connects two or more bones together, making people move freely and coordinate their movements beautifully. Because the joints in different parts of the human body undertake different tasks, the joint shapes are different and the activity sizes are different. According to the difference of connective tissue between two bones, joints are generally divided into three types.

(1) Fibrous joint is also called immobile joint, and the two bones are connected by dense fibrous connective tissue, which has no active function. For example, the skull is composed of eight flat bones, the edges of which are jagged and staggered, separated by periosteum in the middle, and connected into a whole block, which cannot be moved at all; Another example is that teeth are embedded in alveolar fossa, which also belongs to this category.

(2) Cartilage joint is also called fretting joint. Joints are connected by cartilage tissue, and these joints only move partially. For example, the spine consists of vertebral blocks. A cricoid cartilage is placed between the two vertebrae, and then they are tied together with an elastic "rope"-ligament, so that our head, neck, chest and waist can bend and rotate back and forth, but the range of motion between the articular surfaces is very small.

(3) Synovial joints are also called movable joints. This kind of joint has no articular disc or only cartilage plate, which is used to fill the irregularity of articular surface and control the flow of synovial fluid. It has obvious joint cavity with synovial membrane on the cavity wall, and synovial fluid is the lubricant of joint. This kind of joint has a wide range of motion. This kind of joint includes the joints of limbs and most joints in the human body. This kind of synovial joint is a common site of arthritis.