vice, 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).

87 The foot in lifting the body on tiptoe appears at first thought to
be a lever of the second class, the body being the weight and the
toe serving as the fulcrum. However, if the distance which the body
is raised is compared with the distance which the muscle shortens,
it is found that the _supposed_ weight has moved _farther_ than the
power (Fig. 118). It will also be noted that the muscle which
furnishes the power is attached at its upper end to the "weight."
These facts show clearly that we are not here dealing with a lever
of the second class. The foot in this instance acts as a lever of
the first class with the fulcrum at the ankle joint and the toe
pressing against the earth, which is the _actual_ weight.