nical model (fig. 62) will help us to visualise
many complex response phenomena. The molecular model consists of a
torsional pendulum--a wire with a dependent sphere. By the stimulus of a
blow there is produced a torsional vibration--a response followed by
recovery. The writing lever attached to the pendulum records the
response-curves. The form of these curves, stimulus remaining constant,
will be modified by friction; the less the friction, the greater is the
mobility. The friction may be varied by more or less raising a vessel of
sand touching the pendulum. By varying the friction the following curves
were obtained.

(_a_) When there is little friction we get an after-oscillation, to
which we have the corresponding phenomenon in the retinal
after-oscillation (compare fig. 105).

(_b_ and _c_) If the friction is increased, there is a damping of
oscillation. In (_c_) we get recovery-curves similar to those found in
nerve, muscle, plant, and metal.

(_d_) If the friction is still further increased the maximum is reached
much later, as will be seen in the increasing slant of the rising part
of the curve; the height of response is diminished and the period of
recovery very much prolonged by partial molecular arrest. The curve
(_d_) is very similar to the 'molecular arrest' curve obtained by small
dose of chemical reagents which act as 'poison' on living tissue or on
metals (compare fig. 93, _a_).

(_e_) When the molecular mobility is further decreased there is no
recovery (compare fig. 93, _b_).

Still further increase of friction completely arrests the molecular
pendulum, and there is no response.

From what has been said, it will be seen that if in any way the friction
is diminished or mobility increased the response will be enhanced. This
is well exemplified in the heightened response after annealing (fig. 58)
and after preliminary vibration (figs. 81, 82).

Possibly connected with this may be the increased responses exhibited by
the action of stimulants (figs. 89, 90).

#Reduction of molecular sluggishness attended (1) by quickened
recovery.#--Sometimes, after a cell has been resting for too long a
period, especially on cold days, the wire gets into a sluggish
condition, and the period of recovery is thereby prolonged. But
successive vibrations gradually remove this inertness, and recovery is
then hastened. This is shown in the accompanying curves, fig. 63, where
(_a_) exhibits only very partial recovery eve