be unable to
distinguish between an apple and an onion, if one be rubbed on the tongue
after the other. As soon as the nostrils are opened the difference is at
once perceived.

Experiment 142. Put a drop of vinegar on a friend's tongue, or on
your own. Notice how the papillae of the tongue start up.

Experiment 143. Rub different parts of the tongue with the pointed
end of a piece of salt or gum-aloes, to show that the _back_ of the
tongue is most sensitive to salt and bitter substances.

Experiment 144. Repeat the same with some sweet or sour substances,
to show that the _edges_ of the tongue are the most sensitive to these
substances.

Experiment 145. We often fail to distinguish between the sense of
taste and that of smell. Chew some pure, roasted coffee, and it seems to
have a distinct taste. Pinch the nose hard, and there is little taste.
Coffee has a powerful odor, but only a feeble taste. The same is true of
garlic, onions, and various spices.

Experiment 146. Light helps the sense of taste. Shut the eyes, and
palatable foods taste insipid. Pinch the nose, close the eyes, and see
how palatable one half of a teaspoonful of cod-liver oil becomes.

Experiment 147. Close the nostrils, shut the eyes, and attempt to
distinguish by taste alone between a slice of an apple and one of a
potato.

320. Modifications of the Sense of Taste. Taste is modified to a
great extent by habit, education, and other circumstances. Articles of
food that are unpleasant in early life often become agreeable in later
years. There is occasionally a craving, especially with people of a
peculiar nervous organization, for certain unnatural articles (as chalk
and laundry starch) which are eaten without the least repugnance. Again,
the most savory dishes may excite disgust, while the simplest articles may
have a delicious flavor to one long deprived of them. The taste for
certain articles is certainly acquired. This is often true of raw
tomatoes, olives, and especially of tobacco.

The organs of taste and smell may be regarded as necessary accessories of
the general apparatus of nutrition, and are, therefore, more or less
essential to the maintenance of animal life. While taste and smell are
generally maintained until the close of life, sight and hearing are often
impaired by time, and may be altogether destroyed, the other vital
functions remaining unimpaired.

321. Effect of Tobacco and Alcohol up