The connection between two or more bones is called a joint, and joints are generally formed by the relative formation of two adjacent bones, such as three or more bones to participate in the composition of the joint is called a composite joint. Cartilage covers the surfaces of the two bones that make up the joint, forming the articular surface. It is surrounded by a capsule of connective tissue, the joint capsule, which contains a small amount of synovial fluid. The surfaces of the two bones that make up a joint are called articular surfaces, and are generally convex and concave to each other. The presence of cartilage on the articular surfaces makes the head and fossa of the joint more adaptable, its surface is smooth, and there is a little synovial fluid between the joint surfaces, which makes the movement more flexible, and because of the elasticity of the cartilage, it can withstand loads and slow down the vibration. The composition of the joint also includes a number of auxiliary structures, such as ligaments and joint discs. Ligaments are made up of dense connective tissue, which is flat, rounded or membranous, and are generally connected to the joint capsule, forming a localized thickened part of the capsule, while some exist independently. The primary function of the ligaments is to limit the range of motion of the joints, to enhance the stability of the joints, and secondly, to provide attachment points for muscles or tendons. There are also ligaments located within the joint called intra-articular (capsular) ligaments, such as the round femoral ligament and the cruciate ligament of the knee, which are surrounded by a synovial layer. Some joints have a fibrocartilaginous plate, called a disc, which is born in the joint cavity. The circumference of the disk is attached to the joint capsule, and the articular disk separates the joint cavity into upper and lower parts. It serves to make the joint head and fossa more adaptable, and joint movements can be performed in the upper and lower joint cavities, respectively, thus increasing flexibility and variety of movement. In addition it also serves to cushion shock. The incomplete articular disk in the knee joint is two half-moon shaped pieces of cartilage called menisci, whose function is similar to that of the articular disk.