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the spine ; the power is at the sternum-virtually the opposite end of the rib; and the resistance to be overcome lies between the two.

(c) The rising of the body upon the toes, in standing on tiptoe, and in the first stage of making a step forwards. (Fig. 49, II.)

Here the fulcrum is the ground on which the toes rest; the power is applied by the muscles of the calf to the heel (Fig. 2, I.); the resistance is so much of the weight of the body as is borne by the ankle-joint of the foot, which of course lies between the heel and the toes.

9. Three examples of levers of the third order are

(a) The spine, head, and pelvis, considered as a rigid bar, which has to be kept erect upon the hip-joints. (Fig. 2.)

Here the fulcrum lies in the hip-joints; the weight is at the centre of gravity of the head and trunk, high above the fulcrum ; the power is supplied by the extensor, or flexor, muscles of the thigh, and acts upon points comparatively close to the fulcrum. (Figs. 2, 2, and II.)

(6) Flexion of the forearm upon the arm by the biceps muscle, when a weight is held in the hand.

In this case, the weight being in the hand and the fulcrum at the elbow-joint, the power is applied at the point of attachment of the tendon of the biceps, close to the latter. (Fig. 48.)

(c) Extension of the leg on the thigh at the knee-joint.

Here the fulcrum is the knee-joint ; the weight is at the centre of gravity of the leg and foot, somewhere between the knee and the foot; the power is applied by the muscles in front of the thigh (Fig. 2, 2) through the ligament of the knee-cap, or putella, to the tibia, close to the kneejoint.

10. In studying the mechanism of the body, it is very important to recollect that one and the same part of the body may represent each of the three kinds of levers, according to circumstances. Thus it has been seen that the foot may, under some circumstances, represent a lever of the first, in others, of the second order. But it may become a lever of the third order, as when one dances a weight resting upon the toes, up and down, by moving only the foot. In this case, the fulcrum is at the anklejoint, the weight is at the toes, and the power is furnished by the extensor muscles at the front of the leg (Fig. 2, 1), which are inserted between the fulcrum and the weight. (Fig. 49, III.)

11. It is very important that the levers of the body should not slip, or work unevenly, when their movements are extensive, and to this end they are connected together in such a manner as to form strong and definitely arranged joints or articulations.

Joints may be classified into imperfect and perfect.

(a) Imperfect joints are those in which the conjoined levers (bones or cartilages) present no smooth surfaces, capable of rotatory motion, to one another, but are connected by continuous cartilages, or ligaments, and have only so much mobility as is permitted by the flexibility of the joining substance.

Examples of such joints as these are to be met with in the vertebral column- the flat surfaces of the bodies of the vertebræ, being connected together by thick plates of very elastic fibro-cartilage, which confer upon the whole column considerable play and springiness, and yet prevent any great amount of motion between the several vertebræ. In the pelvis (see Plate, Fig. VI.), the pubic bones are united to each other in front, and the iliac bones to the sacrum behind, by fibrous or cartilaginous tissue, which allows of only a slight play, and so gives the pelvis a little more elasticity than it would have if it were all one bone.

(6) In all perfect joints, the opposed bony surfaces which move upon one another are covered with cartilage, and between them is placed a sort of sac, which lines these cartilages, and, to a certain extent, forms the side walls of the joint ; and which, secreting a small quantity of viscid, lubricating fluid-the synoviais called a synovial membrane.

12. The opposed surfaces of these articular cartilages, as they are called, may be spheroidal, cylindrical, or pulley-shaped ; and the convexities of the one answer, more or less completely, to the concavities of the other.

Sometimes, the two articular cartilages do not come directly into contact, but are separated by independent plates of cartilage, which are termed inter-articular. The opposite faces of these inter-articular cartilages are fitted to receive the faces of the proper articular cartilages.

While these co-adapted surfaces and synovial membranes provide for the free mobility of the bones entering into a joint, the nature and extent of their motion is

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Fig. 50.-A Section of the Hip Joint TAKEN THROUGH THE ACETABULUM OR ARTICULAR CUP OF THE PelvIS AND THE MIDDLE OF THE

HEAD AND NECK OF THE THIGH-BONE. L.T. Ligamentum teres, or round ligament. The spaces marked with an

interrupted line (--.-) represent the articular cartilages. The cavity of the synovial membrane is indicated by the dark line between, and as is shown, extends along the neck of the femur beyond the limits of the cartilage. The peculiar shape of the pelvis causes the section to have the remarkable outline shown in the cut. This will be intelligible if compared with Fig. VI. in the Plate. defined, partly by the forms of the articular surfaces, and partly by the disposition of the ligaments, or firm, fibrous cords which pass from one bone to the other.

13. As respects the nature of the articular surfaces, joints may be what are called ball and socket joints, when the spheroidal surface furnished by one bone plays in a cup furnished by another. In this case the motion of the former bone may take place in any direction, but the extent of the motion depends upon the shape of the cup-being very great when the cup is shallow, and small in proportion as it is deep. The shoulder is an example of a ball and socket joint with a shallow cup; the hip of such a joint with a deep cup (Fig. 50).

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FIG. 51.-LONGITUDINAL AND Vertical SECTION THROUGH THE

ELBOW-JOINT. H. humerus ; Ul. ulna ; Tr, the triceps muscle which extends the arm ;

Bi. the biceps muscle which flexes it.

14. Hinge-joints are single or double. In the former case, the nearly cylindrical head of one bone fits into a corresponding socket of the other. In this form of hingejoint the only motion possible is in the direction of a plane perpendicular to the axis of the cylinder, just as a door can.

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only be made to move round an axis passing through its hinges. The elbow is the best example of this joint in the human body, but the movement here is limited, because the olecranon, or part of the ulna which rises up behind the humerus, prevents the arm being carried back behind the straight line; the arm can thus be bent to, or straightened, but not bent back (Fig. 51). The knee and ankle present less perfect specimens of a single hinge-joint.

A double hinge-joint is one in which the articular surface of each bone is concave in one direction, and convex in another, at right angles to the former. A man seated in a saddle is “articulated ” with the saddle by such a joint. For the saddle is concave from before backwards, and convex from side to side, while the man presents to it the concavity of his legs astride, from side to side, and the convexity of his seat, from before backwards.

The metacarpal bone of the thumb is articulated with the bone of the wrist, called trapezium, by a double hingejoint.

15. A pivot-joint is one in which one bone furnishes an axis, or pivot, on which another turns ; or itself turns on its own axis, resting on another bone. A remarkable example of the former arrangement is afforded by the atlas and axis, or two uppermost vertebræ of the neck (Fig. 52). The axis possesses a vertical peg, the so-called odontoia process (6), and at the base of the peg are two, obliquely placed, articular surfaces (a.) The atlas is a ring-like bone, with a massive thickening on each side. The inner side of the front of the ring plays round the neck of the odontoid peg, and the under surfaces of the lateral masses glide over the articular faces on each side of the base of the peg. A strong ligament passes between the inner sides of the two lateral masses of the atlas, and keeps the hinder side of the neck of the odontoid peg in its place (Fig. 52, A). By this arrangement, the atlas is enabled to rotate through a considerable angle either way upon the axis, without any danger of falling forwards or backwards--accidents which would immediately destroy life by crushing the spinal marrow.

The lateral masses of the atlas have, on their upper faces, concavities (Fig. 52, A, a) into which the two convex, occipital condyles of the skull fit, and in which they play

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