As the food approaches the mouth, a message also is telegraphed down the nerves to the stomach, which at once actively sets to work pouring out a digestive juice in readiness, called the "appetite juice." This shows how important are, not merely a good appetite, but also attractive appearance and flavor in our food; for if this appetite juice is not secreted, the food may lie in the stomach for hours before the proper process of digestion, or melting, begins.
The Salivary Glands.

Now, where does this saliva in the mouth come from?
It is poured out from the pouches of the cheeks, and from under the tongue, by some little living sponges, or juice factories, known as _salivary glands_.[2] [Illustration: THE SALIVARY GLANDS In this diagram are shown the three glands (_G_) of the left side.

The duct (_D_) from the parotid gland empties through the lining of the cheek; those from the lower glands empty at the front of the mouth under the tongue (_T_).

_N_, nerve; _A_, artery; _V_, vein.] All the juices poured out by these glands, indeed nearly all the fluids or juices in our bodies, are either _acid_ or _alkaline_.

By acid we mean sour, or sharp, like vinegar, lemon juice, vitriol (_sulphuric acid_), and _carbonic acid_ (which forms the bubbles in and gives the sharp taste to plain soda-water).