KIDNEY.
membranes line all those passages by which internal parts communicate with the sur face. and by which matters are either admitted into or eliminated from the body. As a general ride they are soft and velvety, and of a more or less red color. from their great vascularity; but they present certain structural peculiarities ac cording to the functions which they are required to discharge. In all the principal parts of the mucous tracts we find the mavens membrane to present an external layer of epithelium (q.v.) resting on a thin. transparent, homogeneous membrane, which from its position is termed the basement membrane. and beneath this a stratum of vascular tissue of variable thickness. which usually presents either outgrowths in the form of papilla. and villi, o• 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 alimentary or gastro-intestinal mucous membrane. The mu cous membranes constitute the medium through which nearly all the material changes are effect ed that take place between the living organism and the external world. Thus in the gastro intestinal mucous membrane we find a provision for reducing the food by masts of a solvent fluid poured out from its follicles; while the villi, which are closely set. upon the surface of the small intestine. are specially adapted to absorb the nutrient materials thus reduced to the liquid state. The same membrane, at its lower part, constitutes In outlet through which are cast out not merely the indigestible residuum of the food, but also the excretions from numerous minute glandular in the intestinal wall, which result from the decomposition of the tissues• and which must be separated from them to prevent fur ther decay. Again• the hronchio-pulmonary or re spiratory mucous membrane serves for the intro duction of oxygen from the air. and for the ex
halation of water and c•arbonie avid. And, lastly. the muedins membranes are continuous with the eelblined vesicles or tubes of the various glands, which are the instruments whereby their respec tive products are eliminated from the blood. Although the various kinds of epithelial cells discharge a special office in relation to the pe culiar function of the mucous membrane upon wLirh Pad' kind occurs. yet. they all servo one general purposi—namely, that of Protecting the surfaces on which they are placed. This proteet ing power is increased by the presence of the se cretion known as mucus, which ordinarily forms all extremely thin layer on these membranes, but when they are irritated or inflamed is secreted in very considerable quantity. (See CATARRH.) The exact mode of its formation is still a dis puted 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 high ly sensitive membranes, it has other uses, among which two may be especially mentioned: (1) It communicates to the salivary, and prob ably to other glands, properties which are not possessed either by itself or hy the pure glandular secretions; and (2) it serves to eliminate a eon siderable quantity of nitrogen from the system. This nitrogen is contained in the macin, which forms from 2.4 to 9 per cent. of nasal and bronchial mucus. This nmein contains 12.64 per cent. of nitrogen. and is the substance which gives to mucus its viscid and tenacious charac ter. Normal mucus is devoid of smell and taste, and faintly alkaline in reaction, and hence its constant presence in the mouth gives rise to no disagreeable sensation.