EPITHELIII21. - The free surfaces ocvered by an epithelium are the skin and the membranes, named, from the character of their secretion, mucous membranes. The _Mucous Membranes line internal passages and canals, and are continuous at certain orifices with the skin, - e.y., the mucous membrane of the alimentary canal opens on the surface at the mouth and anus ; the respiratory mucous membrane opens on the surface at the nostrils, and is continuous in the pharynx with the alimentary mucous membrane - it is also prolonged through the Eustachian tube into the tympanum, and is continuous through the nasal duct with the conjunctiva; the genito-urinary mucous membrane opens on the surface at the orifice of the urethra and vagina. Mucous membranes also line the ducts of the various glands which open on the surface either of the skill or the several mucous membranes. The epithelial cells are as a rule arranged in layers or strata, and the shape of the cells is by no means uniform in the different layers. The cells of the deeper strata are usually smaller, softer, more rounded, and more recently formed than those of the superficial strata, though sometimes, as in the bladder, conjunctiva, and some other mucous surfaces, they may be irregular in form and size, or even elongated into short columns. The cells next the free surface have a tendency to be shed, and their place is then taken by the cells of the deeper layers, which become modified in form as they approach the surface. The form of the cells of the superficial layer varies in different localities, which has led to a division of epithelium into groups bearing appropriate names. Epithelium is distinguished further by being devoid of blood-vessels, i.e., it is non-vascular; and also, with some exceptions, devoid of nerves, i.e., non-sensitive.

The epithelial cells, whether arranged in one or several strata, rest upon a subjacent tissue, which, from its relation to the cells, may be called sub-epithelial. The sub-epithelial tissue is a delicate modification of the fibrous form of connective tissue, to be subsequently described, and in it the nerves and the blood and lymph vessels of the skin and mucous membranes ramify; hence it is sometimes described as a fibro-vascular tissue or corium. It was for a long time believed that between the deeper surface of the epithelium and the corium a homogeneous continuous membrane, named by Bowman a basement membrane, intervened. Bowman, however, himself admitted that in some of the localities where this membrane was theoretically supposed to exist it could not satisfactorily be demonstrated; and the general opinion of anatomists now is, that a distinct separable membrane .does not intervene between the epithelium and the fibro-vascular corium, but that the cells of the former rest directly upon the surface of the latter. The corium is also the seat of the numerous glands, with the blood and lymph vessels and the nerves belonging to them, found in connection with both the skin and the mucous membranes ; and the epithelial lining of the glands is continuous at their orifices with the epithelial investment of the corium. The surface both of the skin and mucous membranes is usually more or less undulated - sometimes it is thrown into strong folds or rugme, at others it is elevated into minute, frequently conical, processes, named in some localities papillre, in others villi; but in all these cases the epithehum is prolonged as a continuous covering over the undulating free surface. The free surface of all mucous membranes is kept moist by the secretion or mucus which lubricates it.

Tessellated, pavement, scaly, or squamous epithelium is situated on the free surface of the mucous lining of the mouth, pharynx, cesophagus, vestibular entrance to the nose, ocular conjunctiva, and entrance to the urethra and vagina. It forms, under the special name of the horny layer of the cuticle or epidermis, the superficial investment of the skin. Its cells are nucleated flattened scales, varying layer, being in contact by their edges, form a tessellated, pavement-like arrangement, whilst the cells in adjacent layers have their flattened surfaces in contact with each other. Sometimes the cells have jagged, serrated edges, or fluted surfaces, and usually they contain scattered granular particles. In the forma tion of this epithelium a morphological differentiation of the protoplasm of the rounded cells of the deeper strata into flattened scales, and at the same time a chemical differentiation of their soft contents into a horny material, have occurred.

Columnar or cylindrical epithelium is situated on the free surface of the mucous lining of the alimentary canal from the cesophageal orifice of the stomach to the anus, it is prolonged into the ducts of various glands which open on the alimentary mucous membrane ; it covers the mucous lining of the urethra and the mucous membrane of the gall bladder. Its cells are elongated, cylindrical columns, about 3--hth inch long, placed side by side like a row of palisades, and with their long axes perpendicular to the surface on which the cells rest. Sometimes the cells are uniformly cylindrical ; at other times they are compressed at the sides; at others they vary in circumference, - the broader end, lying next the surface, being rounded or polygonal ; the deeper extremity being narrower and more pointed. The nuclei are distinct, and the cell contents are finely granular. Usually this epithelium forms only a single layer of cells. The columnar cells which cover the intess tinal villi have a clear space at their broad free ends, which is often streaked with fine parallel lines. Intermingled with the cells of the columnar epithelium of the alimentary canal are small goblet-shaped cells.

Ciliated epithelium is situated on the free surface of the nasal mucous membrane, which extends into the air-sinuses within the cranial bones, into the nasal duct and lachrymal sac, into the Eustachian tube and tympanum ; on the free surface of the mucous membrane of the windpipe tts far as the terminal branches of the bronchial tubes ; on the mucous surface of the uterus and Fallopian tubes ; on the mucous lining of the commencement of the vas deferens, and on the lining membrane of the ventricles of the brain and central canal of the spinal cord. It generally consists of columnar cells, which have at their free ends extremely slender, soft, pellucid, hair-like processes, or cilia. These cilia are specially differentiated at the free ends of the epithelium cells from which they project. Beale states that the soft bioplasm (protoplasm) of the body of the cell is prolonged along the axis of each cilium, whilst the periphery possesses the firmer consistence of formed or differentiated material.

During life these processes move rapidly to and fro in the fluid which moistens the surface of the membrane ou which this form of epithelium is situated. In the human body the cilia are not more than from 7,1,--,th to 4--kuth inch in length ; but in various marine invertebrate they are both longer and stronger. Sometimes, as in the lining membrane of the cerebral ventricles and central canal of the spinal cord, the cells carrying the cilia are either spheroidal or cylindrical; but as the cavities lined by these cells are shut off from the air, the cells ought rather to be referred to the endothelial than the epithelial series of structures. Cilia occasion currents in the fluid in which they move, and play an important part in the economy of many animals; in some of the invertebrate they serve as organs of locomotion, in others they propel currents over respiratory surfaces, and in others aid in bringing food within the animal's reach.

Spheroidal or glandular epithelium is situated on the free surface of the follicles or ultimate secreting apparatus of glands, and the commencement of gland ducts. The cells are often spheroidal in form, though not unfrequently they are polyhedral. Their contents are specially differentiated into the secretion of the particular gland in which they are situated.

The epithelial cells of a Secreting Gland rest upon a sub-epithelial tissue. Not unfrequently this tissue has the appearance of a membrane; it represents, indeed, the basement membrane of Bowman, and is called membrana propria. Deeper than this apparent membrane is a delicate connective tissue in which the blood and lymph vessels and the nerves of the gland ramify. The anatomical structures necessary for secretion are cells, blood-vessels, and nerves The blood-vessels convey the blood from which the secretion has to be derived; the cells, as Goodsir showed by a variety of proofs, are the active agents in separating the secretion from the blood ; the nerves regulate the size of the blood-vessels, and therefore the amount of blood which circulates through the gland, and perhaps also exercise some direct influence on the activity of the cells. The connective tissue and the me2nbrana propria are merely supporting structures for the cells, vessels, and nerves. All secreting glands have the same general type of structure, though they differ from each other, as will be pointed out when the individual glands are described, in the degree of complexity in which their constituent parts are arranged.

Transitional epithelium is the name applied to epithelial cells, situated on some free surfaces, which possess transitional forms either between the columnar and tessellated epithelia, or the columnar and spheroidal. The epithelium of the mucous lining of the bladder is transitional between the columnar and scaly varieties ; and in many glands the continuity of the epithelial layer from the spheroidal epithelium of the gland follicles to the columnar epithelium of the duets is preserved by the interposition of intermediate transitional forms of cells.

The epithelial surfaces of the upper part of the mucous lining of the nose and of the back of the tongue are specially modified in connection with the senses of smell and taste localised in those regions, as will afterwards be 'considered when their anatomy is described.