(From membrana, parchment, which it resembles). A membrane; chiton. Winslow describes it to be a pliable texture of fibres, disposed or interwoven together in the same plane. Membranes differ in thickness, according to the smallness of their fibres, and the number of their planes, called lamina. For a description of the cellular membrane, see Cellu-losa Membrana, and Fibra.

The mucous membrane is that covering of the surface of any part, generally within the body, which is exposed to injury from extraneous matter, as the internal membrane of the mouth, nose, lungs, oesophagus, stomach, intestines, and urinary passages, from which they are defended by the mucus, which it secretes.

The common membranes, called the common teguments, are the scarf skin, the true skin, and the cellular membrane; but from this rank the first and last must be excluded, for the scarf skin is an apparently inorganic exudation, inspissated, or a scaly covering, and the tela cellulosa not very different. The membranes which cover particular parts are, the dura and pia mater, pleura, peritonaeum, pericardium, periosteum, membrana propria musculorum, a vascular membrane which covers all the vessels of the body, and those which form the tubes, as the stomach, intestines, arteries, veins, gall bladder, or urinary bladder. Membranes, we have said, (see Fibra, Foetus, and Generatio,) are apparently primordial and nervous, and afford a striking instance where nerves, by a closer and more compact texture lose their sensibility; for membranes, like tendons, are insensible in a sound state. They show, however, their peculiar nature by the extreme pain and irritation felt when stretched or inflamed.

Membrana hyaloidea, the membrane which encloses the vitreous humour of the eye.

Membrana papillaris, the membrane which covers the pupil of a foetus before the sixth month.

Membrana ruyschiana the internal lamina of the choroid.

Membrana tympani. The membrane separating the tympanum of the ear from the meatus externus. (See Auris.) It is said to consist of six laminae, 1st, a production of the epidermis; 2d, of the skin lining the auditory passage; 3d, a cellular membrane supporting reticulated vessels; 4th, a production of the periosteum of the meatus; 5th, like the 3d; 6th, a production of the periosteum of the internal cavity. Muscular fibres have also been discovered in it.