This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol2", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
Manna is the concrete exuded juice of Ornus Europaea, or flowering ash, and of Ornus rotundifolia, growing wild in the South of Italy, and in Sicily, where also one or both trees are cultivated for this product. it is obtained through incisions in the trunk; that which exudes in the hot and dry months of summer being the finest, while that obtained later in the season, when the weather is cool and rainy, is of less value. The former is called flake manna; the latter, which is of variable character, may be divided into two varieties, one of which is a soft almost amorphous mass, named fat manna, and the other seemingly a mixture of fragments resembling the flake manna with a portion less or greater of the fat, and called manna in sorts or common, manna. it is only the flake and the common manna that are found in our shops.
Sensible and Chemical Properties. Flake manna is in irregular pieces of various size, often several inches long, sometimes less than an inch, and frequently concave on one side, as if from concretion upon the trunk of the tree. it is very light, porous, brittle, somewhat granular or crystalline in structure, and of a whitish or yellowish-white colour. Common manna consists of small fragments like those of flake manna, mixed with a soft, viscid, amorphous, darker substance, and varies in appearance as one or the other of these ingredients predominates.
The odour of manna is feeble and peculiar; its taste, sweet, peculiar, and often somewhat nauseous, but in the finest kinds scarcely disagreeable. it melts with heat, and is inflammable. it is soluble, with the exception of impurities, both in water and alcohol. Heated alcohol dissolves it very freely, and on cooling deposits a large proportion in a crystalline form. This is mannite, which may be obtained pure by a second solution and crystallization.
Chief Constituents. The characteristic ingredient of manna is mannite, which constitutes about 75 per cent. of the purer kind. This is a beautifully white, crystalline substance, inodorous and sweetish, and bearing a considerable resemblance to sugar, from which it differs in being insusceptible of the vinous fermentation. Besides this, there is in manna a small proportion of proper sugar, with a little yellow nauseous matter, upon which its cathartic property is thought mainly to depend. Pure mannite, however, has been found to be slightly laxative.
Manna is a very mild laxative, producing soft unirritating stools; though its operation is not unfrequently attended with flatulence, and a little consequent griping. it is adapted to infantile cases, to the constipation of pregnancy, and to hemorrhoidal affections attended with the same condition of bowels. Persons habitually costive, but not dyspeptic, may with advantage carry with them a piece of the finer kind of flake manna, and eat a little of it as required. It is, however, much more used in connection with other cathartics, the taste of which it covers, while it aids their operation. The medicines with which it is most frequently associated are senna and magnesia.
The dose for full effect is one or two ounces; for a child, according to the age, from one to four drachms. A drachm or two, however, will often act as a gentle laxative upon an adult. The flake manna may be given in substance; but the medicine is more frequently administered in solution, either in one of the aromatic waters, as peppermint water, or an aromatic infusion, especially fennel-seed tea.