78 In the production of motion in the body, as well as in the production of any kind of _purposeful_ motion outside of the body, three conditions must be fulfilled. There is required, in the first place, a mechanical device or machine which is so constructed as to produce a certain kind of motion. In the second place, energy is needed to operate this device. And, finally, there must be some controlling force, by means of which the motion is made to accomplish definite results. The driving of a horse hitched to a wagon will illustrate these conditions. The wagon is the mechanical device, the horse furnishes the energy, and the driver supplies the controlling force. In this, as in most cases, the machinery, the source of energy, and the controlling force are disconnected except when at work; but in the body all three occur together in the same structure. 79 The dependence of the outer layers of bone cells upon the periosteum for nourishment causes a destruction of this membrane to affect seriously the bone beneath, producing in many instances a decay of the bone substance. 80 It has been claimed that the introduction of vertical writing has reduced the number of cases of spinal curvature originating in the schoolroom, and statistics appear to prove the claim. It is shown, on the other hand, that unnatural positions also are unnecessary in the slanting system of writing, and that in either system the pupil who is permitted to do so is liable to assume an improper position. 81 Lovett, _Lateral Curvature of the Spine and Round Shoulders_. 82 See "Hygiene of Muscles," Chapter XV. 83 On account of the striations of these cells the muscles which they form are called striated muscles. 84 The striated muscle cells, having many nuclei, are said to be multi-nucleated. 85 Every movement in the body has its opposing movement. This is necessary both on account of the work to be accomplished and for preserving the natural form of the body. 86 The distance from the fulcrum to the power is called the _power-arm_ and the distance from the fulcrum to the weight is called the _weight-arm_ (Fig. 115).