rounded or polyhedral, and those of the deepest layer generally somewhat columnar in shape (fig. 18, c). Moreover, the deeper cells are soft and protoplasmic, and are separated from one another by a system of intercellular channels, which are bridged across by numerous processes passing from cell to cell.

The deeper cells multiply by division, the newly formed cells tending as they enlarge to push those external to them nearer to the surface, from which they are eventually thrown off. As they approach the surface they become hard and horny, and in the case of the epidermis lose entirely their cellular appearance, which can, however, be in a measure restored by the action of potash (§ 2). The cast-off superficial cells of the stratified epithelium of the mouth, which are seen in abundance in the saliva (§ 1), are less altered, and the remains of a nucleus is still visible in them.

Simple scaly or pavement epithelium is found in the saccules of the lungs, in those of the mammary gland when inactive, in the kidney (in the tubes of Henle), and also lining the cavities of serous membranes (fig. 20), and the heart, blood-vessels, and lymphatics. When occurring on internal surfaces, such as those of the serous membranes, blood-vessels, and lymphatics, it is often spoken of as endothelium.

Polyhedral or spheroidal epithelium is characteristic of many secreting glands; columnar and ciliated epithelium are for the most part found covering the inner surface of mucous membranes, which are membranes lining passages in communication with the exterior, such as the alimentary canal and the respiratory and generative passages.

The detailed study of most of these may be reserved until the organs in which they occur are respectively dealt with.

The hairs and nails and the enamel of the teeth are modified epithelial tissues.

LESSON VII.

COLUMNAR AND CILIATED EPITHELIUM, AND
TRANSITIONAL EPITHELIUM.

1. TAKE a piece of rabbit's intestine which has been two days in chromic acid solution (1 part chromic acid to 2,000 normal salt solution). Scrape the inner surface with a scalpel, break up the scrapings in a drop of water on a slide. Add a small piece of hair to avoid crushing, and cover the preparation. Sketch one or two columnar cells and also a row of cells. Measure two or three cells and their nuclei.

To keep this preparation, add a drop of dilute hæmatoxylin (1 drop of the ordinary solution to half a watch-glass-ful of distilled water) at one edge of the cover-glass. When the hæmatoxylin has passed in and has stained the cellnuclei, place a drop of glycerine at the same edge, and allow it slowly to diffuse under the cover-glass. Cement this another day.1

2. Break up in glycerine a shred of epithelium from a piece of frog's intestine that has been treated with osmic acid, and has subsequently macerated in water for a few days. The cells easily separate on tapping the cover-glass. They are larger than those of the rabbit and exhibit certain points of structure better. Measure and sketch one or two cells. The cover-glass may be at once fixed by gold size.

3. Prepare the ciliated epithelium from a trachea that has been in bichromate of potash solution ( per cent.) for two days, in the same way as in § 1. Measure in one or two of the cells (a) the length of the cell, (b) the length of the cilia, (c) the size of the nucleus. Sketch two or three cells.

This preparation is to be stained and preserved as in § 1.

4. Make a similar teased preparation of the epithelium of the urinary bladder. Observe the large flat superficial cells, and the pear-shaped cells of the second layer. Measure and sketch one or two of each kind. Stain and preserve as in §§ 1 and 3.

Columnar epithelium.-The cells of a columnar epithelium (fig. 21) are prismatic columns, which are set closely side by side, so that when seen from the surface a mosaic appearance is produced. They often taper somewhat towards their attached end, which is generally truncated, and set upon a basement membrane. Their free surface is covered by a thick striated border (fig. 22, str), which may sometimes become detached in teased preparations. The protoplasm of the cell is highly vacuolated or reticular, and fine longitudinal striæ may be seen in it, which appear continuous with the striæ of the

Gentian-violet solution (see Appendix) may be employed instead of hæma

toxylin.

free border. The nucleus (n) is oval and reticular.

The lateral borders of the cells are often somewhat irregular or jagged, the result of the pressure of amoeboid lymph-cells, which are generally found between the columnar cells, at least in the intestine. After a

FIG. 21.-A ROW OF COLUMNAR CELLS FROM THE INTESTINE OF THE RABBIT. Smaller cells are seen between the epithelium-cells, probably of the nature of white blood or lymph

corpuscles.

meal containing much fat the cells may be filled with fat-globules, which become stained black in the osmic preparation.

Some of the columnar cells contain mucigen, which may greatly distend the part of the cell nearest the striated border. When the

FIG. 22.-COLUMNAR EPITHELIUM-CELLS OF THE RABBIT'S INTESTINE. The cells have been isolated after maceration in very weak chromic acid. The cells are much vacuolated, and one of them has a fat-globule near its attached end; the striated border (str) is well seen, and the bright disk separating it from the cell-protoplasm; n, nucleus with intranuclear network; a, a thinned-out wing-like projection of the cell which probably fitted between two adjacent cells.

mucigen is extruded as mucus, this border is thrown off, and the cell takes the form of an open cup or chalice (goblet-cell, fig. 23).

Columnar epithelium-cells are found lining the whole of the interior of the stomach and intestines: they are also present in the ducts of most glands, and sometimes also in their secreting saccules. The epithelium which covers the ovary also has a modified columnar shape, but cells having all the structural peculiarities indicated above are found only in the alimentary canal and in its diverticula.

Ciliated epithelium.-The cells of a ciliated epithelium are also usually columnar in shape (fig. 24), but in place of the striated border the cell is surmounted by a bunch of fine tapering filaments which,

during life, move spontaneously to and fro, and serve to produce a current of fluid over the surface which they cover.

The cilia are to be regarded as active prolongations of the cellprotoplasm. The border upon which they are set is bright, and appears formed of little juxtaposed knobs, to each of which a cilium is attached. In the large ciliated cells which line the alimentary canal of some molluscs (fig. 25) the knob may be observed to be prolonged into the protoplasm of the cell as a fine varicose filament, termed the rootlet of the cilium. These filaments may represent the longitudinal striæ often seen in the protoplasm of the columnar cell, the bunch of cilia being homologous with the striated border. The protoplasm and nucleus have a similar vacuolated or reticular structure in both kinds of cell. Goblet cells may also occur in ciliated epithelia.

FIG. 23.-GOBLET CELL.

FIG. 24.-COLUMNAR CILI-
ATED EPITHELIUM-CELLS.

FIG. 25.-CILIATED CELL,
FROM THE INTESTINE
OF A MOLLUSC.

Ciliated epithelium is found throughout the whole extent of the air-passages and their prolongations (but not the part of the nostrils. supplied by the olfactory nerves, nor in the lower part of the pharynx); in the Fallopian tubes and the greater part of the uterus; in some of the efferent ducts of the testicle (where the cilia are much longer than

elsewhere in the body); in the ventricles of the brain, and the central canal of the spinal cord; and in the convoluted tubules of the kidney.

Transitional epithelium is a stratified epithelium consisting of only two or three layers of cells. It occurs in the urinary bladder, the ureter, and the pelvis of the kidney. The superficial cells (fig. 26, a) are large and flattened; they often have two nuclei. On their

FIG. 26.-EPITHELIAL CELLS FROM THE BLADDER OF THE RABBIT. (Magnified 500 diameters.)

a, large flattened cell from the superficial layer, with two nuclei and with strongly marked ridges and intervening depressions on its under surface; b, pear-shaped cell of the second layer adapted to a depression on one of the superficial cells.

under surface they exhibit depressions, into which fit the larger ends of pyriform cells, which form the next layer (fig. 26, b). Between the tapered ends of the pyriform cells one or two layers of smaller polyhedral cells are found.