penetrate into epithelial tissue, although in some cases minute channels may exist. between the cells into which the plasma of the blood, derived from the blood-vessels of the subjacent connective tissue, may pass for the nutrition of the epithelium-cells. Nerves are abundant in many epithelia, the nervous fibrils passing in the form of fine varicose filaments among the epithelium-cells.

In certain situations branched "migratory cells" which may contain pigment lie in the intercellular substance of an epithelium.

Epithelium-cells vary in structure as well as in shape, and some of these differences will be mentioned in speaking of the varieties of epithelium. The nucleus varies, however, far less than the rest of the cell: in most cases it has an intranuclear network and one or more nucleoli. In the division of epithelium-cells, it undergoes the changes which have already been described.

Classification of epithelia.-The varieties of epithelium may be classified in various ways, but none perhaps are altogether satisfactory. Thus we may distinguish an epithelium according to its origin, as epiblastic, mesoblastic, or hypoblastic, and this distinction is partially indicated when a separate term (endothelium) is used to denote mesoblastic epithelium. Or again, the epithelia may be classed according to their function, and in this way we distinguish between the protective, the secreting, the ciliated, and the sense-epithelia. But without failing to recognize that these modes of classification have a certain amount of importance, it will be most convenient here to follow the prevalent custom, and to classify the varieties of epithelium-cells according to their shape and arrangement.

In the first place we may distinguish an epithelium which is composed of only a single layer of cells as a simple epithelium in contra-distinction to a stratified epithelium, in which the layers of cells are numerous. Where, on the other hand, the cells are in more than a single layer, but the two or three layers dove-tail the one into the other, so that the structure is not distinctly stratified, the term transitional may be employed.

Stratified epithelium. In a stratified epithelium the cells are disposed in a number of layers, and it is commonly found that the constituent cells of the various strata exhibit every variety of shape. As a rule the cells of the deepest or

The

c, lowermost columnar cells; p, polygonal cells above these; A, flattened cells near the surface. intercellular channels bridged by minute processes of the cell, are well seen. The lower part of the section on the right is somewhat broken.

attached layer are columnar (fig. 222, c), and the superficial cells are flattened scales (fig. 222, fl) which may be of considerable size, but which do not, like the cells of pavement or simple scaly epithelium, fit together by their edges, but, on the contrary, overlap one another (fig. 223). The cells of the layers immediately external to the columnar layer are rounded in shape or at least only so far modified as to enable them to fit to the columnar cells and to one another (fig. 222, p); but as we trace the strata

towards the surface, we find the component cells becoming more flattened and larger, whilst at the same time undergoing a change in their chemical constitution, so that at first the external part, and afterwards the whole of the protoplasm of the cell, is converted into horny substance, even the nucleus being at last involved.

The conversion into horny substance is in many instances preceded by the deposit of a granular material within the cells, which is termed eleidin (Ranvier) or keratohyalin (Waldeyer). In the epidermis and some other parts the cells which contain this granular material form an almost complete layer between the soft, still protoplasmic, deeper cells and the superficial horny stratum (fig. 224). The layer is termed stratum granulosum and was described by Langerhans.

The deeper protoplasmic cells of a stratified epithelium are continually multiplying by cell-division, and, as the new cells which are thus produced in the deeper parts increase in size, they compress and push outwards those previously formed. In this way cells which were at first deeply seated, become gradually shifted towards the surface, undergoing meanwhile the chemical change above spoken of. The older superficial cells are continually being removed by abrasion, while others rise up to supply their place.

Intercellular bridges and channels.-The deeper layers of a stratified epithelium are not closely applied to one another by their edges, but there exists a system of intercellular channels, which are bridged across by fibres which run from one cell to the other (see figs. 222, 224). When the cells are isolated, the fibres are

Fig. 223.-EPITHELIUM-SCALES FROM THE INSIDE OF THE MOUTH; MAGNIFIED 260 DIAMETERS. (Henle.) Fig. 224.-PORTION OF EPIDERMIS FROM A SECTION OF THE SKIN OF THE FINGER, COLOURED WITH

PICROCARMINE. (Ranvier.)

a horny layer; b, its deepest part (stratum lucidum) with flakes of eleidin; c, eleidin granules in cells of stratum granulosum; d, deeper cells of stratum mucosum with intercellular channels; e, dentations by which the deepest cells are fixed to the surface of the cutis vera.

broken through and appear as spikes or dentations on the surface and edges of the cells (fig. 225). Sometimes the intercellular channels become widened in consequence of an excess of fluid accumulating in them, but usually they are very narrow and but little obvious.

The spikes and ridges upon the deeper cells of a stratified epithelium were first noticed by Max Schultze, who was of opinion that they were for the purpose of effecting, by indenting with those on adjoining cells, a firmer connexion between the cells of the epithelium. The true relations of the structures in question, and the intercellular channels which are bridged across by them, were discovered by Bizzozero. The researches of J. Arnold and of Thoma have shown that similar intercellular channels occur extensively in all varieties of epithelium. The fibrils which bridge across the intercellular spaces are described by Ranvier as passing

through the protoplasm of the cells. According to Ramón y Cajal, they are covered by a prolongation of cell-membrane. A similar view is taken by Ide, who describes the cell-membrane as reticulated. A radiating system of fibrils has also been shown to occur in the flattened epithelium cells which cover the posterior surface of the cornea, and in this case also the fibrils traverse the intercellular spaces, passing from one cell into another.

d

Stratified scaly epithelium occurs in one of its simplest and most typical forms covering the anterior surface of the cornea of the eye (fig. 222). It is found also lining the mouth, the chief part of the pharynx, and the oesophagus, and in the female it lines the vagina and part of the cervix uteri, but its most extensive distribution is over the surface of the skin, where it forms the epidermis. In many parts of the epidermis the layers become very numerous, and their arrangement somewhat complicated, as will be noticed in the description of the skin. It may be remarked. that, in most of the situations where it is found, stratified scaly epithelium is of epiblastic origin, but this is by no means invariably the case and its occurrence depends much more upon the physiological conditions of the parts which it covers. Thus, wherever a surface is liable to undergo friction or abrasion, there we find a development of stratified scaly epithelium.

Fig. 225.-Two"PRICKLE-CELLS " FROM THE DEEPER PART OF THE EPIDERMIS. (Ranvier.) d, space around the nucleus, probably caused by shrinking of the latter.

Fig. 226.-SECTION OF THE TRANSITIONAL EPITHELIUM LINING THE BLADDER. (E. A. S.)
a, superficial; b, intermediate; and c, deep layer of cells.

HIGHLY MAGNIFIED.

(Klein.)

Fig. 227.-EPITHELIAL CELLS FROM THE BLADDER OF THE RABBIT. a, large flattened cell from the superficial layer, with two nuclei, and with strongly marked ridges and intervening depressions on its under surface; a', one of the same cells shown in profile; b, pear-shaped cell of the second layer showing the manner in which it is adapted to a depression on the superficial cell.

kind of cell in each which happens to be most prominent or superficial. The

columnar and ciliated transitional epithelia differ, however, so very slightly from the corresponding simple epithelia-viz., merely in the presence between the fixed ends of the columnar and ciliated cells of smaller and probably younger epithelium cells irregularly disposed-that they do not seem to merit any special description. But the scaly transitional epithelium which is met with lining the urinary bladder and ureters presents several peculiarities. It consists of three or four layers of cells, of which the inner or most superficial are large flattened scales when examined from the distended bladder (fig. 227, a); almost cubical in shape when taken from the collapsed organ; smooth over their free surface, but pitted on the opposite side, being moulded over the rounded ends of the cells which form the next layer. These are pyriform, and the smaller end of the pear is set upon the subjacent connective tissue, whilst the larger end has the position just mentioned (fig. 227). Filling up the intervals between these tapering cells are the smaller irregular cells of the third layer (fig. 226, c). All these cells have distinct nuclei, and in the flattened superficial cells two nuclei may often be seen in the cell. If this is an indication that the cell is about to divide, the mode of growth of this kind of epithelium must be different from that of the stratified scaly variety, in which the multiplication of the cells appears to take place exclusively in the deeper layers.

Fig. 228.-PAVEMENT EPITHELIUM, SCRAPED FROM A SEROUS MEMBRANE.
a, cell-body; b, nucleus; c, nucleoli. (Henle.)

Fig. 229.-PAVEMENT EPITHELIUM (ENDOTHELIUM) FROM THE OMENTUM OF THE RABBIT.
NITRATE OF SILVER STAINING. (E. A. S.)

Pavement epithelium.-In this the cells form polygonal plates or scales, which fit together by their edges like the tiles of a mosaic pavement. The lines of junction of the cells may be straight, or they may be more or less jagged or sinuous. The flattened mesoblastic epithelia (endothelia), such as the epithelium of serous membranes and of the vessels and that lining the anterior chamber of the eye belong to this variety, but it includes also the epithelium lining the alveoli of the lungs which is of hypoblastic origin; that covering the outer surface of the membrana tympani, and that lining the mammary ducts, both of which are epiblastic.

Columnar epithelium.-A second variety of simple epithelium is the columnar,

or cylinder-epithelium, in which the cells have a prismatic figure, and are set upright on the surface which they cover. In profile a row of these cells looks for the most part like a close palisade (fig. 231); but viewed from the surface each cell has a polygonal outline, the cells being flattened where they touch, from mutual com

Fig. 230.-COLUMNAR EPITHELIUM CELLS OF THE RABBIT'S INTESTINE. (E. A. S.) The cells have been isolated after maceration in very weak chromic acid. The protoplasm is reticular and vacuolated; the striated border (str.) is well seen, and the bright disc separating this from the cellprotoplasm; n, nucleus with intranuclear network; a, a thinned out wing-like projection of the cell which probably fitted between two adjacent cells.

Fig. 231.-A ROW OF COLUMNAR CELLS FROM AN INTESTINAL VILLUS OF THE RABBIT. (E. A. S.) str, striated border; w, wander-cells between the epithelium cells.

pression, so that thus again a mosaic pattern is produced. Columnar epitheliumcells vary much in form, in dimensions, and even in structure. Those which may be looked upon as typical are of a long tapering figure, the finer extremity being set

Fig. 232. CROSS-SECTION OF A VILLUS OF THE INTESTINE. (E. A. S.)

e, columnar epithelium with striated border; g, goblet cell, with its mucus partly extended; l, lymph-corpuscles between the epithelium-cells; b, basement membrane; c, sections of blood capillaries; m, section of plain muscular fibres; c. l., central lacteal.

HIGHLY MAGNIFIED.

(Klein.)

upon a surface, and the other and larger end being free. At their sides and edges the columnar cells are often irregular and jagged, especially where, as is often the case, lymphoid or wander-cells are met with between the epithelium-cells (fig. 231, w). Indeed the cells are not by any means so regular in shape as they are often figured, being often compressed laterally, and sometimes extended sideways into flattened lamellæ (fig. 230, a), which fit between the adjacent cells of the epithe