upward and downward. Thus the nodding of the head is effected by the movement of the skull upon the atlas ; while, in turning the head from side to side, the skull does not move upon the atlas, but the atlas slides round the odontoid peg of the axis vertebra.

A The Atlas viewed from above: a a, upper articular surfaces of its lateral masses for the condyles of the skull; b, the peg of the axis vertebra. B. Side view of the axis vertebra: a, articular surface for the lateral mass of the atlas; b, peg or odontoid process.

The second kind of pivot-joint is seen in the forearm. If the elbow and forearm, as far as the wrist, are made to rest upon a table, and the elbow is kept firmly fixed, the hand can nevertheless be freely rotated so that either the palm, or the back, is turned directly upwards. When the palm is turned upwards, the attitude is called supination (Fig. 53, A); when the back, pronation (Fig. 53, B).

The forearm is composed of two bones; one, the ulna, which articulates with the humerus at the elbow by the hinge-joint already described, in such a manner that it can move only in flexion and extension (see § 17), and has no power of rotation. Hence, when the elbow and wrist are rested on a table, this bone remains unmoved.

But the other bone of the forearm, the radius, has its small upper end shaped like a very shallow cup with thick edges. The hollow of the cup articulates with a spheroidal surface furnished by the humerus; the lip of the cup, with a concave depression on the side of the ulna.

The large lower end of the radius bears the hand, and has, on the side next the ulna, a concave surface, which articulates with the convex side of the small lower end of that bone.

Thus the upper end of the radius turns on the double surface, furnished to it by the pivot-like ball of the humerus, and the partial cup of the ulna: while the lower end of the radius can rotate round the surface furnished to it by the lower end of the ulna.

In supination, the radius lies parallel with the ulna, with its lower end to the outer side of the ulna (Fig. 53, A). In

The bones of the right forearm in supination (A) and pronation (B). H. humerus; R. radius; U. ulna.

pronation, it is made to turn on its own axis above, and round the ulna below, until its lower half crosses the ulna, and its lower end lies on the inner side of the ulna (Fig. 53, B).

16. The ligaments which keep the mobile surfaces of bones together are, in the case of ball and socket joints,

strong fibrous capsules which surround the joint on all sides. In hinge-joints, on the other hand, the ligamentous tissue is chiefly accumulated, in the form of lateral ligaments, at the sides of the joints. In some cases ligaments are placed within the joints, as in the knee, where the bundles of fibres which cross obliquely between the femur and the tibia are called crucial ligaments; or, as in the hip, where the round ligament passes from the bottom of the socket or acetabulum of the pelvis to the ball furnished by the head of the femur (Fig. 50).

Again, two ligaments pass from the apex of the odontoid peg to either side of the margins of the occipital foramen, i.e. the large hole in the base of the skull, through which the spinal cord passes to join the brain; these, from their function in helping to stop excessive rotation of the skull, are called check ligaments (Fig. 54, a).

FIG. 54.

The vertebral column in the upper part of the neck laid open, to show -a, the check ligaments of the axis; 6, the broad ligament which extends from the front margin of the occipital foramen along the hinder faces of the bodies of the vertebræ; it is cut through, and the cut ends turned back to show, c, the special ligament which connects the point of the "odontoid" peg. with the front margin of the occipital foramen; I. the atlas ; II. the axis.

In one joint of the body, the hip, the socket or acetabulum (Fig. 50) fits so closely to the head of the femur, and the capsular ligament so completely closes its cavity on

all sides, that the pressure of the air must be reckoned among the causes which prevent dislocation. This has been proved experimentally by boring a hole through the floor of the acetabulum, so as to admit air into its cavity, when the thigh-bone at once falls as far as the round and capsular ligaments will permit it to do, showing that it was previously pushed close up by the pressure of the external air.

17. The different kinds of movement which the levers thus connected are capable of performing, are called flexion and extension; abduction and adduction; rotation and circumduction.

A limb is flexed, when it is bent; extended, when it is straightened out. It is abducted, when it is drawn away from the middle line; adducted, when it is brought to the middle line. It is rotated, when it is made to turn on its own axis; circumducted, when it is made to describe a conical surface by rotation round an imaginary axis.

No part of the body is capable of perfect rotation like a wheel, for the simple reason that such motion would necessarily tear all the vessels, nerves, muscles, &c. which unite it with other parts.

18. Any two bones united by a joint may be moved one upon another in, at fewest, two different directions. In the case of a pure hinge-joint, these directions must be opposite and in the same plane; but, in all other joints, the movements may be in several directions and in various planes.

In the case of a pure hinge-joint, the two practicable movements-viz. flexion and extension-may be effected by means of two muscles, one for either movement, and running from one bone to the other, but on opposite sides of the joint. When either of these muscles contracts, it will pull its attached ends together, and bend or straighten, as the case may be, the joint towards the side on which it is placed. Thus the biceps muscle is attached, at one end, to the shoulder blade, while, at the other end, its tendon passes in front of the elbow-joint to the radius (Figs. 48 and 51); when this muscle contracts, therefore, it bends, or flexes, the forearm on the arm. At the back of the joint there is the triceps (Tr. Fig. 51); when this contracts, it straightens, or extends, the forearm on the arm.

In the other extreme form of articulation-the ball and socket joint-movement in any number of planes may be effected, by attaching muscles in corresponding number and direction, on the one hand, to the bone which affords the socket, and on the other to that which furnishes the head. Circumduction will be effected by the combined and successive contraction of these muscles.

19. It usually happens that the bone to which one end of a muscle is attached is absolutely or relatively stationary, while that to which the other is fixed is moveable. In this case, the attachment to the stationary bone is termed the origin, that to the moveable bone the insertion,

of the muscle.

The fibres of muscles are sometimes fixed directly into the parts which serve as their origins and insertions: but, more commonly, strong cords or bands of fibrous tissue, called tendons, are interposed between the muscle proper and its place of origin or insertion. When the tendons play over hard surfaces, it is usual for them to be separated from these surfaces by sacs containing fluid, which are called bursa; or even to be invested by synovial sheaths, i.e. quite covered for some distance by a synovial bag forming a double sheath very much in the same way that the bag of the pleura covers the lung and the chest wall.

Usually, the direction of the axis of a muscle is that of a straight line joining its origin and its insertion. But in some muscles, as the superior oblique muscle of the eye, the tendon passes over a pulley formed by ligament, and completely changes its direction before reaching its insertion. (See Lesson IX.)

Again, there are muscles which are fleshy at each end, and have a tendon in the middle. Such muscles are called digastric, or two-bellied. In the curious muscle which pulls down the lower jaw, and specially receives this name of digastric, the middle tendon runs through a pulley connected with the hyoid bone; and the muscle, which passes downwards and forwards from the skull to this pulley, after traversing it, runs upwards and forwards, to the lower jaw (Fig. 55).

20. We may now pass from the consideration of the mechanism of mere motion to that of locomotion.

When a man who is standing erect on both feet pro