ischarge impulses into the cerebrum. The result is
to arouse an activity of the mind--a sensation. The steps in the production
of any _special_ sensation naturally involve the following parts:

1. A sense organ where the terminations of the neurons are acted upon by
the stimulus.

2. A chain of neurons which connect the sense organ with the brain.

3. The part of the cerebrum which produces the sensation.

*Sense Organs.*--The sense organs are not parts of the afferent neurons,
but are structures of various kinds, in which the neurons terminate. Their
function is to enable the sensation stimuli to start the impulses. By
directing, concentrating, or controlling the stimuli, the sense organs
enable them to act to the best advantage upon the neurons. When it is
recognized that such widely different forces as light waves, sound waves,
heat, pressure, and odors are enabled by them to stimulate neurons, the
importance of these organs becomes apparent. As would naturally be
inferred, the construction of any sense organ has particular reference to
the nature of the stimulus which it is to receive. This is most apparent
in the sense organs of sight and hearing.

*Simple Forms of Sense Organs.*--The simplest form of a sense organ (if
such it may be called) is one found among the various tissues. It consists
of the terminal branches of nerve fibers which spread over a small area of
cells, as a network or plexus. Such endings are numerous in the skin and
muscles.

Next in order of complexity are the so-called _end-bulbs_. These consist
of rounded, or elongated, connective tissue capsules, within which the
nerve fibers terminate. On the inside the fibers lose their sheaths and
divide into branches, which wind through the capsule. End-bulbs are
abundant in the lining membrane of the eye, and are found also in the skin
of the lips and in the tissues around the joints.

Slightly more complex than the end-bulbs are the _touch corpuscles_. These
are elongated bulb-like bodies, having a length of about one
three-hundredth of an inch, and occupying the papillae of the skin (Fig.
144). They are composed mainly of connective tissue. Each corpuscle
receives the termination of one or more nerve fibers. These, on entering,
lose the medullary sheath and separate into a number of branches that
penetrate the corpuscle in different directions.

[Fig. 144]

Fig. 144--*A touch corpuscle* highly magni