So the circulation has a great deal to do with the intelligent planning and arranging of our work, our meals, and our play.

If we are going to increase our endurance, we must increase the power of our heart and blood vessels, as well as that of our muscles.

The real thing to be trained in the gymnasium and on the athletic field is the heart rather than the muscles.
Fortunately, however, the heart is itself a muscle, alive and growing, and with the same power of increasing in strength and size that any other muscle has.

So that up to a proper limit, all these things which throw strain upon the heart in moderate degree, such as running, working, and thinking, are not only not harmful, but beneficial to it, increasing both its strength and its size.

The heart, for instance, of a thoroughbred race-horse is nearly twice the size, in proportion to his body weight, of the heart of a dray-horse or cart-horse; and a deer has more than twice as large a heart as a sheep of the same weight.
[Illustration: THE SCHOOL PHYSICIAN EXAMINING HEART AND LUNGS] The important thing to bear in mind in both work and play, in athletic training, and in life, is that this work must be kept easily within the powers of the heart and of the other muscles, and must be increased gradually, and never allowed to go beyond a certain point, or it becomes injurious, instead of beneficial; hurtful, instead of helpful.