The energy of our muscles, or of some of them, is within our control, and we can thereby direct other physical energies into desired channels; but we cannot in the slightest degree alter the amount of that energy. We utilise terrestrial energy, by directing and controlling its transformations and transferences, within the limits of our knowledge; but we do it always by moving material objects, and in no other way. For instance, we cannot directly or consciously generate an electric current, or magnetism, or light, or life; for all these things we depend upon partially explored properties of matter, which we can arrange in a certain way so as to achieve a desired end. A multitude of complex processes are constantly occurring in our bodies without any intervention of consciousness; and though we may make a study of the functions of the several organs, and gradually learn something about them, it is a study as of something outside ourselves; the due performance of bodily function is independent of our volition. We can interfere with and damage our organs, and with skill we can so arrange damaged parts that the self-healing process shall have time and opportunity to act; we can also introduce beneficent agencies and stimulating drugs; but our power of direct action is practically limited to muscular and mental activity. _Digression on Rudimentary Physiology_ It is well for children to have some conception of the complex processes constantly occurring in their own organisms. The fact that the heart is a continuously acting pump, urging the blood along arteries to the tissues,—to places where it picks up nutriment, to places where the crudely enriched blood is oxidised, to places where the elaborated material is deposited so as to replenish waste and effect growth—all this should be known; and the partial analogy with the sap of trees, rising in the trunk to be elaborated in the leaves by means of sunshine and air, and then descending ready to be deposited as liquid wood, can be pointed out. The function of the lungs, wherein the blood dispersed throughout a spongy texture is exposed in immense surface to the air, without loss or leakage other than what properly transpires through the membranes, and the consequent advantage of deep breathing and of fresh clean air,—all this has a practical as well as a theoretical interest. The lungs are more under voluntary control than the heart, but the way exercise increases the circulation, and generally blows the fires of the body, is also of practical interest.