MUCOUS MEMBRANES AND MUCUS. Under the term Mucous SYSTEM, anatomists include the skin, mucous membranes, and true glands, all of which are continuous with one another, and are essentially composed of similar parts. As the skin and the glands are described in special articles, it only remains to speak of the great internal mucous tracts. These are the alimentary mucous membrane, the respiratory mucous membrane,. and the genitourinary mucous membrane.
The alimentary raucous membrane commences at the lips, and not only forms the inner coat of the intestinal canal from the mouth to the anus, but gives off prolonga tions which after lining the ducts of the various glands (the salivary glands, the liver, and the pancreas) whose products are discharged into this canal, penetrate into the inner most recesses of these glands, and constitute their true secreting element. Besides these laro•er offsets we find in the stomach and small intestine an infinite series of minute tubular prolongations, the anatomical arrangement and function of which are described in the article DIGESTION.
The respiratory mucous membrane begins at the nostrils, and under the name of sahneiderian or pituitary membrane, lines the nasal cavities, from whence it sends' on either side an upward prolongation through the lachrymal duct to form the conjunctiva of the eye; backwards, through the posterior nares(the communication between the nose and the throat), it sends a prolongation through the Eustachian tube to the middle ear (the cavity of the tympanum), and is continuous with the pharyngeal mucous membrane (which is a portion of the alimentary tract); it then, instead of passing down the ceso phagus, enters and forms a lining to the larynx, trachea, and bronchial to their terminations. From the continuity of these two tracts, some writers describe them as a single one, under the name of the tract.
The genito-urinary mucous membrane commences at the genitourinary orifices, lines the excretory passages from the generative and urinary organs, and is the essential con stituent of the glands of both. See KIDNEY, for example: We thus see that mucous membranes line all those passages by which internal parts communicate with the surface, and by which matters are either admitted into or elimi nated from the body. As a general rule, tney are soft and velvety, and of a more or less red color, from their great vascularity, but they present certain structural peculiarities according to the they are requirmid MdiScharge.: • In!all the principal parts of the mucous "tiActs we find the mucous an externallayer of epithelium (q.v.) resting on a thin, transparent., homogenous membrane, which from its position is termed the easement membrane, and beneath this a stratum of vascular tissue of variable thickness, which usually presents either outgrowths in the form of papilhe and villi, or depressions or inversions in the form of follicles or glands, or both. The
follicles are almost invariably present, but the papilla! and villi are limited to the alimen ta•y or gast•o-intestival mucous membrane. " The MUCOUS membranes," says Dr. Car penter, "constitute the medium through which nearly all the material changes are .
effected that take place between the living organism and the external world. 'Thus, in the gastrointestinal mucous membrane we find a provision for reducing the food by means of a solvent fluid poured out from its follicles: whilst the villi, or root-like file melds, which are closely set upon its surface towards its upper part, are specially adapted to absorb the nutrient materials thus reduced to the liquid state. The same membrane, at its lower part. constitutes an outlet through which are cast out not merely the indigestible residuum of the food, but also the excretions from numerous; minute glandube in the intestinal wall, which result front the decomposition of the tissues, and which must be separated from. them to prevent further decay. Again, the bronehio pulmonary. or respiratory mucous membrane, serves for the introduction of oxygen from the air, and for the exhalation of water and carbonic acid. And, lastly, the mucous membranes are continuous with the cell-lined vesicles or tubes of the various glands, which are the instruments whereby their respective products are eliminated from the blood." Although the various kinds of epithelial cells discharge a special office in rela tion to the peculiar function of the mucous membrane upon which each kind occurs, yet they all serve one general purpose—namely, that of protecting the surfaces on which they are placed. This protecting !timer is increased by the presence of the secretion known as mucus, which ordinarily forms an extremely thin layer on these membranes, but wlien they are irritated or inflamed, is secreted in very considerable quantity. The exact mode of its formation is still a disputed question, but it is generally believed to be the product of the gradual solution of the uppermost epithelial cells. Besides acting both mechanically and chemically as a shield to highly sensitive membranes, it has other uses, amongst which two may be especially mentioned.-1. It communicates to the salivary, and probably to other glands, properties which are not possessed either by itself or by the pure glandular secretions; and 2. It serves to eliminate a considerable quantity of nitrogen from the system. This nitrogen is contained in the mucio, which forms front 2.4 to 9 per cent of nasal and bronchial mucus. This mucin contains 12.G-1 per cent of nitrogen, and is the substance which gives to mucus its viscid and tenacious character. Normal mucus is devoid of smell and taste, and almost, if not quite, neutral; and hence its constant presence in the mouth gives rise to no disagreeable sensation.