On the heart and circulation the effects are stimulant unless large doses are given, when the pulse becomes slow and blood-pressure much lessened.
Physostigmine, the active principle of the Calabar bean, acts chiefly as a stimulant to voluntary and involuntary muscles, and at the same time exercises a depressing effect upon the spinal cord.
In China it was an old internal remedy for leprosy and struma, and is accredited with stimulant, tonic, sedative, astringent and vulnerary properties.
If two poisons act on the same tissue, one stimulating and the other paralysing it, the paralysing substance removes the action of the stimulant substance, not by bringing the tissue back to its normal state, but by abolishing its excitability; hence, although life may be saved by such an action, yet it can only be so within certain limits of dosage, because the antagonism is never complete at every point.
When dissolved in water, however, carbonic acid gas is a gentle stimulant to the mouth, stomach and bowel, the mixture being absorbed more rapidly than plain water; hence its greater value in assuaging thirst.