The unions between the bones of the skeleton are very varied in function and character. They may be classed as: i. Sutures, in which the bones are firmly united by rugged surfaces without the interposition of any cartilage. They are practically only the lines of union of different bones, which grow together to form a single bone.

Diagrams showing the mode of action of the three orders of levers (numbered from above downward) illustrated by the action of the elbow joint.

Fig.194. Diagrams showing the mode of action of the three orders of levers (numbered from above downward) illustrated by the action of the elbow joint.

2. Symphyses, in which two bony substances are strongly cemented together by ligaments, and a more or less thick adherent layer of fibro-cartilage, are joints allowing of some movement, which is, however, very limited.

3. Arthroses, or true movable joints, such as are commonly met with in the extremities. They are characterized by a synovial sac lining the surrounding ligaments, and two smooth surfaces of cartilage which cover over the bony extremities taking part in the articulation, and form what are called the articular surfaces. The synovial sac is strengthened by a loose membranous covering - the capsular ligament - which is attached round the edge of the cartilages next to the periosteum, which here ceases.

The articular surfaces are always in exact and close contact, being pressed together by the following influences: (1) The elastic tension and tonic contraction of the surrounding muscles, which exert considerable traction on them. (2) The traction of the surrounding ligaments, which in some cases holds the bones firmly together, no'matter what their relative positions may be. This can be well seen in the knee joint, in which a comparatively small number of the ligaments suffice to keep the articular surfaces in contact. (3) The atmospheric pressure also tends to hold the bones in close apposition, as may be seen in the hip joint, which is not easily disarticulated, even when all the surrounding structures and the ligaments have been severed.

The synovial joints may be classified according to the form of their surfaces, or their mode of motion as follows: -

1. Flat articular surfaces held together by a short rigid capsule, allowing of but very slight gliding movement; examples of this form of joint are to be found in the tarsus and the articular processes of the vertebrae.

2. Hinge joints, in which the surfaces are so adapted that only one kind of motion can take place. A groove-like cavity in one bone fits closely and glides around the axis of a roller on the other bone, while the sides of the joint are kept tightly together by means of small lateral ligaments. Examples of this form of joint are to be found between the phalanges of the digits and at the humero-ulnar joint.

3. The rotary hinge, or pivot joint, in which a part moves round the axis of the bone, instead of the axis of rotation being at right angles to both bones, forming the joint as in an ordinary hinge. Such joints are seen at the head of the radius and at the articulation between the atlas and the odontoid process of the axis.

4. A saddle-shaped joint is a kind of double hinge, in which each of the articulating bones forms a partial socket and roller, and hence there are two axes of rotation, placed more or less at right angles one to the other. A good example of this kind of joint occurs between the thumb and one of the wrist bones.

5. Spiral articulations are modifications of the hinge, in which the surface of the roller does not run "true," but becomes eccentric, so that the surface of the roller forms, really, part of a spiral, by means of which the bone articulating with it is forced away from the central axis of rotation and becomes jammed, as if stopped by a wedge. The best example of this is the knee. In this joint the axis of rotation (c) is near the posterior surfaces of the bones, and passes transversely through the condyles of the femur, the surfaces of which form an arc, the centre corresponding to the axis of motion. In ordinary flexion the head of the tibia (f) moves on the arc around the axes so as to partially relax the lateral ligament and allow of some rotation on the axis of the tibia. When the head of the tibia moves forward, in extension (e), it becomes wedged against the anterior part of the articular surface of the femur (w), which presents an eccentric, spiral-like curve, departing more and more from the centre of rotation as the articular surface of the tibia proceeds forward. The effect of this is, that in extension of the leg the ligaments are made tense, and the bones are firmly locked together. Owing to the inequality between the size of the internal and external condyles, the axis of rotation is not at right angles to the axis of the femur, but is at such an angle that extreme extension causes a slight amount of outward motion of the leg.

Diagram of the action of the knee joint.

Fig. 195. Diagram of the action of the knee joint. W = articular surface of femur. E = tibia in position of extension. F = tibia in position of flexion. C = centre of rotation.

6. In the ball and socket joints - the name of which implies their mechanism - the most varied movements occur. (Hip and shoulder).