GROUP XIII  . Local Irritants.—Although some of the drugs already considered have a local irritant action, they produce other more important effects, but the substances here ranged under this heading depend entirely for their action on their local irritant effects . a . Those which act upon the alimentary canal: Simple bitters such as quassia wood, columbo root, taraxacum, gentian, chiretta, and many others, irritate gently the mucous membrane of the stomach and bowels, and by increasing the secretions improve the appetite and digestion . The aromatic bitters such as chamomile flowers, cascarilla bark, hops, orange peel and others contain in addition small quantities of essential oils which increase their local action . The active principles in some of these bitters have been isolated pure, and have been found to be alkaloids or neutral compounds . Substances like pepper, cayenne pepper, mustard, horse-radish and ginger irritate the stomach and bowel much in the same way, but are more pungent, and are consequently used as condiments . Some of these have a similar but less marked effect upon the skin . The large number of vegetable substances used as purgatives owe their action to an irritating effect upon the mucous membrane and the neuro-muscular apparatus of the bowel, whereby the secretions and peristalsis are more or less increased, as the result of which diarrhoea ensues . Some of them cause so much irritation that the discharge is very watery (hydragogue cathartics), while others, for example aloes, by acting gently on the lower part of the bowel and on its muscular coat, produce simply a laxative effect . A few of them, such as aloin and colocynthin, are also purgative when injected subcutaneously or into the blood, probably owing to their being excreted into the intestinal canal . b .

Those which act on the skin: The best known of these is

cantharides (Spanish fly), the active principle of which is a colour-less crystalline body—cantharidin—which is extremely irritating . On a mucous membrane or a delicate skin it exerts an irritant action, which occurs more quickly than on a thickened epidermis, such as the scalp, and according to the strength and period of application there may result redness, a blister, or an ulcer . Many other substances, such as chrysarobin, mustard, pepper, &c., are also capable of irritating the skin, the effect produced varying from mere dilatation of the cutaneous vessels to destruction of tissue . GaouP XIV . Male-fern.—This includes the male-fern, santonin, cusso, pomegranate bark, pumpkin seeds and many other substances containing active principles which have a specific poisonous action on intestinal parasitic worms . Apart from this their actions vary considerably, but are of little practical importance . Geoup XV . Ethereal Oils.—This includes a very large number of substances which owe their action to the fact that they contain ethereal or essential oils . The best known of these are cloves, pimento (allspice), myrtle, eucalyptus, caraway, fennel, dill, coriander, rosemary, lavender, peppermint, spearmint, nutmeg, cinnamon, sandal-wood, turpentine, juniper berries, valerian and sumbul . In this group may be included the oleo-resins, such as copaiba, cubebs and Canada balsam; the gum-resins, such as asafetida, myrrh, ammoniacum and galbanum; and the true balsams, such as benzoin, storax, balsam of Tolu and balsam of Peru . The resins when taken internally have much the same action as essential oils, which are closely allied chemically, while the benzoic and cinnamic acids in the balsams modify their actions very slightly . Although individual essential oils may differ somewhat in action, chemically and pharmacologically they are fundamentally similar .

They all have a poisonous action on

protoplasm, which makes them useful in medicine as antiseptics, disinfectants, germicides, anti-fermentalives and parasiticides; when locally applied they are more or less irritating, and, when very dilute, astringent . When swallowed in small doses they slightly irritate the mouth and gastric mucous membrane, increasing the secretions and producing a feeling of warmth . At the same time they increase the movements of the stomach, and also in this way hasten digestion, an action which extends to the upper part of the bowel . They are readily absorbed into the blood, and they are excreted chiefly by the kidneys in a more or less altered form, and probably also by the different mucous membranes, and even by the skin . After absorption their action, speaking generally, is exerted on the brain and spinal cord, and is at first slightly stimulant and afterwards depressing, even to the causing of sleepiness and stupor . Locally applied they depress the terminations of sensory nerves, and may thereby lessen pain . On the heart and circulation the effects are stimulant unless large doses are given, when the pulse becomes slow and blood-pressure much lessened . During excretion they irritate the kidneys and the sweat-glands, and thereby increase the excretion of urine and of sweat . They also increase the number of leucocytes in the blood,and the more irritating of them increase the flow of blood to the pelvic organs, and may thus stimulate the uterus, or in large doses cause abortion . The various camphors, such as laurel camphor, Borneo camphor, menthol and cumarin, are oxidized derivatives of essential oils, and differ only superficially from them in their action .