the body that do not possess
sense organs. The members of the first class--and these include the
sensations of touch, temperature, taste, smell, hearing, and sight--are
known as the _special_ sensations. The others, including the sensations of
pain, hunger, thirst, nausea, fatigue, comfort, discomfort, and those of
disease, are known as _organic_, or general, sensations. These two classes
of sensations differ in their purpose in the body as well as in the manner
of their origin.

*Purposes of Sensations.*--Any given sensation is related to the stimulus
which excites it as an _effect_ to a _cause_. It starts up or stops,
increases in intensity or diminishes, according to the action of the
exciting stimulus. As the stimuli are outside of the nervous system, and
in the majority of cases outside of the body, the sensations indicate to
the mind what is taking place either in the body itself or in its
surroundings. They supply, in other words, the means through which the
mind acquires information. By means of the special sensations, a knowledge
of the physical surroundings of the body is gained, and through the
organic sensations the needs of the body and the state of the various
organs are indicated. In general, sensations are made to serve two great
purposes in the body, as follows:

1. They provide the necessary conditions for intelligent and purposeful
action on the part of the body.

2. They supply the basis for the higher mental activities, as perception,
memory, thought, imagination, and emotion.

Intelligent action is impossible without a knowledge both of the bodily
organs and of the body's surroundings. Protection and the regulation of
the work of an organ necessitate a knowledge of its condition, while the
adapting and adjusting of the body to its surroundings require a knowledge
of what those surroundings are. The dependence of all the higher forms of
mental activity upon sensations is recognized by psychologists and is
easily demonstrated by a study of the manner in which we acquire
knowledge. "Without sensation there can be no thought."

*Steps in the Production of Sensations.*--The steps in the production of
sensations are not essentially different from those in the production of
reflex action. First of all, external stimuli act upon the fiber
terminations in the sense organs, or elsewhere, starting impulses in the
neurons. These pass into the central nervous system and there excite
neurons which in turn d