; it does not come to us, but is aroused in us
by the stimulus. It is the stimulus that comes to us, and the
sensation is our own act, aroused by the stimulus. Sensation means the
activity of the receiving organ (or sense organ), of the sensory
nerves, and of certain parts of the brain, called the sensory centers.
Without the brain response, there is apparently no conscious
sensation, so that the activity of the sense organ and sensory nerve
is preliminary to the sensation proper. Sensation may be called the
first response of the brain to the external stimulus. It is usually
only the first in a series of brain {188} responses, the others
consisting in the recognition of the object and the utilization of the
information so acquired.

Sensation, as we know it in our experience, goes back in the history
of the race to the primitive sensitivity (or irritability) of living
matter, seen in the protozoa. These minute unicellular creatures,
though having no sense organs--any more than they have muscles or
digestive organs--respond to a variety of stimuli. They react to
mechanical stimuli, as a touch or jar, to chemical stimuli of certain
kinds, to thermal stimuli (heat or cold), to electrical stimuli, and
to light. There are some forces to which they do not respond:
magnetism, X-rays, ultraviolet light; and we ourselves are insensitive
to these agents, which are not to be called stimuli, since they arouse
no response.

The Sense Organs

In the development of the metazoa, or multicellular animals,
specialization has occurred, some parts of the body becoming muscles
with the primitive motility much developed, some parts becoming
digestive organs, some parts conductors (the nerves) and some parts
becoming specialized receptors or sense organs. A sense organ is a
portion of the body that has very high sensitivity to some particular
kind of stimulus. One sense organ is highly sensitive to one stimulus,
and another to another stimulus. The eye responds to very minute
amounts of energy in the form of light, but not in other forms; the
ear responds to very minute amounts of energy in the form of sound
vibrations, the nose to very minute quantities of energy in certain
chemical forms.

There is only one thing that a sense organ always and necessarily
contains, and that is the _termination of a sensory nerve_. Without
that, the sense organ, being isolated, would have no effect on the
brain or muscles or any other {189} part of th