In all true joints the bones glide one over the other, and in certain joints that is the only movement possible, as, for instance, between the bones in the wrist. In other joints an angular motion is also allowed, and these are called hinge - joints-such are the elbow-joint, the knee-joint, and the finger-joints.

In others one bone turns round upon another, and such are called pivot-joints; an example of this is found between the first and second vertebrae-the first vertebra, with the head upon it, turns round on the pivot stuck out of the second vertebra,-it is called the motion of rotation,.

Another example of the movement of rotation is found in the joint between the radius and ulna.

Besides these, there is the ball-and-socket joint, which allows of the greatest amount of movement. In this, the head of one bone is globular, shaped like a ball, and fits into a cavity or hollow of another. As examples of this, I may refer to the shoulder-joints and the hip-joints. In these there is yet another movement allowed which we have not mentioned, and that is the movement by which thewhole limb is moved round an imaginary line; this is called the movement of circumduction.

The movement of rotation is much greater in the case of the thigh-bone than it is in that of the main-bone of the arm, because the thigh-bone is bent so that it can be turned round much more readily than the arm-bone, which is very nearly straight; this makes up for the want of rotation in the fore-leg; on the other hand, rotation of the main-bone of the arm is not needed, because we have the maximum power of rotation in the fore-arm.

I have before explained that one of the uses of the bones in the body, and especially of the long bones, is to act as levers by which the parts of the body can be moved. The bones cannot move by themselves, and I am now going to describe the substance, the office of which is to move the bones, and so to move the different parts of the body. That substance we call flesh or muscle. Mesh or muscle is made up of bundles of fibres, which are surrounded by strong, tough, fibrous structure: a set of such bundles again surrounded by another strong fibrous structure goes by the name of a muscle, so that a muscle consists of bundles of fibres, of what we call muscular tissue, enveloped in a strong fibrous membrane.

As a rule, this strong fibrous membrane at each end of the muscle is fixed into bone, and frequently at one or both ends of the muscle, assumes the form of a cord, or, as we call it, a tendon or sinew.

Flesh or muscle has a peculiar property, which is that it contracts when it is irritated; contracts, as we say, under the application of stimuli; this irritation may be mechanical, chemical, or electrical; cooling of the muscular tissue produces a like effect.

When either of these kinds of irritation is applied to a muscle it contracts, i.e. it shortens, the distance between its two ends becomes less, and it becomes thicker. Not only does the stimulus affect the whole muscle, but each fibre.

It is clear when this happens that one or both of the bones to which the muscle is attached must move, or else it would be torn away from its attachment to the bones. The bones are moved because the muscle joined to the bones shortens, and therefore the two ends of the muscle must be brought nearer together, and that can only happen when either one or both of the bones is moved.

There is another stimulus by which the muscles we are now considering, viz. the muscles that constitute the flesh of the body, the muscles by which the levers are caused to move the different parts of the body, are made to contract, and that other stimulus is the will.

The stimulus caused by the will acts through an apparatus nbt yet described, namely the nervous system. The stimulus exercised by the will can cause these muscles to contract, and therefore the different parts of the body to be moved, and so these muscles that are under the influence of the will go by the name of voluntary muscles; they are muscles over which we have control by means of the will, so that we can make them contract whenever we choose. We shall see that there are a great many muscles over which we have no control by means of the will, and these are called involuntary muscles; there is a considerable difference in structure between voluntary and involuntary muscles, which need not be described here.

There are a great many voluntary muscles over which we have no control, but that is our fault, and no fault of the muscles. We all have muscles, for instance, which, if we could use them, would make our ears move; if we had exercised those muscles from childhood we should now be able to move our ears at pleasure, and there are a number of people who can.

The voluntary muscles then are placed at the outside of the body, in its walls and in the limbs, on both sides of them. Another function that they have to perform is to help to protect the joints, and they are to a certain extent a protection to some other parts of the body, for instance to some of the blood-vessels.

Muscles that by contracting bend a part, are called flexors. Muscles that by contracting straighten a part, are called extensors, so that the flexors and extensors are opposed to one another. Muscles, when they move one bone round another, are called rotators. Muscles which move a whole limb round an imaginary line are called circumductors. Muscles which bring a limb towards the body are called adductors. Muscles which take a limb away from the body are called abductors.

Although I must tell you that every single muscle has its own separate name, yet these six names are sufficient for our purpose.

By muscles we are enabled to maintain our bodies in an upright position. The skeleton will not stand upright, and a dead body will not stand upright, although the muscles are there; and that shows that the muscles are continually being exerted, in order to enable us to balance ourselves.

The muscles of the lower limbs, and of the trunk, are continually exerting an opposing influence to one another. We could not stand upright if certain of the muscles of the lower limbs and trunk were not more or less in a state of contraction.

The large muscles in the calf of the leg are in a sufficient state of contraction to prevent us falling forwards, and the muscles in front of the thigh are in a sufficient state of contraction to prevent us falling backwards, and so on with the muscles of the trunk.

So you see that these muscles keep the body upright by being in a state of contraction, exerting an opposing power to one another, and in that way they manage to balance the body. You can now see, too, why we cannot go on standing up always; supposing a dead person, or a skeleton, could stand upright, there would be no reason why we could not stand upright for a week. But we cannot do this, because we could not support the fatigue that these muscles would have to undergo by such continual contraction.