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 cirmmductors. 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.