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We come now to speak of the Second Class,—of those Poisons which enter the circulation, and act through that medium on the heart, brain, and alimentary canal. These organs, however, are affected in very different degrees by different poisons, or even by the same poisons, under different circumstances. Mr. Brodie has shewn that vegetable poison, although when introduced into the alimentary canal affect life, in consequence of the nervous sympathy which subsists between these surfaces and the common sensorium, yet, that the same poisons applied externally to a wound, produce their effects exclusively through the medium of the circulation, being conveyed to the brain only by mixing with the blood in its vessels, and not by being conveyed through the lymphatics, for a ligature upon the great blood-vessels prevents their producing deleterious effects; whereas a ligature upon the thoracic duct, or general canal through which all the absorbents pour their contents into the blood, does not in the least retard or prevent the operation of the poison. There are also several of the mineral poisons which, whether introduced into the stomach, or applied externally to a wound, poison the animal in consequence of being carried into the circulation. It had long been supposed that Arsenic occasioned death by inflaming the stomach; but Mr. Brodie has very satisfactorily shewn that its influence arises from its absorption, and that it must be regarded rather as a vital, than as a chemical agent. In the first place, he has found the inflammation of the stomach, in several cases, so slight, that on a superficial examination it might have been easily overlooked; and, in most of his experiments with Arsenic, death took place in too short a time to be considered as the result of inflammation; and in the next place, in whatever manner the poison is applied, whether externally to a wound, or internally to the membrane of the stomach, the inflammation is confined to the stomach and intestines; and, indeed, it is commonly more violent, and even more immediate, when applied to a wound, than when internally administered; and it also precedes any inflammation of the wound. This important fact was proved by an experiment made by Mr. Hunter and Sir Everard Home, and subsequently by the repeated investigations of Mr. Brodie.