At first there was just a little pocketing or pouching down of the mucous lining, like the finger of a glove; then a couple of smaller hollow fingers budded off from the bottom of the first finger; then four smaller fingers from the bottom of these; and so on, until a regular little hollow tree or shrub of these tiny tubes was built up, all discharging through the original hollow stem, which has now become what we call the _duct_ of the gland.

Every secreting gland in the body--the stomach (or peptic) glands, the salivary glands, the liver, the pancreas--is built up upon this simple plan.

The saliva and the juice of the pancreas and that of the liver (bile) are alkaline, as are also the blood and most juices of the body.
The stomach juice is acid, as also are the urine and the perspiration.
[3] It is wonderfully elastic and constantly changing in size, contracting till it will scarcely hold a quart when empty, and expanding, as food or drink is put into it, until it will easily hold two quarts, or even a gallon or more when greatly distended, as by gas.
[4] If you take some pepsin which has been extracted from the stomach of a pig or a calf, melt it in water in a glass tube, then drop one or two little pieces of meat or hard-boiled white of egg into it, you can see them slowly melt away like sugar in a cup of coffee.

If you add a few drops of hydrochloric acid, the melting will go on much faster; and if you warm up the tube to about the heat of the body, it will proceed faster still.

So nature knew just what she was doing when she provided pepsin and acid and warmth in the stomach.
[5] The liver and the bile are more fully described in chapter XVII.
[6] Tiny plant cells, known also as _germs_, which cause fermentation, decay, and many diseases..