1. Alexandrian Senna—This is furnished by Cassia acutifolia and is imported in large bales. It used formerly always to arrive in a very mixed and dirty state, containing, in addition to leaflets of senna, a variable proportion of leafstalks and broken twigs, pods and flowers; besides which there was almost invariably an accompaniment of the leaves, flowers and fruits of Solenostemma Argel Hayne ([p. 220]), not to mention seeds, stones, dust and heterogeneous rubbish. Such a drug required sifting, fanning and picking, by which most of these impurities could be separated, leaving only the senna contaminated with leaves of argel. But Alexandrian Senna has of late been shipped of much better quality. Some we have recently seen (1872) was, as taken from the original package, wholly composed of leaflets of C. acutifolia in a well-preserved condition; and even the lower qualities of senna are never now contaminated with argel to the extent that was usual a few years ago.
The leaflets, the general form of which has already been described ([p. 216]), are ¾ to 1¼ inches long, rather stiff and brittle, generally a little incurled at the edges, conspicuously veined, the midrib being often brown. They are covered with a very short and fine pubescence which is most dense on the midrib. The leaves have a peculiar opaque, light yellowish green hue, a somewhat agreeable tea-like odour, and a mucilaginous, not very marked taste, which however is sickly and nauseous in a watery infusion.
2. Arabian Moka, Bombay or East Indian Senna—This drug is derived from Cassia angustifolia, and is produced in Southern Arabia. It is shipped from Moka, Aden and other Red Sea ports to Bombay, and thence reaches Europe.
Arabian senna is usually collected and dried without care, and is mostly an inferior commodity, fetching in London sometimes as low a price as ½d. to ¼d. per lb. Yet so far as we have observed, it is never adulterated, but consists wholly of senna leaflets, often brown and decayed, mixed with flowers, pods, and stalks. The leaflets have the form already described ([p. 217]); short adpressed hairs are often visible on their under surface.
3. Tinnevelly Senna—Derived from the same species as the last, but from the plant cultivated in India, and in a state of far greater luxuriance than it exhibits in the drier regions of Arabia where it grows wild. It is a very superior and carefully collected drug, consisting wholly of the leaflets. These are lanceolate, 1 to 2 inches in length, of a yellowish green on the upper side, of a duller tint on the under, glabrous or thinly pubescent on the under side with short adpressed hairs. The leaflets are less rigid in texture than those of Alexandrian senna, and have a tea-like, rather fragrant smell, with but little taste.
Tinnevelly senna has of late fallen off in size, and some importations in 1873 were not distinguishable from Arabian senna, except from having been more carefully prepared. The drug is generally shipped from Tuticorin in the extreme south of India.
Chemical Composition—The analysis of senna with a view to the isolation of its active principle has engaged the attention of numerous chemists, but as yet the results of their labours are not quite satisfactory.
Ludwig (1864) treated an alcoholic extract of senna with charcoal, and obtained from the latter by means of boiling alcohol two bitter principles, Sennacrol, soluble in ether, and Sennapicrin, not dissolved by ether.
Dragendorff and Kubly (1866) have shown the active substance of senna to be a colloid body, easily soluble in water but not in strong alcohol. When a syrupy aqueous extract of senna is mixed with an equal volume of alcohol, and the mucilage thus thrown down has been removed, the addition of a further quantity of alcohol occasions the fall of a dark brown, almost tasteless, easily alterable substance, which is indued with purgative properties. It was further shown that this precipitate was a mixture of calcium and magnesium salts of phosphoric acid and a peculiar acid. The last named, separated by hydrochloric acid, has been called Cathartic Acid; it is a black substance which in the mouth is at first insipid, but afterwards tastes acid and somewhat astringent. In water or strong alcohol it is almost insoluble, and entirely so in ether or chloroform; but it dissolves in warm dilute alcohol. From this solution it is precipitable by many acids, but not by tannic.
Groves[844] in 1868, unaware of the researches of Dragendorff and Kubly, arrived at similar results as these chemists, and proved conclusively that a cathartate of ammonia possesses in a concentrated form the purgative activity of the original drug.