SYNOVIA.--Synovia or joint oil, is a viscid, transparent fluid, colorless, or pale yellow, physically resembling oil, but it contains very little fatty material, consisting chiefly of albumen, salts and water; it is secreted by the cells of the inner surface of the synovial membrane. When an animal is in active exertion, there is a greater demand for joint oil than when at rest, consequently there is an increased secretion of it. CLASSES OF JOINTS.--Joints may be divided into three classes--Immovable, movable and mixed. MOTION IN JOINTS.--The following terms express the various movements allowed by joints--_Extension_ tends to bring two bones as nearly into a straight line as the structure of the joint will permit; _flexion_ is the reverse of this, and diminishes the angle that extension increases; _abduction_ expresses the outward movement of a limb or bone from the central line of the body; _adduction_ is the reverse action; rotation signifies the partial _revolution_ of a bone or number of bones, as it were, on their own axis; _circumduction_ implies the movement of the lower end of a bone or limb, where it describes a curve, as the bow of a circle, _ellipse_, etc. The term gliding explains itself, and is peculiar to movable and other joints having no frictional surfaces. Important points of joints to be considered will be described on Plate I by Roman figures to correspond with those given below. JOINTS OF THE FRONT LEG. I. SHOULDER JOINT.--This joint is formed by the lower end of the scapula, or shoulder blade, and the upper end of humerus or shoulder bone. This is a ball-and-socket joint, and is held in its place by ligaments and muscles. This belongs to the class of rotation joints, its action is outward, inward, backward and forward. II. ELBOW JOINT.--This joint is formed by the lower part of the humerus and the upper portion of the radius and ulna. It is covered with ligaments and muscles similar to the above mentioned joint. This is a hinge joint the action of which is only forward and backward, or flexion and extension. It has no lateral movement.