The brain is an organ of the body exactly in the same way that the heart, the lungs, and the liver are, and therefore is subject to the same changes which belong to every other part of the frame. It will be at its best when there is circulating through it a full supply of rich red arterial blood, for that means a continual renewal of fresh material to it, and a speedy removal of worn-out products. It is by exercise mainly, whether it be voluntarily undertaken, or whether it pertain to the calling, that the body is kept at the pink of condition, and the brain benefited accordingly. Another great and important result from improving the bodily health is the increased power of what we call the will. The undertaking, say, of a long walk or climb involves the possession of a certain amount of determination, and many people, though perfectly aware of the good to be obtained by a few hours' exercise outside the house, have not the determination to carry it into effect. Once the disinclination to move is overcome, the effort required is less each time, and ultimately the will gains a distinct mastery. When the muscles are put into action, what is termed their contractility is called into play--that is, the force which was dormant before is roused into activity. This is effected through the nervous system, and it is the will which emanates from the brain and is carried along certain nerves to accomplish definite actions. During the contraction of a muscle its individual fibres change in form, producing an alteration in the shape of the whole muscle; thus it becomes shorter and thicker. At the same time, while it is in action more blood flows through it, hence we see that one of the beneficial effects of exercise is to stimulate the circulation through the muscular system. It has also been ascertained by experiments, that the venous blood which comes from a muscle in action is darker in colour than that from a muscle in repose. When the circulation is quickened by movement, and the blood stream hastened, the vigour of the body is increased, because the used up material is all the quicker taken away, and a freshly created supply of nutrition brought to every part.