Irritability, again, although a notable quality of living substance, is not peculiar to it, for many inorganic substances respond to external stimulation by definite changes.
Local hypertrophy may also be due to stimulation resulting from friction or intermittent pressure, as one may see in the thickenings on the skin of the artisan's hands.
The action of atropine in dilating the pupil is also aided by a stimulation of the fibres from the sympathetic nervous system, which innervate the remaining muscle of the iris - the dilator pupillae.
In both these cases the stimulation is followed, not only by movement, but by the secretion of an acid liquid containing a digestive juice, by virtue of which the insect is digested after being killed.
The action of aconitine on the circulation is due to an initial stimulation of the cardio-inhibitory centre in the medulla oblongata (at the root of the vagus nerves), and later to a directly toxic influence on the nerve-ganglia and muscular fibres of the heart itself.
With some of them, such as chloral and chloroform, the stimulation period is short compared with the narcotic period, while with others, such as ether, the reverse is the case.
Jalap is a typical hydragogue purgative, causing the excretion of more fluid than scammony, but producing less stimulation of the muscular wall of the bowel.
Small, stimulating doses, and repeated, are good, but stimulation can be more effectively produced by ammonia or strychnine.
The initial action of digitalis is a stimulation of the cardiac terminals of the vagus nerves, so that the heart's action is slowed.
A similar stimulation of the non-striped muscle in the alimentary canal results in violent vomiting and purging, if a large dose has been taken.