You have overworked them all, and such overwork is the more fatiguing in proportion to what is done than any other form, except overwork of the brain." And the overtired stomach and heart and lungs tire the brain, of course.
Of the work that is given to the brain itself to overtire it we must speak later.

So much now for that which prevents the body from keeping rested inside, in the finer working of its machinery.
It is easy to find out what and how to eat.

A very little careful thought will show us that.

It is only the plain common sense of eating we need.

It is easy to see that we must not eat on a tired stomach, and if we have to do so, we must eat much less than we ordinarily would, and eat it more slowly.