It is the shrunken and shriveled remains of a large pouch of the intestine which once opened into the cecum, and was used originally as a sort of second stomach for delaying and digesting the remains of the food.

The reason why it gives rise to so much trouble is that it is so small--scarcely larger than will admit a knitting-needle--and so twisted upon itself that germs or other poisonous substances swallowed with the food may get into it, start a swelling or inflammation, get trapped in there by the closing of the narrow mouth of the tube, and form an abscess, which leaks through, or bursts into, the cavity of the body, called the _peritoneum_.

This causes a very serious and often fatal blood poisoning.
Fortunately, _appendicitis_, or inflammation of the appendix, is not a very common disease, causing only one in one hundred of all deaths that occur; and these are mostly cases that were not treated promptly.

Yet, if you have a severe, constant pain, rather low down in the right-hand corner of your abdomen, and if, when you press your hand firmly down in that corner, it hurts, or you feel a lump, it is decidedly safest to call a doctor and let him see what the condition really is, and advise you what to do.
FOOTNOTES: [1] The term _salts_ includes, as will be explained later, a large number of substances, like ordinary table salt, baking soda, and the laxative salts.
[2] There are three pairs of these: one just below the ears and behind the angles of the jaw, known as the _parotid_; one under the middle of the lower jaw known as the _submaxillary_; and a small pair just under the tip of the tongue, called the _sublingual_.

These glands have grown up from the very simplest of beginnings.