In the discharge of energy, summation plays an important role.
Summation is attained by the repetition of stimuli at such a rate
that each succeeding stimulus is applied before the nerve-cells
have returned to the resting stage from the preceding stimulus.
If drops of water fall upon the skin from a sufficient height to cause
the slightest unpleasant sensation, and at such a rate that before
the effect of the stimulus of one drop has passed another drop
falls in precisely the same spot, there will be felt a gradually
increasing painful sensation which finally becomes unbearable.
This is summation of stimuli. When, for a long time, a patient
requires frequent painful wound dressings, there is a gradual
increase in the acuteness of the pain of the receptors.
This is caused by summation. In a larger sense, the entire behavior
of the individual gives considerable evidence of summation, _e.
g_., in the training of athletes, the rhythmic discharge of muscular
energy at such intervals that the resting stage is not reached
before a new exercise is given results in a gradual ascent
in efficiency until the maximum is reached. This is summation,
and summation plays a large role in the development of both normal
and pathologic phenomena.
We have now pointed out the manner in which at least
a part of the nervous energy of man may be discharged.
The integrative action of the nervous system and the discharge
of nervous energy by phylogenetic association may be illustrated
by their analogy to the action of an electric automobile.
The electric automobile is composed of four principal parts:
The motor and the wheels (the muscular system and the skeleton);
the cells of the battery containing stored electricity
(brain-cells, nervous energy); the controller, which is connected
with the cells by wiring (the receptors and the nerve-fibers);
and an accelerator for increasing the electric discharge
(thyroid gland?). The machine is so constructed that it acts
as a whole for the accomplishment of a single purpose.
When the controller is adjusted for going ahead (adequate stimulus
of a receptor), then the conducting paths (the final common path)
for the accomplishment of that purpose are all open to the flow of
the current from the battery, and the vehicle is integrated to go ahead.
It spends its energy to that end and is closed to all other impulses.
When the controller is set for reverse, by this adequate stimulus
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.