Articulation, a term in anatomy, denoting the various modes of union between the bones of the skeleton. We may class articulations under three general heads, viz., movable joints, immovable joints, and joints of a mixed order, being somewhat movable, without much relative displacement of the contiguous surfaces. Movable joints are the most complex and various in structure; immovable, the most simple. Movable joints are common in the limbs, and the articulation of the lower jaw with the skull; immovable joints are common in the head and face and lower portion of the trunk; mixed forms of articulation are common in the spinal column and the upper portions of the trunk. The hinge joints of the elbows and the knees, allowing free movements in one plane only, form one order of the movable class; the i ball-and-socket joints of the hip and shoulder, allowing free movements in a circular direction, form a second order of the movable class; and different combinations of these two orders, as seen in the articulations of the lower jaw with the skull, of the hands and feet with the arms and legs at the wrists and the ankles, and also of the bones of the hands and fingers, feet, and toes, form a third order of the movable class.

The elbow joint, in fact, is of a compound order, being of the hinge-joint form with reference to the cubital movement of the forearm on the arm, and of the ball-and-socket form with reference to the radial movement of the forearm on the arm, in what are termed the supination and pronation of the hand and arm. - The class of immovable joints may also be subdivided into different orders and varieties. In the sacrum and the pelvis many bones which are distinct at first literally grow together in some subjects, so as to efface all trace of original separation, while in others traces remain visible of former separation. In the cranium and the face there are numerous modes of junction between bones connected by immovable articulation. The most prominent order of this class in the cranium is the serrated suture, the firmness of the union being increased by alternate notches or indentations and projections like the teeth of a saw formed on the edges of the bones, the teeth of the one being adapted to the indentations of the other. In this manner the bones of the skull unite at the top of the head and in the centre of the forehead. In other cases bevelled edges overlap each other, and in this manner the temporal bones are joined to the parietal bones of the skull.

Another form of fixed articulation is the ridge-and-groove, a ridge being formed on the edge of one bone and a grooved fissure in another to receive it. By this means the bony part of the septum of the nose is inserted into the floor of the nasal cavity to divide the nostrils, and thus form a double cavity by means of a partition wall. - The mixed class of articulations contains many varieties of adaptation. The mode in which ribs are attached to the spinal column behind and to the sternum in front forms one simple order of the mixed class; the mode in which the vertebrae are connected with each other in the spinal column, another, more complex; and the mode in which the slightly yielding portions of the pelvic articulations are connected, a third and simple order of this class. - The movable articulations, being the most complex in form and structure, will give the best idea of the various elements of an articulation; and the ball-and-socket joint, being the most simple of this kind, will serve the purpose of a simple illustration. In the hip joint we have a kind of ball, or rounded surface, at the head of the thigh bone, which hemispherical surface is capped with a thin layer of cartilage, somewhat elastic in structure, and exceedingly smooth on its external surface.

In the bones of the pelvis a socket is formed, called the acetabulum, exactly shaped for the reception of this hemispherical head of the thigh bone, and this socket is lined with a thin layer of dense, elastic, and polished eartilage, so that in the joint two polished surfaces meet together and allow free movement, with the least possible amount of friction; but to lessen the effect of friction, and facilitate the movements of these surfaces one upon the other, a delicate membrane surrounds the external borders of the articular cartilages, and secretes a viscid fluid which lubricates the surfaces, preventing actual contact and destructive friction of the cartilaginous tissues. This lubricating fluid is technically called synovia, and the secreting membrane the synovial sac or synovial membrane. To prevent the dislocation of the joint, a strong rope of fibrous tissue, very similar in structure to that part of an oyster which cannot easily be removed from the shell, connects the top of the ball with the bottom of the socket, in a somewhat loose but very strongly attached manner. This is termed the round ligament; it is very short and very strong.

The outer surfaces of the ball and socket (not in the socket, but outside) are connected by means of a strong ligamentous band of fibrous tissue, loosely connecting the head of the thigh bone with the pelvic bones, on the outer rim of the socket, but strongly attached to the bones themselves, which it binds together firmly, while permitting a considerable freedom of motion or rotation in the joint. In other joints of the movable class the outer ligaments are not always continuous and circular bands as in this case, but take the form of distinct fibrous ropes, strongly attached to the bones, and forming strong, flexible bands, as strips of leather nailed to the body and the lid of a box serve as ligaments where there are no hinges. Thin, dense, elastic layers of cartilage cap the articular edges and surfaces of bones in the great majority of joints; strong, fibrous, and flexible ligaments connect the bones externally; and, where the joints are very movable, synovial membranes surround the articulating surfaces, and the synovia which they secrete lubricates the surfaces exposed to contact, friction, and mobility.

FIG. 1.   Elbow joint, showing the hinge' like articulation of the humerus with the ulna.

Fig. 1. - Elbow joint, showing the hinge'-like articulation of the humerus with the ulna.

1. Lower extremity of the humerus, or bone of the upper arm.

2, 3. Upper extremity of the ulna, or bone of the forearm.

Fig. 2.   A section of the hip joint taken through the acetabulum and the middle of the head and neck of the thigh bone.   L. T. Ligamentum teres, or round ligament.

Fig. 2. - A section of the hip joint taken through the acetabulum and the middle of the head and neck of the thigh bone. - L. T. Ligamentum teres, or round ligament.

Fro. 3   Diagram of a longitudinal section of an articulation. A. Bones. B. Articular cartilage. C. Periosteum, D,E. Synovial membrane.

Fro. 3 - Diagram of a longitudinal section of an articulation. A. Bones. B. Articular cartilage. C. Periosteum, D,E. Synovial membrane.