TEXTURES.] ANATOMY 847 EPITHELIUM. The free surfaces covered by an epithe lium 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.g., 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 skin 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 super ficial strata, though sometimes, as in the bladder, conjunc tiva, 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. Epithe lium is distinguished further by being devoid of blood vessels, i.e., it is non- vascular; and also, with some excep tions, 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 rela tion 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 theoreti cally supposed to exist it could not satisfactorily be demon strated ; 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 g^nds is con tinuous at their orifices with the epithelial investment of the corium. The surface both of the skin and mucous mem branes is usually more or less undulated sometimes it is thrown into strong folds or rugce, at others it is elevated into minute, frequently conical, processes, named in some locali ties papillae, in others villi ; but in all these cases the epithe lium is prolonged as a continuous covering over the undulat ing 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, oesophagus, 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 j. FIG. 31. Columnar epithelium. A, side view of a group of cells; B, larger free end of a group of cells; C, a striated columnar cell from intestinal villus. in diameter from -J^th to To^th inch. Those in the same layer, being in contact by their edges, form a tessellated, pave ment-like arrangement, whilst the cells in adjacent layers have their flattened surfaces in con tact 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 morpho logical 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 -y-g-oth 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. Some times the cells are uni formly cylindrical ; 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 intes tinal villi have a clear space at their broad free ends, which is often streaked with fine parallel lines. Inter mingled 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 lachry mal sac, into the Eustachian tube and tympanum ; on the free surface of the mucous membrane of the windpipe as far as the terminal branches of the bronchial tubes; oil the mucous surface of the uterus and Fallopian tubes ; on the mucous lining of the commencement of the vas def erens, 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 peri phery possesses the firmer consistence Fio.32.-ciiiated epithelium of formed or differentiated material. tells - During life these processes move rapidly to and fro in the fluid which moistens the surface of the mem brane on which this form of epithelium is situated. In the human body the cilia are not more than from ^^ T th to j^V^l"- i ncn i n length; but in various marine iii- vertebrata 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
