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 vertebræ.

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 cor

FIG. 195.

C

E

Diagram of the action of the knee joint. Warticular surface of femur.

E tibia in position of extension.

F= tibia in position of flexion.

C centre of rotation.

responding 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.

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

STANDING.

(Hip and

In order that an elongated rigid body may stand upright, it is only necessary that a line drawn vertically through its centre of gravity should pass within its basis of support, and, if the latter be sufficiently wide, the object will remain permanently in that position. The human body, in the first place, is not rigid, and in the second place the basis of support is too small to insure a satisfactory degree of steadiness. The act of standing must, therefore, be accomplished by the action of certain muscles, which are employed in preventing the different joints from bending, and in so balancing the various parts of the body as to keep the whole frame from toppling over.

In order to economize muscular energy while standing, we may lock the more important joints, and thus depend rather on the passive ligaments than upon muscular action for the rigidity of the body. With this object we are taught to place the heels together, turn out the toes, bring the legs parallel by approximating them, and, extending the knees to the utmost, to straighten and to throw back the trunk so as to render tense the anterior hip ligaments, to direct the face straight forward so as to balance the head evenly, and to let the arms fall by the sides.

In this position, as a soldier stands at attention, the knee and hip joints remain fixed, without any effort on the part of the muscles, but it is far from being the most comfortable attitude one can assume for prolonged standing, and hence the position known best by the order "stand at ease" is adopted if more complete rest is desired. In this position the weight of the body is usually allowed to rest on one leg, while the other lightly touches the ground to form a kind of stay, and relieve the muscles which surround the supporting ankle from too great an effort of balancing. At the same time the knee is extended, and the pelvis becomes somewhat oblique, so as to bring it more directly

over the head of the femur. In ordinary easy standing, the joints are not usually kept locked by the tension of the ligamentous structures, but their position is constantly being very slightly altered, so as to vary the muscles employed in preserving the balance and thus prevent fatigue.

The joints most exercised in the erect posture are the following:

1. The ankle has to support the weight of the entire body, while the joint is neither flexed nor extended to its utmost, and cannot be fixed in this position by ligamentous arrangements. The foot being placed on the ground, resting on the heel and the balls of the great and little toes, is supported in an arch-like form by strong though elastic ligaments, which allow but little motion in the numerous joints. The bones of the leg can move in the freest way, backward or forward, over the articular surface of the astragalus, which forms the roller of the hinge, lateral motion being prevented by the malleoli. The line passing through the centre of gravity of the body generally falls slightly in front of the axis of rotation of the ankle joint, so that the entire body tends to fall forward at the ankles. This tendency is checked by the powerful calf muscles, which, attached to the calcaneum by means of the strong tendo-Achillis, keep the parts in such a position that an exact balance is almost constantly kept up.

2. The knee joint, when completely extended, requires no muscular action to prevent it from bending, because the line of gravity then passes in front of the axis of rotation, and the weight of the body tends to bend the knee backward. This is impossible, on account of the strong ligaments which exert their traction behind the axis of rotation. As a rule, these ligaments are not put on the stretch in this way, but the joint is held, by muscular power, in such a position that the line of gravity passes just through, or very slightly behind, the axis of rotation of the joint, so that, if anything, there is a slight tendency for the knee to bend. This is completely checked, and the body balanced, by the powerful extensor muscles of the thigh.

3. In the hip joints, which have to support the trunk and head, the line of gravity falls just behind the line uniting the joints.

when the person is perfectly erect, so that here the body has a tendency to fall backward. This is prevented by the strong iliofemoral ligament. When, however, the knee is not straightened to the full extent, so that the line of gravity passes through or a little behind the axis of rotation of that joint, then the pelvis is very slightly flexed on the femora, so that the axis of the joints lies exactly in or a little behind the line of gravity, and thus the body inclines rather to fall forward. This tendency is prevented by the powerful glutei muscles, which also enable us to regain the erect posture after bending the trunk forward.

The motions of which the pelvis and vertebral column are capable are too slight to deserve attention here. The vertebral column, wedged in as it is between the two innominate bones, may be taken, together with the pelvis, as forming a very yielding and elastic, but practically jointless pillar, the upper part of which can alone be bent to such an extent as to require mention in discussing the mechanism of station.

The individual joints between the cervical vertebræ permit but a slight amount of movement when taken separately, but by their aggregate motion they enable considerable extension and flexion of the neck to take place. These motions follow so closely, and are so inseparably associated with those of the head on the upper vertebra, that there is no need to consider them separately from the latter.

The atlanto-occipital joints admit of some little lateral movement, but that in the antero-posterior direction is much the more important, but even this would be insignificant were it not associated with the movements between the other cervical vertebræ.

The cranium has then to be balanced on the top of a flexible column, and rests immediately in a kind of socket, which can move as a double hinge around two axes at right angles one to the other. The vertical line from the centre of gravity of the cranium must vary with every forward, backward, or lateral movement of the head or neck, but in the erect posture it passes a little in front of the axis of rotation of the atlanto-occipital joint, and somewhat behind the curve of the cervical vertebræ, so that the head may be said to be poised on the apex of the verte