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Encyclopedia Britannica - Main :: BUN-CAL |
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CALABAR BEAN , the seed of a leguminous plant, Physostigma venenosum, a native of tropical Africa. It derives its scientific name from a curious beak-like appendage at the end of the stigma, in the centre of the flower; this appendage though solid was supposed to be hollow (hence the name from 4vQa, a bladder, and stigma). The plant has a climbing habit like the scarlet runner, and attains a height of about 50 ft. with a stem an inch or two in thickness. The seed pods, which contain two or three seeds or beans, are 6 or 7 in. in length; and the beans are about the size of an ordinary horse bean but much thicker, with a deep chocolate-brown colour. They constitute the E-ser-e or ordeal beans of the negroes of Old Calabar, being administeredto persons accused of witchcraft or other crimes. In cases where the poisonous material did its deadly work
The bean usually contains a little more than 1 % of alkaloids. Of these two have been identified, one called calabarine, and the other, now a highly important drug, known as physostigmineor occasionally as eserine. The British pharmacopoeia contains an alcoholic extract of the bean, intended for internal administration; but the alkaloid is now always employed. This is used as the sulphate, which has the empirical formula
Physostigmine has no action on the unbroken skin. When swallowed it rapidly causes a great increase in the salivary secretion, being one of the most powerful sialogogues known. It has been shown that the action is due to a direct influence on the secreting gland-cells themselves. After a few minutes the salivation is arrested owing to the constricting influence of the drug upon the blood-vessels that supply the glands. There is also felt a sense of constriction in the pharynx, due to the action of the drug on its muscular fibres. A similar stimulation of the non-striped muscle in the alimentary canal results in violent vomiting and purging, if a large dose has been taken. Physostigmine, indeed, stimulates nearly all the non-striped muscles in the body
tissue in the bronchial tubes, and by a " water-logging " of the lungs due to an increase in the amount of bronchial secretion. It may here be stated that the non-striped muscular tissue of the bladder, the uterus and the spleen
ordinary convulsions. The action is a direct one upon the muscular tissue (cf. the case of the gland-cells), sinceit occurs in an animal whose motor nerves have been paralysed by curare.Consciousness is entirely unaffected by physostigmine, there being apparently no action on any part of the brain above the medulla oblongata. But the influence of the alkaloid upon the spinal cord is very marked and characteristic. The reflex functions of the cord are entirely abolished, and it has been experimentally shown that this is due to a direct influence upon the cells in the anterior cornua. It is precisely the reverse of the typical action of strychnine. Near the termination of a fatal case there is a paralysis of the sensory columns of the cord, so that general sensibility is lowered. 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. Besides the secretions already mentioned as being stimulated, the bile, the tears and the perspiration are increased by. the exhibition of this drug. There remains only to consider its highly important action upon the eye. 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. That this action is a direct and not a nervous one is shown by the fact that if the eye be suddenly shaded the pupil will dilate a little, showing that the nerves which cause dilatation are still competent after the administration of physostigmine. Besides the sphincter pupillae, the fibres of the ciliary muscle are stimulated. There is consequently spasm of accommodation, so that clear vision of distant objects becomes impossible. The intra-ocular tension is markedly lowered. This action, at first sight somewhat obscure, is due to the extreme pupillary contraction which removes the mass of the iris from pressing upon the spaces of Fontana, through which the intraocular fluids normally make a very slow escape from the eye into its efferent lymphatics. There is a marked antagonism in nearly all important particulars between the actions of physostigmine and of atropine. The details of this antagonism, as well as nearly all our knowledge of this valuable drug, we owe to Sir Thomas Fraser, who introduced it into therapeutics. The clinical uses of physostigmine are based upon the facts of its pharmacology, as above detailed. It has been recommended in cases of chronic constipation, and of want of tone in the muscular wall
Toxicology.The symptoms of Calabar bean poisoning have all been stated above. The obvious antidote is atropine, which may often succeed; and the other measures
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