at responds to matter in the gaseous
state, and is, for this reason, the only natural means of detecting
harmful constituents of the atmosphere. In this connection it has been
likened to a sentinel standing guard over the air passages. Many gases
are, however, without odor, and for this reason cannot be detected by the
nostrils. It is of especial importance that gases which are likely to
become mixed with the air supply to the body have odor, even though the
odor be disagreeable. The bad odors of illuminating gas and of various
compounds of the chemical laboratory, since they serve as danger signals
to put one exposed to them on his guard, are of great protective value.

*Sight and Hearing.*--The sense organs of sight and hearing are highly
complicated structures, and will be considered in the chapters following.

*Summary.*--Sensations are certain activities of the mind that result from
excitations within the body or at its surface. These cause the neurons to
discharge impulses which on reaching the cerebrum cause the sensations.
Sensations are necessary for intelligent and purposeful action and for
acquiring all kinds of knowledge. To enable the stimuli to act to the best
advantage in starting the impulses, special devices, called sense organs,
are employed. These receive the terminations of the neurons, and by their
special structure enable the most delicate stimuli to start impulses. The
simpler forms of sense organs are those of touch, temperature, taste, and
smell.

*Exercises.*--1. Compare sensations and reflex actions with reference to
their nature and cause. Give steps in the production of each.

2. Give examples of sensation stimuli. State the purpose of sense organs.

3. How do general sensations differ from special sensations?

4. Of what value is pain in the protection of the body?

5. Show that sensations lead to the higher forms of mental activity, such
as emotion and imagination.

6. Of what value to the body is the "localizing of the sensation"?

7. What kinds of sense organs are found in the skin? State the purpose of
each.

8. Through what sense avenues is one made aware of solids, of liquids, and
of gases?

9. Of what special protective value is the sense of smell?

PRACTICAL WORK

*To demonstrate the Pacinian Corpuscles.*--Spread out the mesentery from
the intestine of a cat and hold it between the eye and the light: Pacinian
corpuscles will appear as small translucent bodies hav