Ciliated epithelium. The cells of a ciliated epithelium are also usually columnar in shape (fig. 31), 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. 32), 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 perhaps represent the longi

FIG. 30.-GOBLET-CELL
FROM THE TRACHEA.

(Klein.)

FIG. 31.-COLUMNAR CILI-
ATED EPITHELIUM-CELLS
FROM THE LOWER PART
OF THE NASAL PASSAGES.
EXAMINED FRESH IN
SERUM. (Sharpey.)

FIG. 32.-CILIATED CELL,
FROM

THE INTESTINE
OF A MOLLUSC. (Engel-
mann.)

tudinal 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 and reticular structure in both kinds of cell.

Ciliated epithelium is found throughout the whole extent of the air-passages and their prolongations (but not in 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 longer than elsewhere in the body); in the ventricles of the brain, and the central canal of the spinal cord; and, according to some authorities, 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

FIG.

cells (fig. 34, a) are large and flattened; m, m2, m3, mucus-secreting cells in

they often have two nuclei. On their

under surface they exhibit depressions,

into which fit the larger ends of pyriform

various stages of mucigen formation. The preparation was treated with dilute chromic acid in the manner recommended in the instructions for practical work.

cells, which form the next layer (fig. 34, b).

Between the tapered

ends of the pyriform cells one or two layers of smaller polyhedral

FIG. 34.-EPITHELIAL-CELLS FROM THE BLADDER OF THE RABBIT. (Klein.)
(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.

cells are found. The epithelium is renewed by division of these deeper cells.

C

LESSON VIII.

STUDY OF CILIA IN ACTION.

1. MOUNT in sea-water one or two bars of the gill of the marine mussel (fig. 35). Study the action of the large cilia. Now place the preparation upon the copper warm stage (see Lesson V.) and observe the effect of raising the temperature.

FIG. 35.-VALVE OF MUSSEL (MYTILUS EDULIS) SHOWING br, br, THE EXPANDED GILLS OR BRANCHIA, WHICH, OWING TO THE LITTLE BARS OF WHICH THEY ARE COMPOSED, PRESENT A STRIATED ASPECT.

ml, mantle; m, cut adductor muscle; i, mass of viscera; the dark projection just above is the foot.

Keep this preparation until the end of the lesson, by which time many of the cilia will have become languid. When this is the case pass a drop of dilute potash solution (1 part KHO to 1,000 of sea-water) under the coverglass and observe the effect.

FIG. 36.-MOIST CHAMBER ADAPTED FOR PASSING A GAS OR VAPOUR TO A
PREPARATION UNDER THE MICROSCOPE.

2. Cement with sealing-wax a piece of small glass tubing to a slide so that one end of the tube comes nearly to the centre of the slide. To do this

effectually the slide must be heated and some sealing-wax melted on to it and allowed to cool. The glass tube is then made hot and applied to the slide, embedding itself as it does so in the sealing-wax. On this put a ring of putty or modelling wax (half an inch in diameter and rising above the glass tube) so as to include the end of the tube. Make a deep notch in the ring opposite the tube. Place a small drop of water within the ring (fig. 36). Put a bar from the gill upon a cover-glass in the least possible quantity of sea-water; invert the cover-glass over the putty ring, and press it gently down. The preparation hangs in a moist chamber within which it can be studied through the cover-glass, and into which gases or vapours can be passed and their effects observed.

Pass CO, through the chamber, and after observing the effect replace it by air (see fig. 37). Repeat with chloroform vapour instead of CO2.

The movement of cilia.-When in motion a cilium is bent quickly over in one direction with a lashing whip-like movement, immediately recovering itself. When vigorous the action is so rapid, and the rhythm so frequent (ten or more times in a second) that it is impossible to follow the motion with the eye. All the cilia upon a ciliated surface are not in action at the same instant, but the move

f.F.COLLINGS

FIG. 37.-METHOD OF SUBJECTING A PREPARATION TO A STREAM OF CARBONIC

ANHYDRIDE.

b, bottle containing marble and hydrochloric acid: b', wash-bottle, connected by indiarubber tube, t, with the moist chamber, s.

ment travels in waves over the surface. If a cell is detached from the general surface, its cilia continue to act for a while, but at once cease if they are detached from the cell.

The rhythm is slowed by cold, quickened by warmth, but heat beyond a certain point kills the cells. The movement will continue for some time in water deprived of oxygen. Both CO, gas

and chloroform vapour arrest the action, but it recommences on re

storing air. Dilute alkaline solutions quicken the activity of cilia, or may even restore it shortly after it has ceased.

Various attempts have been made to explain the manner in which cilia act, some supposing that they are themselves contractile, others that their movement is a passive one, and that the real movement is at their rootlets in the protoplasm of the cell. The bending-over action can also be supposed to be due to the alternate flowing and ebbing of hyaloplasm from the body of the cell into hollow permanent cellprocesses, i.e. the cilia; if we assume that one side of each cilium is less extensible than the other, it must necessarily be bent over in the manner usually observed. Some cilia, however, have a spiral action rather than the simple to and fro movement; in this case we may assume that the line of less extensibility passes not straight along one side of the cilium, but spirally round it.

This hypothesis has the advantage that it permits ciliary motion to be brought into the same category as amoeboid movements, in so far that both are explicable by the flowing of hyaloplasm out of and into the reticulum of spongioplasm.