Physiology and Hygiene for Secondary Schools

by Francis M. Walters

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.

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.

Lovett, Lateral Curvature of the Spine and Round Shoulders.

See "Hygiene of Muscles," Chapter XV.

On account of the striations of these cells the muscles which they form are called striated muscles.

The striated muscle cells, having many nuclei, are said to be multi-nucleated.

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.

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).

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. Since the earth is immovable, the body is lifted or pushed upward, somewhat as a fulcrum support is made to move when it is too weak to hold up the weight that is being lifted. In other words, we have the same lever action in the foot in lifting the body as we have when one lies face downward, and, bending the knee, lifts some object on the toes.

Walking is considered one of the very best forms of counter-active exercise for the brain worker (page 328).