CALABAR BEAN, the seed of a leguminous plant, Physo stigma venenosum, a native of tropical Africa. The plant is a climber and attains a height of about Soft. with a stem an inch or two in thickness. The seed pods, which contain two or three seeds or beans, are 6 or Tin. in length ; and the beans are about the size of an ordinary horse bean but much thicker, with a deep chocolate-brown colour. Although highly poisonous, the bean has nothing in external aspect, taste or smell to distinguish it from any harmless leguminous seed, and disastrous effects have resulted from its being incautiously left in the way of children.
The bean usually contains a little more than 1% of alkaloids. Of these two have been identified, calabarine, and physostigmine or eserine. Physostigmine is used as the sulphate, which has the empirical formula of plus an unknown num ber of molecules of water. It occurs in small yellowish crystals, turned red by exposure to light or air, readily soluble in water or alcohol and with a bitter taste. The dose is grain, and should be administered by hypodermic injection. For the use of the oculist, it is also prepared in lamellae for insertion within the conjunctival sac. Each of these contains one-thousandth part of a grain of physostigmine sulphate.
Physostigmine has no action on the unbroken skin. When swallowed it causes great increase in salivary secretion, due to a direct influence on the secreting gland-cells. After a few min utes salivation is arrested owing to constriction of the blood vessels that supply the glands. Secretion of bile, tears and sweat is also increased. Physostigmine raises the blood pressure, in duces constriction of the pharynx and leads to violent vomiting and purging. It also acts upon the bladder, uterus, spleen and iris (see below). The terminals of the vagus nerve are also stimulated, causing the heart to beat more slowly. Later in its action, the drug depresses thg intra-cardiac motor ganglia, causing prolongation of diastole and finally arrest of the heart in dilatation. The respiration is at first accelerated, but is afterwards slowed and ultimately arrested. The initial hastening is due to stimula tion of the vagus terminals in the lung. The final arrest is due to paralysis of the respiratory centre in the medulla oblongata, hastened by a quasi-asthmatic contraction of the non-striped muscular tissue in the bronchial tubes, and by a "water-logging" of the lungs due to an increase in the amount of bronchial secre tion. It is only in very large doses that the voluntary muscles are poisoned, there being induced in them a tremor which may simulate ordinary convulsions. The action is a direct one upon the muscular tissue (cf. the case of the gland-cells) , since it 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 great. The reflex functions of the cord are abolished owing to a direct influence upon the cells in the anterior cornea. Near the termination of a fatal case there is a paralysis of the sensory columns of the cord, so that general sensibility is lowered. Calabarine on the other hand, stimulates these functions.
Physostigmine causes a contraction of the pupil more marked than any other known drug. Besides the sphincter pupillae the fibres of the ciliary muscle are stimulated. Consequently clear vision of distant objects becomes impossible. The intra-ocular tension is markedly lowered. This action is due to the extreme pupillary contraction which removes the mass of the iris from pressing upon the spaces of Fontana, through which the intra ocular fluids normally make a very slow,escape from the eye into its efferent lymphatics.
Physostigmine is used chiefly, but not exclusively, by the oculist. He uses it for at least six purposes. Its stimulant action on the iris and ciliary muscle is employed when they are weak or paralysed. It is used in all cases where one needs to reduce the intra-ocular tension, and for this and other reasons in glaucoma. It is naturally the most efficient agent in relieving the discomfort or intolerable pain of photophobia; and it is the best means of breaking down adhesions of the iris, and of preventing prolapse of the iris after injuries to the cornea.