Hemlock is the product of Conium maculalum, an umbelliferous, herbaceous, biennial plant, indigenous in Europe and Asia, and naturalized in the United States, in some parts of which it grows in considerable abundance. One of the most remarkable characters of the plant, from which it acquired its specific name of maculatum, is the appearance of the stem, covered with purple spots. The whole plant exhales a fetid odour, compared to the smell of mice, especially in its flowering period in July and August. It is said to be most energetic as a medicine in hot dry seasons, in sunny situations, and in warm climates. Though poisonous to man, it is eaten with impunity by horses, goats, and sheep. Two parts of it are officinally recognized; the leaves, namely, and the fruit.

The student should guard against the use of the name cicuta, which, after the Roman authors, has frequently been applied to this plant; and continues to be applied by many. Botanists having given the Greek name zwyetoy to the genus to which it belongs, and the Latin name cicuta to a wholly different genus; and the former having been adopted in the officinal codes as the title of the medicine; it becomes the profession to abandon the latter altogether, in order to avoid confusion. This is the more important in the United States, as we have an indigenous poisonous plant, which has received the name of Cicuta maculata, or American hemlock, and might, without some caution, be confounded with the genuine hemlock of the old continent. The Cicuta maculata has a purple stem, less spotted than the Conium, and grows in low meadowy or swampy grounds, while the latter flourishes on the upland. Several cases of poisoning have occurred among children, from eating by mistake the root of American hemlock.

The leaves only are recognized by the U. S. Pharmacopoeia, the fruit having been abandoned in the present edition. The British Pharmacopoeia recognizes both the leaves with the young branches, and the fruit; designating the former as conium, the latter as conii fructus.

The leaves are bipinnate or tripinnate, with channeled footstalks, and small, incised leaflets, which are deep-green above, and paler beneath. When gathered, if intended for keeping, they should be deprived of the footstalk, and dried at a temperature not exceeding 120° F. They may be kept whole or in powder, and should be excluded from the air and light, as they rapidly deteriorate on exposure. With the greatest care, they undergo a gradual deterioration, and should not be used when more than a year old.

The dried leaves, when in powder, have a fine deep-green colour, and a strong, peculiar, but not disagreeable odour, which differs from that of the growing plant, and is probably unconnected with their medicinal efficiency. Their taste is bitterish and nauseous. Water distilled from them has their odour, but not their narcotic properties. if good, they emit the odour of mice when rubbed with solution of potassa.