51.  Synovial Membrane.  A very delicate connective tissue, called the synovial membrane, lines the capsules of the joints, and covers the ligaments connected with them.  It secretes the synovia, or joint oil, a thick and glairy fluid, like the white of a raw egg, which thoroughly lubricates the inner surfaces of the joints.  Thus the friction and heat developed by movement are reduced, and every part of a joint is enabled to act smoothly.

52.  Ligaments.  The bones are fastened together, held in place, and their movements controlled, to a certain extent, by bands of various forms, called ligaments.  These are composed mainly of bundles of white fibrous tissue placed parallel to, or closely interlaced with, one another, and present a shining, silvery aspect.  They extend from one of the articulating bones to another, strongly supporting the joint, which they sometimes completely envelope with a kind of cap (Fig. 28).  This prevents the bones from being easily dislocated.  It is difficult, for instance, to separate the two bones in a shoulder or leg of mutton, they are so firmly held together by tough ligaments.

While ligaments are pliable and flexible, permitting free movement, they are also wonderfully strong and inextensible.  A bone may be broken, or its end torn off, before its ligaments can be ruptured.  The wrist end of the radius, for instance, is often torn off by force exerted on its ligaments without their rupture.

The ligaments are so numerous and various and are in some parts so interwoven with each other, that space does not allow even mention of those that are important.  At the knee joint, for instance, there are no less than fifteen distinct ligaments.

53.  Imperfect Joints.  It is only perfect joints that are fully equipped with the structures just mentioned.  Some joints lack one or more, and are therefore called imperfect joints.  Such joints allow little or no motion and have no smooth cartilages at their edges.  Thus, the bones of the skull are dovetailed by joints called sutures, which are immovable.  The union between the vertebrae affords a good example of imperfect joints which are partially movable.

[Illustration:  Fig. 26.—­Elastic Tissue from the Ligaments about Joints.  (Highly magnified.)]

54.  Perfect Joints.  There are various forms of perfect joints, according to the nature and amount of movement permitted.  They an divided into hinge joints, ball-and-socket joints and pivot joints.

The hinge joints allow forward and backward movements like a hinge.  These joints are the most numerous in the body, as the elbow, the ankle, and the knee joints.

In the ball-and-socket joints—­a beautiful contrivance—­the rounded head of one bone fits into a socket in the other, as the hip joint and shoulder joint.  These joints permit free motion in almost every direction.