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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).

a

FIG. 54.

The vertebral column in the upper part of the neck laid open, to show -a, the check ligaments of the axis; b, 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

ceeds to walk, beginning with the right leg, the body is inclined so as to throw the centre of gravity forward; and, the right foot being raised, the right leg is advanced for the length of a step, and the foot is put down again. In the meanwhile, the left heel is raised, but the toes of the left foot have not left the ground when the right foot has reached it, so that there is no moment at which both

[graphic][subsumed]

FIG. 55. THE COURSE OF THE DIGASTRIC MUSCLE.

D, its posterior belly; D', its anterior belly; between the two is the tendon passing through its pulley connected with Hy. the hyoid bone.

feet are off the ground. For an instant, the legs form two sides of an equilateral triangle, and the centre of the body is consequently lower than it was when the legs were parallel and close together.

The left foot, however, has not been merely dragged away from its first position, but the muscles of the calf, having come into play, act upon the foot as a lever of the second order, and thrust the body, the weight of which rests largely on the left astragalus, upwards, forwards, and to the right side. The momentum thus communicated to the body causes it, with the whole right leg, to describe an arc over the right astragalus, on which that leg rests below. The centre of the body consequently rises to its former height as the right leg becomes vertical, and descends again as the right leg, in its turn, inclines forward.

When the left foot has left the ground, the body is supported on the right leg, and is well in advance of the left foot; so that, without any further muscular exertion,

the left foot swings forward like a pendulum, and is carried by its own momentum beyond the right foot, to the position in which it completes the second step.

When the intervals of the steps are so timed that each swinging leg comes forward into position for a new step without any exertion on the part of the walker, walking is effected with the greatest possible economy of force. And, as the swinging leg is a true pendulum,-the time of vibration of which depends, other things being alike, upon its length (short pendulums vibrating more quickly than long ones), it follows that, on the average, the natural step of short-legged people is quicker than that of longlegged ones.

In running, there is a period when both legs are off the ground. The legs are advanced by muscular contraction, and the lever action of each foot is swift and violent. Indeed, the action of each leg resembles, in violent running, that which, when both legs act together, constistute a jump, the sudden extension of the legs adding to the impetus, which, in slow walking, is given only by the feet.

21. Perhaps the most singular motor apparatus in the body is the larynx, by the agency of which voice is produced.

The essential conditions of the production of the human voice are :

a. The existence of the so-called vocal chords.

b. The parallelism of the edges of these chords, without which they will not vibrate in such a manner as to give out sound.

c. A certain degree of tightness of the vocal chords, without which they will not vibrate quickly enough to produce sound.

d. The passage of a current of air between the parallel edges of the vocal chords of sufficient power to set the chords vibrating.

22. The larynx is a short tubular box opening above into the bottom of the pharynx and below into the top of the trachea. Its framework is supplied by certain cartilages more or less moveable on each other, and these are connected together by joints, membranes and muscles. Across the middle of the larynx is a transverse partition,

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