Characters. - About the size of a very large horse-bean, with a very firm, hard, brittle shining integument of a brownish-red, pale chocolate, or ash-grey colour. Irregularly kidney-shaped, with two flat sides, and a furrow running longitudinally along its convex margin, ending in an aperture near one end of the seed. Within the shell is a kernel consisting of two cotyledons, weighing on an average about 46 grains, hard, white, and pulverisable, of a taste like that of the ordinary edible leguminous seeds, without bitterness, acrimony, or aromatic flavour. It yields its virtues to alcohol, and imperfectly to water.

Composition. - Besides the ordinary constituents of beans, the seed of physostigma contains an active principle, physostigmin or eserin, C15H11N3O2, an alkaloid, combining with acids, and variously obtained as colourless crystals, or an amorphous or syrupy body.

Dose, in powder. - 1 to 4 gr.

Preparation. Extractum Physostigmatis. - Spirituous. 45 in 1. Dose, 1/16 to 1/4 gr.

1. Immediate Local Action And Uses

Extract of physostigma or preparations of eserine are readily absorbed by the conjunctiva, and produce the specific contraction of the pupil to be presently noticed.

Taken by the mouth, calabar bean in moderate doses sometimes causes sickness and colic, and in larger doses diarrhoea, all from increased and irregular peristalsis, apparently of local origin. The extract is therefore occasionally used in habitual constipation.

2. Action In The Blood

Eserin enters the blood unchanged, and passes thence into the tissues.

3. Specific Action And Uses

Eserin is found in all the organs. Along with the gastrointestinal symptoms first described, moderate doses of the bean give rise to a sense of weakness, faintness, and shortness of breath; larger doses to an aggravation of the same symptoms, with contraction of the pupil, frontal headache, salivation, diaphoresis, slowing and weakening of the pulse. These are short of truly poisonous effects.

On analysis it is found that consciousness is not lost, though impaired by large doses, showing comparative freedom of the convolutions. The cord is the part principally affected by calabar bean, the chief symptoms being of the nature of motor paralysis from depression of the anterior cornua, and thus of reflex irritability also. The respiratory muscles necessarily fail from this cause. The posterior cornua (sensory portions) of the cord are paralysed to a degree, so that sensibility is diminished in the limbs. The motor nerves and muscles are but slightly affected directly. Occasional twitchings occur, whether direct or spinal in origin. The sensory nerves are not directly influenced. The medulla is decidedly affected by physostigma. Thus the respiratory centre, after brief (probably reflex) stimulation, is depressed, and death occurs chiefly by asphyxia. The cardiac centre is first stimulated, so that the heart beats more powerfully and less frequently; but at last, or after large doses, depression ensues. Therewith the intracardiac branches of the vagus are probably stimulated at first, and the ganglia paralysed at last. The blood pressure rises with the increased cardiac action, and falls later on. Whether there is any direct action of eserin on the vaso-motor apparatus is unsettled.

Contraction of the pupil and spasm of accommodation are striking and highly important effects of eserin, whether it be given internally or applied locally. Both phenomena are due to irritation of the fibres of the third nerve, and not to central disturbance as in the contraction caused by opium, or to paralysis of the sympathetic. These effects are accompanied by fall of the intraocular tension, and can be removed by atropia. The salivary secretion is increased through the centre of the chorda, but ceases after large doses from arrest of the circulation in the glands.

The specific uses of calabar bean depend on its action on the cord and the eye. It has been frequently given in tetanus, and other convulsive diseases referable to irritation or disease of the spinal centres, and apparently with success, although many of these cases recover spontaneously, and others resist the eserin. The alkaloid should be given subcutaneously in doses of gr. 1/60 to 1/12 in solution; or gr. 1/3 of the extract may be given subcutaneously, or gr. 1 by the mouth, repeated in two hours, and followed by doses of gr. 1/16 to 1/4 every few hours. For the convulsions of strychnia poisoning calabar bean is of little or no use. Neither is it of much real service in the treatment of poisoning by atropia or chloral, as was once expected.

In diseases of the eye eserin is now much used. A drop of a solution of the sulphate (2 gr. to 1 ounce of water) is applied locally to diminish intraocular pressure in glaucoma, perforating keratitis, etc.; in paralysis of the iris and ciliary muscle, e.g. after diphtheria (1/2 gr. to 1 ounce); to counteract the effects of belladonna; or to diminish the entrance of light in painful diseases of the eye, photophobia, etc.

4. Remote Local Action

Eserin is excreted by the liver and salivary glands, but has never been found in the urine.