Of these two have been identified, one called calabarine, and the other, now a highly important drug, known as physostigmine- or occasionally as eserine.
For the use of the oculist, who constantly employs this drug, it is also prepared in lamellae for insertion within the conjunctival sac. Each of these contains one-thousandth part of a grain of physostigmine sulphate, a quantity which is perfectly efficient.
Physostigmine has no action on the unbroken skin.
Physostigmine, indeed, stimulates nearly all the non-striped muscles in the body, and this action upon the muscular coats of the arteries, and especially of the arterioles, causes a great rise in blood-pressure shortly after its absorption, which is very rapid.
The respiration is at first accelerated by a dose of physostigmine, but is afterwards slowed and ultimately arrested.
Consciousness is entirely unaffected by physostigmine, there being apparently no action on any part of the brain above the medulla oblongata.
The alkaloid calabarine is, on the other hand, a stimulant of the motor and reflex functions of the cord, so that only the pure alkaloid physostigmine and not any preparation of Calabar bean itself should be used when it is desired to obtain this action.
Whether administered in the form of the official lamella or by subcutaneous injection, physostigmine causes a contraction of the pupil more marked than in the case of any other known drug.
There is a marked antagonism in nearly all important particulars between the actions of physostigmine and of atropine.
The clinical uses of physostigmine are based upon the facts of its pharmacology, as above detailed.
Unfortunately the antagonism between physostigmine and atropine is not perfect, and Sir Thomas Fraser has shown that in such cases there comes a time when, if the action of the two drugs be summated, death results sooner than from either alone.
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.
It is a physiological antagonist of chloral hydrate, morphine and physostigmine, and may be given in poisoning by these drugs.
Thus atropine will save life after three and a half times the fatal dose of physostigmine has been taken, but will hasten the end if four or more times the fatal dose has been ingested.