to that observed upon a single muscle. It becomes
weak, lacks energy, and in many instances actually loses weight when
exercise is omitted. This shows exercise to supply an actual need and to
be in harmony with the nature and plan of the body.

*How Exercise benefits the Body.*--In accounting for the healthful effects
of exercise, it must be borne in mind that the body is essentially a
motion-producing structure. Furthermore, its plan is such that the
movements of its different parts aid indirectly the vital processes. The
student will recall instances of such aid, as, for example, the assistance
rendered by muscular contractions in the circulation of the blood and
lymph, due to the valves in veins and lymph vessels, and the assistance
rendered by abdominal movements in the propulsion of materials through the
food canal. A fact not as yet brought out, however, is that _exercise
stimulates nutritive changes in the cells_, thereby imparting to them new
vigor and vitality. While this effect of exercise cannot be fully
accounted for, two conditions that undoubtedly influence it are the
following:

1. Exercise causes the blood to circulate more rapidly.

2. Exercise increases the movement of the lymph through the lymph vessels.

The increase in the flow of the blood and the lymph causes changes to take
place more rapidly in the liquids around the cells, thereby increasing the
supply of food and oxygen, and hastening the removal of waste.

*One should plan for Exercise.*--Since exercise is demanded by the nature
and plan of the body, to neglect it is a serious matter. People do not
purposely omit exercise, but from lack of time or from its interference
with the daily routine of duties, the needed amount is frequently not
taken. Especially is this true of students and others who follow sedentary
occupations. People of this class should plan for exercise as they plan
for the other great needs of the body--food, sleep, clothing, etc. It is
only by making a sufficient amount of muscular work or play a regular part
of the daily program that the needs of the body for exercise are
adequately supplied.

*Amount and Kind of Exercise.*--The amount of exercise required varies
greatly with different individuals, and definite recommendations cannot be
made. For each individual also the amount should vary with the physical
condition and the other demands made upon the energy. One in health should
exercise sufficiently to keep the musc