_Faba Calabarica_, _Faba Physostigmatis_; _Calabar Bean_, _Ordeal Bean
of Old Calabar_, _Eseré Nut_, _Chop-nut_; F. _Fève de Calabar_; G.
_Calabarbohne_.
=Botanical Origin=—_Physostigma venenosum_ Balfour, a perennial plant
resembling the common Scarlet Runner (_Phaseolus multiflorus_ Lam.)
of our gardens, but having a woody stem often an inch or two thick,
climbing to a height of 50 feet or more. It grows near the mouths of
the Niger and the Old Calabar River in the Gulf of Guinea.
The imported seeds germinate freely, but the plant, though it thrives
vigorously in a hothouse, has not yet, we believe, flowered in Europe.
It has already been introduced into India and Brazil. In the latter
country Dr. Peckolt, late of Cantagallo, has raised plants which have
blossomed abundantly, producing racemes of about 30 flowers each,
pendent from the axils of the ternate leaves.
The flower, which is fully an inch across and of a purplish colour, has
the form of _Phaseolus_, but is distinguished from that genus by two
special characters, namely that it has the style developed beyond the
stigma backwards as a broad, flat, hooked appendage,[756] and the seeds
half surrounded by a deeply grooved hilum.
=History=—The pagan tribes of Tropical Western Africa compel persons
accused of witchcraft to undergo the ordeal of swallowing some
vegetable poison. One of the substances employed in this horrid custom
is the seed under notice, which is administered in substance or in
the form of emulsion, or even as a clyster. It was first made known
in England by Dr. W. F. Daniell about the year 1840, and subsequently
alluded to in a paper read by him before the Ethnological Society in
1846.[757] The highly poisonous effects of the bean were observed in
1855 by Christison[758] in his own person, and in 1858 by Sharpey, who
administered it to frogs.
Before the seed became an object of commerce, it was regarded by the
natives with some mystery and was reluctantly parted with to Europeans.
It was moreover customary in Old Calabar to destroy the plant whenever
found, a few only being reserved to supply seeds for judicial purposes,
and of these seeds the store was kept in the custody of the native
chief. In 1859, the Rev. W. C. Thomson, a missionary on the West Coast
of Africa, forwarded the plant to Professor Balfour of Edinburgh, who
figured and described it as a type of a new genus.[759]
[756] The name of the genus, from ϕύσα, a bladder, was formed under the
notion that this appendage is _hollow_, which is not the fact.—Mucuna
cylindrosperma Welwitsch, from Angola, is probably the same plant. See
Holmes, _Pharm. J._ ix. (1879) 913.
[757] Edinb. _New Phil. J._ xl. (1846) 313.
[758] _Edinb. Journ. of Medical Science_, xx. (1855) 193; _Pharm.
Journ._ xiv. (1855) 470.
[759] _Trans. Roy. Soc. of Edinb._ xxii. (1861) 305. t. 16-17; see also
Baillon, _Hist. des Plantes_, ii. 206. figg. 153-155, and Bentley and
Trimen, _Med. Plants_, part 6 (1876).
Fraser of Edinburgh (about 1863 or earlier) discovered the specific
power of the seed in contracting the pupil, when the alcoholic extract
is applied to the eye. These myotic effects, counteracting those
of atropine and hyoscyamine, were further examined by many other
experimenters on mammals or birds. The action of the poison when taken
internally was found rapidly to affect the cardiac contractions and
finally to paralyze the heart.
=Description=—The fruit of _Physostigma_ is a dehiscent, oblong legume
about 7 inches in length, containing 2 or 3 seeds. The latter, commonly
known as _Calabar Beans_, are 1 to 1⅜ inches long, about ⁶/₈ of an inch
broad, and ⁴/₈ to ⅝ of an inch in thickness, weighing on an average
twenty seeds, 67 grains each.
They have an oblong, subreniform outline, one side being straight or
but slightly incurved, the other boldly arched. The latter is marked
by a broad furrow, ⅛ of an inch wide, bordered with raised edges, and
running from the micropyle, which is a small funnel-shaped depression,
quite round the opposite end of the seed. In the middle of this
remarkable furrow the raphe is seen as a long raised suture running
from end to end. The surface of the seed is somewhat rough, but has a
dull polish; it is of a deep chocolate-brown, passing into a lighter
tint on the ridges bordering the furrow. The latter is black, dull, and
finely rugose.
When the seed is broken the cotyledons are found adherent to the
testa, with a large cavity between them. The air thus included causes
the seeds to float on water, but they sink immediately when broken.
After digestion for some hours in warm water, the testa having been
previously cracked, the whole seed softens and swells so that its
structure may be easily studied. Each cotyledon is then seen to be
marked on the hilum-side by a long shallow furrow, at one end of
which, just below the micropyle, lies the plumule and radicle. A dark
brown inner membrane, constituting part of the testa, surrounds the
cotyledons.
The seeds have scarcely any taste, or not more than an ordinary bean;
nor in the dry state have they any odour. After being boiled, or when
their alcoholic tincture is evaporated, an odour suggesting cantharides
is developed.
=Microscopic Structure=—The cotyledons are built up of large globular
or ovoid cells, those of the outermost layer being smaller and of
rather cubic form. This parenchyme is loaded with starch granules,
frequently as much as 50 mkm. in diameter. Their interior part is less
distinctly stratified than the outer; the hollow centre radiates in
various directions around the axis of the ovate granule. Polarized
light does not show a cross as in other more globular starch granules,
but two elliptic curves approaching one another near the axis of the
granule. Similar starch granules are commonly met with in the seeds of
_Leguminosæ_.
In the Calabar seeds the starch is accompanied by numerous particles
of albuminous matter becoming distinctly perceptible by addition of
iodine, which imparts to them an orange colouration.
The shell of the seed is built up of four different layers; the
prevailing layer consists of very long, simply cylindrical cells,
densely packed so as to form only one radial row. Tison[760] has
endeavoured to ascertain in what region of the seed the active
principle is lodged; and he has arrived at the conclusion that its seat
is the granular protoplasmic particles, which alone acquire an orange
tint by the action of weak caustic alkalis.
[760] _Histoire de la Fève de Calabar_, Paris, 1873. 38.
=Chemical Composition=—Jobst and Hesse[761] proved in 1863 that the
poisonous nature of Calabar bean depends upon an alkaloid, to which
they gave the name _Physostigmine_. It is obtained by the method
generally adopted for extracting analogous substances, that is, by
precipitating one of its salts from an aqueous solution by bicarbonate
of sodium, and dissolving out the base with ether or benzol. As
extracted by these chemists, physostigmine is an amorphous mass of
decidedly alkaline reaction, soluble in much water and in acids.
On exposure to the air the solution soon becomes red, or sometimes
intensely blue, a partial decomposition of the alkaloid taking place.
The red coloration may even be observed in the aqueous infusion of a
few cotyledons. It disappears by sulphuretted hydrogen or sulphurous
acid, but returns if these reducing agents are allowed to evaporate.
Hesse[762] ascertained (1867) that physostigmine consists of
C₃₀H₂₁N₃O₄; he now obtained it perfectly colourless and tasteless,
softening at 40° C., fusing at 45°, but not supporting a heat of 100°
C., without decomposition, which is manifested by a red coloration.
In 1865 Vée and Leven,[763] by treating the powdered unpeeled seed in
nearly the same way, prepared an alkaloid which they called _Eserine_.
It differs from Hesse’s physostigmine in that it forms colourless,
rhomboidal, tabular crystals of a bitter taste, melting at 90° C. It
dissolves easily in ether, alcohol, or chloroform, but very sparingly
in water. The last named solution is alkaline, and reddens by exposure
to the air.
It is assumed by some writers, as Tison,[764] that eserine is only the
pure form of physostigmine; but at present we feel hardly warranted in
admitting the identity of the two substances.
Harnack and Witkowski in 1876 ascertained the presence of another
alkaloid in the seed, which they called _Calabarine_. It is nearly
insoluble in ether and also very different from physostigmine in its
physiological action, but somewhat similar to strychnine. Calabarine
is consequently not to be found in those preparations of calabar bean
which have been obtained or purified by means of ether.
Hesse (1878) exhausted the cotyledons of Physostigma with petroleum
ether, and obtained crystals of a new indifferent substance C₂₆H₄₄+OH₂,
which he called _Phytosterin_. It is closely allied to Cholesterin,
but, in its solution in chloroform, devoid of rotatory power and
melting at 133°. Cholesterin melts at 145°, and deviates, in its
ethereal solution, the ray of polarized light to the left. Phytosterin
also occurs in peas; Hesse suggests that the crystallized appearance
of alkaloids as prepared by former observers was perhaps due to
phytosterin.
From the cotyledons _per se_, cold water extracts mucilage,
precipitable by neutral acetate of lead. The watery infusion contains
also albumin, which may be coagulated by heat or by alcohol. The
infusion is colourless, does not redden litmus, nor does it contain
sugar in appreciable proportion; a few drops of solution of potash
cause it to assume an orange colour. An infusion of the shell of the
seed is already of this colour, but the tint is intensified by caustic
alkali.
[761] Liebig’s _Annalen der Chem. u. Pharm._ 129 (1864) 115.
[762] _Ibid._ 141 (1867) 82; _Chem. News_, 22 March 1867, 149.
[763] _Comptes Rendus_, lx. (1865) 1194.
[764] _Op. cit._ chap. 2.
The cotyledons yield to boiling ether ½ to ⅓ per cent. of fatty oil,
and after exhaustion by ether and alcohol, afford to cold water 12 per
cent. of albuminous and mucilaginous constituents. The proportion of
starch according to Teich[765] amounts to 48 per cent., the albuminous
matter to 23 per cent. The entire seed furnishes 3 per cent. of ash,
chiefly phosphate of potash. These constituents do not widely differ
in proportion from those found in the common bean, which yields 23 to
25 per cent. of albuminous matters, and 32 to 38 per cent. of starch,
besides 1 to 3 per cent. of oil.
[765] _Chemische Untersuchung der Calabarbohne_.—Inauguralschrift, St.
Petersburg, 1867. We calculate the albuminous matters with reference to
_Teich’s_ analysis, which proved the kernels to contain 3·65 per cent.
of nitrogen.
The shells of Calabar bean are stated by Fraser to be by no means
devoid of active principle.
Vée asserts that if to a solution of eserine, a little potash, lime,
or carbonate of sodium be added, there is developed a red colour which
rapidly increases in intensity. This colour is transient, passing into
yellow, green and blue. If chloroform is shaken with such coloured
solution, it takes up the colour; ether on the other hand remains
uncoloured.
=Uses=—Calabar has been hitherto chiefly employed as an ophthalmic
medicine, for the purpose of contracting the pupil. It has however been
occasionally administered in tetanus and in neuralgic, rheumatic, and
other diseases.
=Adulteration=—Other seeds are sometimes fraudulently mixed with
Calabar beans. We have noticed in particular those of a _Mucuna_ and
of the Oil Palm, _Elæis guineensis_ Jacq. The slightest examination
suffices for their detection.