History—[See p. 299].
Description—Hemlock in its first year produces only a tuft of leaves; but in its second a stout erect stem which often grows to the height of 5 or 9 feet, is much branched in its upper part, and terminates in small umbels, each having about 12 rays. The lower leaves, often a foot in length, have a triangular outline, and a hollow stalk as long as the lamina, clasping the stem at its base with a membranous sheath. Towards the upper portion of the plant, the leaves have shorter stalks, are less divided, and are opposite or in cohorts of 3 to 5. The involucral bracts are lanceolate, reflexed, and about a ¼ of an inch long. Those of the partial umbel are turned towards the outside, and are always 3 in number. The larger leaves are twice or thrice pinnate, the ultimate segments being ovate-oblong, acute, and deeply incised.
The stem is cylindrical and hollow, of a glaucous green, generally marked on its lower part with reddish-brown spots. The leaves are of a dull dark green, and like the rest of the plant quite glabrous. They have when bruised a disagreeable fœtid smell.
For medicinal purposes the plant should be taken when in full blossom.[1152]
Chemical Composition—The leaves of hemlock contain, though in exceedingly small proportion, the same alkaloids as the fruits. Geiger obtained from the fresh herb not so much as one ten-thousandth part of conine. It is probable however that the active constituents vary in proportion considerably, and that a dry and sunny climate promotes their development.
The same observer, as well as Pereira, has pointed out that hemlock leaves when dried are very frequently almost devoid of conine, and the observation is supported by the more recent experiments of Harley (1867). It has also been shown by the last named physician, that the inspissated juice known in pharmacy as Extractum Conii usually contains but a mere trace of alkaloid, the latter having in fact been dissipated by the heat employed in reducing the juice to the required consistence. On the other hand, Harley has proved that the juice of fresh hemlock preserved by the addition of spirit of wine, as in the Succus Conii of the Pharmacopœia, possesses in an eminent degree the poisonous properties of the plant.
The entire amount of nitrogen in dried hemlock leaves was estimated by Wrightson (1845) at 6·8 per cent.; the ash at 12·8 per cent. The latter consists mainly of salts of potassium, sodium, and calcium, especially of sodium chloride and calcium phosphate.
A ferment-oil may be obtained from Conium; it is stated to have an odour unlike that of the plant and a burning taste, and not to be poisonous.[1153]
Uses—Hemlock administered in the form of Succus Conii, has a peculiar sedative action on the motor nerves, on account of which it is occasionally prescribed. It was formerly much more employed than at present, although the preparations used were so defective that they could rarely have produced the specific action of the medicine.
Plants liable to be confounded with Hemlock—Several common plants of the order Umbelliferæ have a superficial resemblance to Conium, but can be discriminated by characters easy of observation. One of these is Æthusa Cynapium L. or Fool’s Parsley, a common annual garden weed, of much smaller stature than hemlock. It may be known by its primary umbel having no involucre, and by its partial umbel having an involucel of 2 or 3 linear pendulous bracts. The ridges of its fruit moreover are not wavy or crenate as in hemlock, nor is its stem spotted.