the machine is integrated to back, and the battery is closed to all
other impulses. Whether integrated for going forward or backward,
if the battery be discharged at a proper rate until exhausted,
the cells, though possessing no more power (fatigue), have sustained
no further impairment of their elements than that of normal wear
and tear. Furthermore, they may be restored to normal activity
by recharging (rest). If the vehicle be placed against a stone wall,
and the controller be placed at high-speed (trauma and fear),
and if the accelerator be used as well (thyroid secretion?), though
the machine will not move, not only will the battery soon be exhausted,
but the battery elements themselves will be seriously damaged
(exhaustion--surgical shock).

We have now presented some evidence that nervous energy is
discharged by the adequate stimulation of one or more of the various
receptors that have been developed in the course of evolution.
In response to an adequate stimulus, the nervous system is
integrated for a specific purpose by the stimulated receptor,
and but one stimulus at a time has possession of the final common path--
the nerve mechanisms for action. The most numerous receptors
are those for harmful contact; these are the nociceptors.
The effect of the adequate stimulus of a nociceptor is like that of
pressing an electric button that sets great machinery in motion.

With this conception, the human body may be likened to a
musical instrument--an organ--the keyboard of which is composed
of the various receptors, upon which environment plays the many
tunes of life; and written within ourselves in symbolic language
is the history of our evolution. The skin may be the "Rosetta Stone"
which furnishes the key.