Source, Etc

Calabar beans are the ripe seeds of Physostigma vene-nosum, Balfour (N.O. Leguminosoe), a woody climbing plant indigenous to the west coast of Africa, especially near the mouths of the Old Calabar and Niger rivers. It ascends trees and, drooping down, bears pendulous racemes of flowers. These are succeeded by legumes about 15 cm. in length, in each of which two or three large seeds are contained. Calabar beans have long been used on the west coast of Africa as ' ordeal beans. They became known in England in 1840; their power of contracting the pupil of the eye was discovered by Fraser in 1862.

Source Etc 134Source Etc 135Fig. 83.   Calabar bean. A, side view, showing the sub reniform shape. B, edge, showing the long hilum. C, seed split open, showing the concave cotyledons. (Maisch.)

Fig. 83. - Calabar bean. A, side view, showing the sub-reniform shape. B, edge, showing the long hilum. C, seed split open, showing the concave cotyledons. (Maisch).

Description

Calabar beans are dark reddish brown or chocolate-brown in colour and average about 25 mm. in length and 12 mm. in thickness. They are usually oblong-reniform in shape, being nearly flat or only slightly convex on one side, but boldly curved on the other. Along nearly the entire length of the curved side, and passing completely round one end of the seed, runs a broad, deep groove, the lips of which are thickened and paler in colour, and the bottom of which is black and bears a distinct brown furrow in its centre. This deep groove is the hilum, and in it may frequently be found portions of a white, papery funiculus; at one end a minute depression (the micropyle) can be detected.

The outer seed-coat (testa) is hard and thick, and appears nearly smooth to the naked eye, but under the lens is seen to be rather rough. After the seeds have been well soaked in water they can easily be split longitudinally, and exhibit then two firm white starchy cotyledons, which are curved so as to include between them a large lenticular cavity filled with air; this enables the seed to float when thrown on the surface of water. Near the micropyle is the small white radicle attached to one of the cotyledons; there is no endosperm.

The seeds have no marked odour or taste beyond those of an ordinary bean; they are nevertheless extremely poisonous.

The student should observe

(a) The reniform (not cylindrical) shape, and the hilum passing round one end of the seed,

(b) The white cotyledons and small radicle directed towards the micropyle.

Constituents

The principal constituent of the Calabar bean is the alkaloid physostigmine (also called eserine, from ' esere,' the native name of the bean); it is present in the cotyledons only and to the extent of about 0.2 (0.04 to 0.3) per cent. The bean also contains the alkaloids eseramine (trace only), physovenine (Salway), and geneserine (Polo-nowski, 1915); stigmasterol, sitosterol, and the dihydric alcohols trifolianol and calabarol, and abundance of starch.

The drug yields about 4 per cent, of ash.

Physostigmine, C15H11N302, forms colourless rhombic crystals melting at 105°; aqueous solutions of its salts rapidly become pink, due to conversion of the physostigmine into rubreserine; it powerfully contracts the pupil of the eye.

Physovenine, C14H1803N2, has been obtained in colourless crystals, strongly mydriatic; traces only.

Eseramine crystallises in needles melting at 238°.

Geneserine yields a crystalline salicylate; the physiological action differs from that of physovenine.

Calabarine and eseridine, formerly considered constituents of the seed are now regarded as non-existent.

Substitutes

P. cylindrospermum, Holmes; seed nearly cylindrical, hilum shorter; were imported in 1879 and are said to contain physostigmine.

Fig. 89.   S e e d of Physostigma cylindrospermum, showing the shorter hilum. (Maisch.)

Fig. 89. - S e e d of Physostigma cylindrospermum, showing the shorter hilum. (Maisch).

Mucuna urens, de Candolle (horse-eye beans); brownish and rounded.

Entada scandens, Bentham (garbee beans); flattened, discoid, 5 cm. in diameter.

Pentaclethra macrophylla, Bentham, mussel-shaped; 7 cm. long, 5 cm. wide.

Uses

Calabar beans are chiefly used as a source of the alkaloid physostigmine, which is much employed to produce contraction of the pupil of the eye. Both the drug and the alkaloid have been employed in tetanus, locomotor ataxy, and as an antidote in cases of strychnine poisoning; large doses produce an increase of blood-pressure, retardation of respiration, and finally death by asphyxia.