Most cases of imagined heart trouble are really due to indigestion.
The Lining Surface of the Stomach.

Now let us look more carefully at the lining surface of the stomach, for it is very wonderful.

Like all other living surfaces, it consists of tiny, living units, or "body bricks" called _cells_, packed closely side by side like bricks in a pavement.

We speak of the _mucous membrane_, or lining, of our food tube, as if it were one continuous sheet, like a piece of calico or silk; but we must never forget that it is made up of living ranks of millions of tiny cells standing shoulder to shoulder.
These cells are always actively at work picking out the substances they need, and manufacturing out of them the ferments and acids, or alkalies, needed for acting upon the food in their particular part of the tube, whether it be the mouth, the stomach, or the small intestine.
[Illustration: A SECTION OF THE LINING SURFACE OF THE STOMACH (Greatly magnified) Showing the mouths of the stomach glands, and the furrows, or folds, of the lining.] The Peptic Juice.

The cells of the stomach glands manufacture and pour out a slightly sour, or acid, juice containing a ferment called _pepsin_.