tatary, or _taste stimulus_, is some chemical or physical condition
of substances which is manifested only when they are in a liquid state.
For this reason _only liquid substances can be tasted_. Solids to be
tasted must first be dissolved.

The different taste sensations are described as bitter, sweet, sour, and
saline, and in the order named are recognized as the tastes of quinine,
sugar, vinegar, and salt. As to how these different tastes are produced,
little is known. Flavors such as vanilla and lemon, and the flavors of
meats and fruits, are really smelled and not tasted. Taste serves two main
purposes: it is an aid in the selection of food and it is a means of
stimulating the digestive glands (page 161).

[Fig. 147]

Fig. 147--*Sense organ of smell.* _A._ Distribution of nerves in outer wall
of nasal cavity. 1. Turbinated bones. 2. Branch of fifth pair of nerves.
3. Branches of olfactory nerve. 4. Olfactory bulb. _B._ Diagram showing
connection of neurons concerned in smell.

*The Sensation of Smell.*--The sense organs of smell are found in the
mucous membrane lining the upper divisions of the nasal cavities. Here are
found two kinds of cells in great abundance--column-shaped epithelial cells
and the cells which are recognized as the sense organs of smell. These
olfactory cells are spindle-shaped, having at one end a slender,
thread-like projection which reaches the surface, and at the other end a
fiber which joins an olfactory nerve (B, Fig. 147). In fact, the olfactory
cells resemble closely the cell-bodies of neurons, and are thought to be
such. The divisions of the olfactory nerve pass through many small
openings in the ethmoid bone to connect with the olfactory bulbs, which in
turn connect with the cerebrum (A, Fig. 147).

*The Olfactory Stimulus.*--Only substances in the gaseous state can be
smelled. From this it is inferred that the stimulus is supplied by gas
particles. Solids and liquids are smelled because of the gas particles
which separate from them. The substance which is smelled must be kept
moving through the nostrils and made to come in direct contact with the
olfactory cells. There is practically no limit to the number of distinct
odors that may be recognized.

*Value of Smell.*--Although the sense of smell is not so acute in man as in
some of the lower animals, it is, nevertheless, a most important and
useful gift. It is the only sense th