Source, Etc

Coto bark was first sent to Europe about 1873 from Bolivia under the name of coto-cinchona. Its botanical source is unknown, but it appears to be derived from a tree belonging to the natural order Laurineoe, and probably closely allied to the genus Cryptocarya (Hartwich, 1899). The bark was employed in the form of powder or alcoholic tincture for diarrhoea and for neuralgia, and rapidly gained a reputation in Europe as an astringent. Owing, however, partly, to the difficulty of obtaining the genuine bark, it fell into disuse, and is now seldom prescribed.

This, the true coto bark, was in commerce but a very short time. In 1876 it was replaced by a very similar bark, also imported from Bolivia. This later bark was found on examination to yield constituents resembling, but not identical with, those of true coto; one of these was called ' paracotoin,' and the bark that yielded it ' paracoto bark.'

Paracoto bark, as it is properly termed, is at present the current variety of coto; it is commonly sold as coto bark, and to it the following description refers.

Description

Paracoto bark occurs in flat or curved pieces commonly up to 30 cm. long, 8 or 10 cm. wide, and 10 to 15 cm. thick. They are hard, heavy, and of a cinnamon-brown colour; some of the pieces bear on their outer surfaces patches of a thin whitish, others of a brownish cork; they are frequently nearly smooth, or bear transverse wrinkles, or are irregularly chequered by longitudinal fissures and transverse cracks. The inner surface is brown and distinctly and coarsely striated, this being due to projecting strands of sclerenchymatous tissue.

The bark breaks with a fibrous, splintery fracture, coarse stiff strands of sclerenchymatous tissue projecting from the fractured surface. The inner part easily separates into coarse longitudinal strips, on the surface of which, especially after they have been exposed to the air for some time, glistening prismatic crystals can be observed even with the naked eye.

Fig. 137.   Paracoto bark, showing a fissured cork and splintery fibrous fracture. Natural size.

Fig. 137. - Paracoto bark, showing a fissured cork and splintery-fibrous fracture. Natural size.

The section is very characteristic. The outer layer, a thin brownish cork, encloses a narrow brown cortex separated by a distinct paler line (of sclerenchymatous cells) from the very thick bast which in older barks constitutes nine-tenths of the drug. This bast contains an abundance of sclerenchymatous tissue arranged in rounded or tangentially elongated groups. The section resembles that of sassy bark, but the groups of sclerenchymatous cells are much smaller, and the colour is reddish brown.

The drug has a very characteristic odour and a pungent taste.

The student should observe

(a) The brownish cork and brown striated inner surface,

(b) The characteristic transverse section, and

(c) The distinctive odour; and should compare this bark with

(i) Sassy bark, which has a nearly black inner surface, darker outer surface, and fewer and larger groups of sclerenchymatous cells;

(ii) Bebeeru bark, which is smoother and exhibits distinct medullary rays, but very inconspicuous masses of sclerenchyma;

(iii) Flat red cinchona bark, which is characterised by its rusty red colour, reddish warts, and bitter astringent taste.

Constituents

Paracoto bark contains several crystalline bodies, the chief of which, paracotoin, C12H804, occurs in pale yellow scales melting at 152°, and give a yellowish brown colour with nitric acid; it is dioxymethylenephenylcumalin. Other crystalline bodies present in the bark are phenylcumalin (leucotin), hydrocotoin (dimethyl ether of benzoylphloroglucinol), methylhydrocotoin, protocotoin (dioxy-methylene-phloroglucinol), and methylprotocotoin (oxyleucotin).

The bark also contains piperonylic acid, resin, tannin, and a little volatile oil.

Uses

The drug has been employed with success for diarrhoea, but is not much used now.

Varieties

True coto bark closely resembles paracoto; it is said to be distinguished by its odour, which recalls cinnamon, that of paracoto recalling nutmeg, and by its taste, which is more pungent. The most definite distinctive characters lie in the constituents. True coto contains cotoin and phenylcumalin, the substance formerly known as dicotoin being probably a mixture of these two substances. Cotoin, C4H1204, is the monomethyl ether of benzoylphloroglucinol; it crystallises in pale yellow prisms (melting point 130°) which colour nitric acid blood red.

The following simple test is the only definite means of distinguishing these two varieties of coto bark: Shake 10 grammes of the powdered bark with 100 grammes of ether frequently during an hour. Pour off the ethereal solution, add 50 grammes of water and distil off the ether. Cool, shake with 30 grammes of petroleum spirit, transfer to a separator (disregarding the resin which adheres to the sides of the flask), filter the aqueous solution into a porcelain dish, evaporate to dryness, dissolve the residue in a little glacial acetic acid, and add a drop of fuming nitric acid. True coto thus tested will afford a blood red coloration (Caesar and Loretz).

Resaldol, the ethyl ester of resorcinol-benzoyl carbonic acid is a synthetic product allied to cotoin and having a similar action.