QUERCUS PEDUNCULATA (Willd.) The bark of this species, the true British oak, is used in medicine. Its appearance varies with the age of the bark, as well as of the tree from which it is obtained. Its excellence is determined by the period of the year when the tree has been barked. The best is obtained from coppice oaks, not from old or full-grown trees, and is better from the branches than the trunk. According to Sir H. Davy, the greatest quantity of tannin exists in the bark in spring, at the time when the buds begin to open Chemistry,' 4th ed., p. 87), which is confirmed by Biggins. This is the period usually selected to decorticate the trees, owing to the greater ease with which it is then effiected, the cambium which exists between the bark and wood allowing of their easier separation. The interior cortical layers are much richer in the active principles than the external ones.
Oak-bark is commonly met with in pieces from one to two feet long. That from young stems or branches is generally thin, tolerably smooth, having a silvery or ash-gray cuticle, and frequently clothed with lichens, chiefly of the genus l'erruraria or Opeyrapha. When newly detached it is whitish internally, but by exposure to the air it deepens into brown, and becomes fibrous and cracked. The bark of old stems is much thicker, very rough and cracked, and greatly inferior in quality.
The chief chemical constituents are : tannin (impure tannic acid), tannates of lime, magnesia, potash, &c., gallic acid, uncrystallisable sugar, pectin, and lignin. It has the peculiar and well-known odour,of tan, and a bitter astringent, but afterwards sweetish, mucilaginous taste.
Oak-bark possesses all the properties of astringent substances, except the power of decomposing tartar-emetic. [ASTRINGENTS ; ANTIMONY.] It is more employed externally than internally as a wash or injection. By long-continued application it can almost tan the living tissue, and either strengthen a weak part of the akin, or convert a secreting into a non-secreting surface, as in cases of prolapsed or inverted uterus. One of the most valuable applications, and at the same time cheapest, to surfaces threatened with gangrene from pressure, as in fever& or old bedridden persona, is a wash formed by digesting oak-lark in weak distilled vinegar, and, after straining, adding hydrochloride of amnionia. It is not only unnecessary, but injurious, to make a decoction of oak bark : no tanner would think of boiling his oak-bark before steeping his hides in it.
Powdered oak-bark with chamomile flowers is a cheap and often effectual mode of checking the paroxyams of agues.
(Cullen's, Mat. Ned.; Pereira's Mat. Med.)