This is a joint in which a small space called a synovial cavity or joint cavity is present between the articulating surfaces of the bones. The bone ends are covered with a smooth hyaline cartilage which is lubricated by the synovial fluid. This is a clear, yellowish, viscous fluid which contains mucin. Its function is to nourish the hyaline cartilage and to lubricate the joints. The fluid prevents the friction between the solid surfaces in contact with each other. The synovial fluid is secreted by the synovial membrane which lines the joint capsules. The membrane contains fat cells which may form fat pads forming cushions to the joint. The synovial joints are freely movable and are made stronger by ligaments. The ligaments may be within or partly outside the joint cavity or may be inside the joint cavity (e.g., hip and knee of mammals).

On the basis of variations in the shape of articulating surfaces the synovial joints are sub divided into following types. (Figure 21.4)

(i) Gliding joint. Articulating surfaces are usually flat, with gliding or non axial movement (e.g., between the carpal bones or vertebrae).

(ii) Hinge joint. The spool-shaped surface fits into the concave surface and allow limited movement in one plane about a single axis (Uniaxial), like a hinge-door movement (e.g., knee and elbow joints). This resists dislocation.

(iii) Pivot joint. In this arch-shaped joint the surface rotates about a rounded or peg-like pivot, and allows rotation of one bone around its own long axis (uniaxial), (e.g. between atlas and axis and between radius and ulna).

(iv) Condyloid joint. An oval-shaped condyle fits into an elliptical cavity. The movement is in two planes (biaxial), at right angles to each other, back and forth and side to side.

(v) Saddle joint. A saddle-shaped bone fits into a socket that is concave-convex in opposite directions . It is a modified condyloid joint. Movement is biaxial (e.g. thumb, between metacarpal and carpal bones, heterocoelous cervical vertebrae of birds).

(vi) Ball-and-socket joint. Ball-shaped head fits into a concave socket. It allows movement in many directions including rotation (triaxial) (e.g., hip joint and shoulder joint). These joints are prone to sudden-strain and are easily separated or dislocated.

(vii) Snap joints are made stable by their ligaments which are lateral in position. Snap joints are found at the elbow and ankle of large mammals. Snap joints provide passive support in the standing position.

(viii) Plane joints have more or less flat surfaces and allow various motion. These joints are found between the carpal bones of large mammals and between pre and post-zygapophysis.

(ix) Radial joints are those in which one bone turns across another, as and when the radius of the forearm turns upon its own axis across the ulna in rotating the arm (Figure 21.5) to turn the palm upwards (supination) or downwards (pronation).