Membrane, a general term applied to thin layers of tissue, more or less elastic, whitish or reddish, lining either closed cavities or canals opening externally, absorbing or secreting fluids, and enveloping various organs. The simple membranes are either mucous, serous, or fibrous. - The mucous membranes are so called from the peculiar fluid or mucus which they secrete; they line the passages of the body which communicate externally, and by which foreign substances are taken "in or the secretions and excrementitious matter carried off; they are continuous with the skin, perform many of its offices internally, and at the points of contact, as in the lips, can hardly be separated by a distinct demarcation. Soft and smooth or velvety, reddish and very vascular, attached to muscle, cartilage, or even periosteum, their free surface is lined with a layer of epithelial cells covering the vascular parts. They present papilla) upon the tongue, villosi-ties and folds in the alimentarv canal, and de-pressions for glands almost everywhere. The three divisions of the mucous membranes are those lining the digestive, respiratory, and genitourinary passages.

The digestive mucous membrane begins in the mouth, extends through the oesophagus to the stomach, and through the intestinal canal to the anus, sending prolongations into the ducts of the salivary glands, liver, pancreas, and gall bladder. The respiratory mucous membrane lines the nose and the cavities and sinuses connected therewith, the eyelids, middle ear, larynx, trachea, and the bronchial ramifications. The genitourinary mucous membrane extends externally from the uriniferous tubes of the kidney, and into and through the reproductive organs. In each of these tracts the membranes present some slight modifications adapted for special functions. Mucous membranes are generally endowed with keen sensibility at their points of origin from the skin, as on the lids, lips, etc, but gradually become less sensitive and finally almost insensible, in a healthy state, in the interior of the organs. Besides being the seat of various secretions and absorptions, they assist in the functions of digestion, respiration, and reproduction. (See Epithelium, Gland, and Intestine.) - Serous membranes are formed of fibro-cellular tissue, covered with epithe- lial cells; they are very thin, smooth, transparent, and extensible, not having the folds, papillae, and glands of mucous membrane; they are closed sacs, and are found wherever internal organs come in contact with each other, or lie in cavities where more or less motion is required; they consist of two layers, the first surrounding the organ itself, and the second reflected upon the parts with which it is in contact and on which it moves; the cavity is lubricated by a serous fluid, exuded from the surface of the membrane.

They are of two kinds: those which line the visceral cavities, as the peritoneum in the abdomen, the pleura and pericardium in the chest, and the arachnoid of the brain and spinal cord; and the synovial membranes, which line the joints, sheaths of tendons and ligaments, and bursa? interposed between muscles and points of bone over which they glide. They are all shut sacs, except where the Fallopian tubes in most vertebrates open into the abdominal cavity. Serous and synovial membranes by their polished and well lubricated surfaces secure the free movement of contiguous organs, as in the intestines, lungs, and joints; in health their fluid is only sufficient for this purpose, but in a state of inflammation the amount is largely increased, as in the dropsical effusions of peritonitis, pleurisy, pericarditis, hydrocephalus, and synovitis; their sensibility in the normal state is nothing, but in diseased conditions may become acute, as in pleurisy and peritonitis. - Bichat gives the name of fibrous membranes to the aponeuroses of muscles, the capsules of the joints, the sheaths of the tendons, the periosteum, the dura mater of the brain, the sclerotic coat of the eye, etc.; these are never free, but are in contact with and adherent to the parts surrounding, and not moistened by - creted fluid; they are whitish, of a pearly and often shining lustre, and may form sacs, sheaths, or extended layers of thin tissue; possessing elastic and inelastic fibrous tissue, they afford strength to organs, retain the muscles and tendons in place, give the shape to the limbs, favor the movements of the skin and superficial muscles, and assist the venous circulation. - Membranes, especially the serous, may be formed as the accidental products of disease, as in cysts in various parts of the body.

False membranes are layers of coagulated rib-rine or lymph exuded upon inflamed surfaces, presenting the external form of true membranes, but destitute of organization; under the influence of violent or special inflammations they may endanger life by closing passages, as in the false membrane thrown out in croup. (See Lymph.) The membranes of the foetus are alluded to under Embryology, and several other membranes under the names of the organs to which they specially belong.