Fig. 62.—Model showing the Effect of Friction
Molecular Model.—We have seen that the electric response is an outward expression of the molecular disturbance produced by the action of the stimulus. The rising part of the response-curve thus exhibits the effect of molecular upset, and the falling part, or recovery, the restoration to equilibrium. The mechanical 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 even after the expiration of 60 seconds, whereas when a few vibrations had been given recovery was entirely completed in 47 seconds (b). There was here little change in the height of response.