The Joints.
50. Formation of Joints. The various bones of the skeleton are connected together at different parts of their surfaces by joints, or articulations. Many different kinds of joints have been described, but the same general plan obtains for nearly all. They vary according to the kind and the amount of motion.
The principal structures which unite in the formation of a joint are: bone, cartilage, synovial membrane, and ligaments. Bones make the chief element of all the joints, and their adjoining surfaces are shaped to meet the special demands of each joint ([Fig. 27]). The joint-end of bones is coated with a thin layer of tough, elastic cartilage. This is also used at the edge of joint-cavities, forming a ring to deepen them. The rounded heads of bones which move in them are thus more securely held in their sockets.
Besides these structures, the muscles also help to maintain the joint-surfaces in proper relation. Another essential to the action of the joints is the pressure of the outside air. This may be sufficient to keep the articular surfaces in contact even after all the muscles are removed. Thus the hip joint is so completely surrounded by ligaments as to be air-tight; and the union is very strong. But if the ligaments be pierced and air allowed to enter the joint, the union at once becomes much less close, and the head of the thigh bone falls away as far as the ligaments will allow it.
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 vertebræ affords a good example of imperfect joints which are partially movable.