This is the product of Physostigma venenosum of Balfour, a climbing leguminous plant, with a ligneous stem, rising upon trees, especially on the borders of streams, into which the fruit falls when ripe, and, being carried down with the floods, is collected by the natives on the banks of the river below, where it is deposited. The plant is a native of the Western Coast of Africa, where its fruit is used by the negroes as an ordeal poison, by which the innocence or guilt of the accused is determined. The name of Calabar was given to it from the particular region where it is used for this purpose. A portion of it, having been taken to Edinburgh, was examined by Drs. Fraser, Christison, and others, and found to possess curious and valuable properties, which are likely to render it important for certain purposes in medicine.

Properties

The seed, which is the part used, is about as large as a large horse-bean, irregularly kidney-form, with a longer convex, and shorter concave edge, two flat sides, and a furrow running from one end along the convex border, to an opening near the other end. Within a hard, brittle, shining integument, of a brownish-red, light-chocolate, or ash-gray colour, is a hard, white kernel, with two cotyledons, pulverizable, and of a taste like that of edible leguminous seeds, and neither bitter nor acrid. it yields its virtues readily to alcohol, and but slightly to water. The shell constitutes, according to Dr. Edwards, 30 per cent., the kernel TO per cent. of the bean. The latter only is active. The virtues of the bean are thought to reside in a peculiar proximate principle, belonging to the alkaloids, and found only in the kernel. As first obtained by Jobst and Hesse, who named it physosligmin, it was a complex substance, containing the active principle, but not consisting of it exclusively. Subsequently the pure alkaloid has been isolated by MM. Amedee Vee and Manuel Leven, and named by them eserin.

Physostigmin (Physostigmia). Eserin {Eserina). The chemists last named procured it by exhausting the powdered bean with cold alcohol of 95° (centigrade), very carefully evaporating the alcohol, adding to the resulting extract a strong solution of tartaric acid, diluting the mixture with water, filtering, then adding powdered bicarbonate of potassa in excess, again filtering, and shaking with ether, which, on evaporation, yields the alkaloid in an impure state. To obtain it pure, the residue was deprived of moisture by exposing it in a bell-glass over strong sulphuric acid, then treating it with ether, and allowing the ethereal solution to evaporate spontaneously. it still, however, contained a small proportion of red colouring matter, which adhered to it with great tenacity, giving it a pinkish hue, but which might be separated by repeated solution and crystallization with ether or alcohol. When quite pure, the alkaloid is colourless, crystallizable in thin rhomboidal plates, of a taste slightly bitter and slowly developed, soluble in ether, alcohol, and chloroform, and very slightly in water, to which, however, it imparts a decided alkaline reaction. it is readily dissolved by the acids; and its solutions are precipitated by the reagents which generally precipitate the alkaloids. it melts with heat, and, at a high heat, sends forth copious white vapours, and burns without residue. Almost all its salts are soluble. in solution it acts promptly on the pupil; and a single drop of a solution containing only 1 part in 1000, introduced beneath the eyelids, produces an excessive and persistent contraction.*

* Comparative experiments have been made on animals upon the relative effects of the extract and alkaline principle of Calabar bean. Of the extract, 20 milligrammes (about one third of a grain) were injected under the skin of a guinea-pig. in ten minutes, great weakness of the hinder limbs came on, and in thirty minutes weakness of the anterior limbs, without contraction of the pupil, though the eyes were turned up. Of physostigmia or eserina, 1.5 milligrammes (.023 gr.), similarly applied, produced palsy of the hind legs in five minutes, convulsive movements of the same in ten minutes, palsy of the fore legs in fifteen minutes, and death in half an hour; the pupils being dilated at the time of death. Under the skin of a rabbit, one centigramme (between one-sixth and one-seventh of a grain) produced weakness of all the limbs, without change of pupil, in fifteen minutes, general paralysis in twenty minutes, with intense contraction of the pupil, followed by irregular respiration and death. The action of the heart had ceased, and there was no excitability of the organ. The brain and spinal marrow were normal, offering no signs of congestion. (Vee and Leven, Ann. de Thérap., 1865, p. 70.) As a general result of these experiments. M. Vee states that injection of eserina into the areolar tissue produces paralysis, purging, alternate contraction and relaxation of the muscles, slowness of the pulse, extreme oppression of breathing, and death. The contraction of the pupil from the poison thus administered is extremely uncertain. After death, the lungs are found bloodless, and the heart soft and more or less filled with black blood. Eserina may cause death by absorption from the conjunctiva. In relation to its effects on man, one milligramme (about one seventieth of a grain), injected into the cellular tissue, and four times as much (one-seventeenth of a grain), introduced into the stomach, produced symptoms of intolerance; and a larger dose would endanger serious results. (Vée, Ibid., 1866, p. 50.)-Note to the third edition.

A peculiarity of this alkaloid is that an aqueous solution of it or one of its salts, exposed to the air in the presence of a little potassa, soda, or lime, assumes a red colour, owing to the absorption of oxygen. The redness soon becomes very deep, but is not permanent, being gradually changed to yellow, green, or blue. Chloroform takes up the red colouring substance, which is probably the oxidized alkaloid, and leaves a colourless solution. This property of becoming red under the agency of the alkalies, had previously been shown to be possessed by the alcoholic extract of the bean, by Dr. Edwards, of Liverpool. Less than the one hundred thousandth part of eserina may be detected by this test. Another test is suggested by the fact, that an alcoholic extract of the contents of the stomach of a boy, poisoned by the bean at Liverpool, produced a marked contraction of the pupil when applied to the eye of a rabbit. (Ann. de Thérap., 1865, p. 106; also ibid,, 1866, p. 46.)