When, therefore, the mind wills to move a flexor muscle of the arm, the gray matter sends out the stimulus through the nerves to the cells of each individual fibre of that muscle, and they contract. When this is done, the nerve of sensation reports it to the brain and mind. If the mind desires to return the arm to its former position, then follows the willing, and consequent stimulus sent through the nerves to the corresponding muscle; its cells contract, and the limb is restored.
When the motion is a compound one, involving the action of several muscles at the same time, a multitude of impressions are sent back and forth to and from the brain through the nerves. But the person acting thus is unconscious of all this delicate and wonderful mechanism. He wills the movement, and instantly the requisite nervous power is sent to the required cells and fibres, and they perform the motions required. Many of the muscles are moved by the sympathetic system, over which the mind has but little control.
Among the muscles and nerves so intimately connected, run the minute capillaries of the blood, which furnish nourishment to all.
[Illustration: Fig. 54]
Fig. 54 represents an artery a a, which brings pure blood to a muscle from the heart. After meandering through the capillaries at c, to distribute oxygen and food from the stomach, the blood enters the vein, b, loaded with carbonic acid and water taken up in the capillaries, to be carried to the lungs or skin, and thrown out into the air.
The manner in which the exercise of the muscles quickens the circulation of the blood will now be explained. The veins abound in every part of every muscle, and the large veins have valves which prevent the blood from flowing backward. If the wrist is grasped tightly, the veins of the hand are immediately swollen. This is owing to the fact that the blood is prevented from flowing toward the heart by this pressure, and by the vein-valves from returning into the arteries; while the arteries themselves, being placed deeper down, are not so compressed, and continue to send the blood into the hand, and thus it accumulates. As soon as this pressure is removed, the blood springs onward from the restraint with accelerated motion. This same process takes place when any of the muscles are exercised. The contraction of any muscle presses some of the veins, so that the blood can not flow the natural way, while the valves in the veins prevent its flowing backward. Meantime the arteries continue to press the blood along until the veins become swollen. Then, as soon as the muscle ceases its contraction, the blood flows faster from the previous accumulation.
If, then, we use a number of muscles, and use them strongly and quickly, there are so many veins affected in this way as to quicken the whole circulation. The heart receives blood faster, and sends it to the lungs faster. Then the lungs work quicker, to furnish the oxygen required by the greater amount of blood. The blood returns with greater speed to the heart, and the heart sends it out with quicker action through the arteries to the capillaries. In the capillaries, too, the decayed matter is carried off faster, and then the stomach calls for more food to furnish new and pure blood. Thus it is that exercise gives new life and nourishment to every part of the body.
It is the universal law of the human frame that exercise is indispensable to the health of the several parts. Thus, if a blood-vessel be tied up, so as not to be used, it shrinks, and becomes a useless string; if a muscle be condemned to inaction, it shrinks in size and diminishes in power; and thus it is also with the bones. Inactivity produces softness, debility, and unfitness for the functions they are designed to perform.
Now, the nerves, like all other parts of the body, gain and lose strength according as they are exercised. If they have too much or too little exercise, they lose strength; if they are exercised to a proper degree, they gain strength. When the mind is continuously excited, by business, study, or the imagination, the nerves of emotion and sensation are kept in constant action, while the nerves of motion are unemployed. If this is continued for a long time, the nerves of sensation lose their strength from over-action, and the nerves of motion lose their power from inactivity. In consequence, there is a morbid excitability of the nervous, and a debility of the muscular system, which make all exertion irksome and wearisome.
The only mode of preserving the health of these systems is to keep up in them an equilibrium of action. For this purpose, occupations must be sought which exercise the muscles and interest the mind; and thus the equal action of both kinds of nerves is secured. This shows why exercise is so much more healthful and invigorating when the mind is interested, than when it is not. As an illustration, let a person go shopping with a friend, and have nothing to do but look on. How soon do the continuous walking and standing weary! But, suppose one, thus wearied, hears of the arrival of a very dear friend: she can instantly walk off a mile or two to meet her, without the least feeling of fatigue. By this is shown the importance of furnishing, for young persons, exercise in which they will take an interest. Long and formal walks, merely for exercise, though they do some good, in securing fresh air, and some exercise of the muscles, would be of triple benefit if changed to amusing sports, or to the cultivation of fruits and flowers, in which it is impossible to engage without acquiring a great interest.