lium. There is always a distinct oval nucleus which contains a network of chromatoplasm. The nucleus may cause a bulging in the part of the cell in which it is situated, and the nuclei of adjacent cells are on this account often seated in different planes. The substance of the cell usually appears granular, but on closer inspection with higher powers it may be seen that the granular appearance is caused by vacuolation and reticulation of the protoplasm. The cell may contain fatty globules and other substances, among which the most deserving of mention is mucin, the chief organic constituent of mucus. The mucin (or mucigen) usually takes the form of a granular deposit within the cell, especially in the part nearest the free border; when fully formed the granules swell, and their substance escapes in the form of mucus; the nucleus is often pressed down towards the finer extremity of the cell. Columnar epithelium cells which are thus altered by distension of the outer or free part of the cell by mucus are termed from their shape "goblet or chalice cells" (figs. 232, 233). In typical columnar epithe

Fig. 234.-ISOLATED CELLS FROM THE CONVOLUTED TUBULES OF THE RAT'S KIDNEY. (Heidenhain). Fig. 235.-SECTION OF A RACEMOSE GLAND, SHOWING THE COMMENCEMENT OF A DUCT IN THE ALVEOLI. MAGNIFIED 425 DIAMETERS. (E. A. S.)

a, one of the alveoli, several of which are in the section shown grouped around the commencement of the duct, d'; a', an alveolus, not opened by the section; b, basement membrane in section; c, interstitial connective tissue of the gland; d, section of a duct which has passed away from its alveoli, and is now lined with characteristically-striated columnar cells; s, semilunar group of darkly-stained cells at the periphery of an alveolus.

something different from the cell-protoplasm. The border does not however, appear to offer a greater resistance than does the cell-protoplasm to the action of reagents, for those which destroy the protoplasm of the cell destroy also the striated border. After having been hardened by reagents it may be detached from the rest of the cell, and since the striated free borders of adjacent cells often adhere together, a continuous membrane may thus be obtained, marked by a mosaic of fine lines indicating the division between the cells from which this "cuticula" has become detached. The fine striæ appear to be caused by the existence of fibrils or septa (perhaps spongioplasmic). The striated cuticula is not immediately in contact with the protoplasm of the cell, but is separated from it by a thin disk composed of a substance which refracts the light even more than the striated border. This disk (shown in fig. 230) corresponds in situation to the bright border of the ciliated epithelium cells (see below), and it is possible that the striated border is the morphological equivalent of the bunch of cilia upon those cells. Columnar epithe

lium-cells are met with in their most characteristic form lining the mucous membrane of the intestines.

Some columnar epithelium-cells are very long, others very short, so as to look cubical when seen in profile. They vary in form, moreover, according to the shape of the surface which they cover, thus they may be larger at the base than at the free end, as when they line a tube or duct, and in a section of this they then appear wedge shaped.

Some epithelium cells, which must be reckoned in with this variety, have a peculiar striated aspect in the basal or fixed half of the cell. This is the case with the cells which line the smaller ducts of the salivary glands and some of the tubules of the kidney (fig. 234 and fig. 235, d).

In the human subject, columnar epithelium is chiefly, but by no means exclusively, derived from the hypoblast.

Glandular epithelium. This variety of epithelium is chiefly characteristic of the terminal recesses or alveoli of secreting glands. In form the cells are columnar, cubical, polyhedral or spheroidal, and are usually set round a tubular or saccular cavity, into which the secretion is poured (fig. 235, a). The protoplasm of the cells is generally occupied by the materials which the

gland secretes. This epithelium will be more fully described in the chapter on secreting glands.

Ciliated epithelium.—In this form of epithelium, the cells, which are generally columnar, bear at their free Fig. 236.-COLUMNAR CILIATED EPITHELIUM CELLS FROM THE HUMAN NASAL MEMBRANE; MAGNIFIED 300 DIAMETERS. (Sharpey.) extremities little hair-like processes, which are agitated incessantly during life, and for some time after systemic death,

with a lashing or vibrating motion. These minute and delicate moving organs are named cilia. They exist very extensively throughout the animal kingdom; and the movements which they produce are subservient to very varied purposes in the animal economy.

Distribution and use. In the human body ciliated epithelium occurs in the following parts, viz:-1. On the mucous membrane of the air-passages and its prolongations. It commences at a little distance within the nostrils, covers the membrane of the nose (except the proper olfactory part) and of the adjoining bony sinuses, and extends up into the nasal duct and lachrymal sac. From the nose it spreads backwards a certain way on the upper surface of the soft palate, and over the upper or nasal region of the pharynx; thence along the Eustachian tube and lining membrane of the tympanum, of which it covers the greater part. The lower part of the pharynx is covered by scaly epithelium; but the ciliated epithelium begins again in the larynx a little above the glottis, and continues throughout the trachea and the bronchial tubes in the lungs to their smallest ramifications. Over the vocal cords, however, the epithelium is of the stratified scaly variety. 2. On the mucous lining and in the glands of the body of the uterus and extending along the Fallopian tubes, even to the peritoneal surface of the latter at their fimbriated extremities. 3. In the testicle lining the vasa efferentia, coni vasculosi, and first part of the tube of the epididymis. 4. Lining the ventricles of the brain, except the fifth ventricle, and throughout the central canal of the spinal cord. 5. In the excretory ducts of certain small racemose glands of various parts (tongue, pharynx, &c.) 6. In the embryo, lining the oesophagus and parts of the stomach and extending also over the whole of the pharynx.1

1 Cilia have also been described in some mammals at the commencement of the tubules of the kidney (Klein), a situation where in lower vertebrates they have long been known to exist.

In other mammiferous animals, as far as examined, cilia have been found in nearly the same parts. To see them in motion, a portion of epithelium may be scraped off any ciliated mucous membrane and examined in a drop of weak solution of salt (0.6 per cent.) or serum of blood. When it is now viewed with a magnifying power of 200 diameters or upwards, a very obvious agitation will be perceived at the edge of the detached piece of epithelium; this appearance is caused by the moving cilia, with which the surface of the membrane is covered. Being set close together, and moving simultaneously or in quick succession, the cilia, when in brisk action, give rise to the appearance of a bright transparent fringe along the margin of the membrane, agitated by such a rapid and incessant motion, that the single threads which compose it cannot be perceived. The motion here meant, is that of the cilia themselves; but they also set in motion the adjoining fluid, driving it along the ciliated surface, as is indicated by the agitation of any little particles that may accidentally float in it. The fact of the conveyance of fluids and other matters along the ciliated surface, as well as the direction in which they are impelled, may also be made manifest by immersing the membrane in fluid, and dropping on it some finelypulverised substance (such as charcoal in fine powder), which will be slowly but steadily carried along in a constant and determinate direction; and this may be seen with the naked eye, or with the aid of a lens of low power (Sharpey).

Cilia have been shown to exist in almost every class of animals, from the highest to the lowest. The immediate purpose which they serve is, to impel matter, generally more or less fluid, along the surfaces on which they are attached; or, to propel through a liquid medium the ciliated bodies of minute animals, or other small objects which are provided with cilia; as is the case with many infusorial animalcules, in which the cilia serve as organs of locomotion like the fins of larger aquatic animals. In many of the lower tribes of aquatic animals, cilia acquire a high degree of importance: producing the flow of water over the surface of their organs of respiration, indispensable to the exercise of that function; enabling the animals to seize their prey, or swallow their food, and performing various other offices of greater or less importance in their economy. In man and the warm-blooded animals, their use is apparently to impel secreted fluids or other matters along the ciliated surface, as, for example, the mucus of the wind-pipe and nasal sinuses, which they carry towards the outlet of these cavities.

Structure. The cells of a ciliated epithelium contain oval nuclei, exhibiting for the most part a distinct intra-nuclear network, and one or more bright nucleoli. Viewed with a moderate magnifying power, their protoplasm looks granular, although the free border of the cell through which the cilia pass presents a clear aspect (fig. 236). The cells have most generally an elongated form, like the particles of the columnar epithelium, which they resemble too in arrangement, but they are often of greater length and more pointed at their lower end ; and this is not unfrequently irregularly forked in those parts where a deeper layer of cells exists below the ciliated cells (fig. 237). The cilia are attached to their broad or superficial end, each cell bearing a tuft of these minute hair-like processes. In some cases, the cells are shorter and cubical in figure, and when completely detached may appear spheroidal.

It has been shown by Engelmann that in large ciliated cells (fig. 238) such as those which line the alimentary canal of some mollusks, e.g., the mussel and oyster, it is possible to make out that the highly refracting free border of the cell to which. the cilia are attached is in reality formed of a number of small juxtaposed fusiform or cylindrical knobs (basal knobs). To each of these a cilium is attached on the one side, and from the other end there passes towards the end of the cell a fine,

1 The Arthropoda offer a singular exception, and it is remarkable that in many of them the spermatozoa are also devoid of a vibratile filament.

varicose filament; these filaments are termed by Engelmann the rootlets of the cilia. They approach one another as they traverse the length of the cell, and may be united towards the extremity into a single thread. They are not connected with the nucleus. The cilia are attached to the basal knobs, each one by a somewhat narrowed portion or neck (intermediate segment of Engelmann). It is here that the cilia usually break off from the cell (see fig. 238). Beyond the neck the cilium swells out into a small bulbous enlargement, and from this the shaft tapers gradually to its extremity. The rootlets, as well as the cilia themselves, are said by Engelmann to be doubly refracting (anisotropous), whereas the basal knob

Fig. 238.-A CILIATED EPITHELIUM-CELL (Engelmanu.)

OF A MOLLUSK.

The columnar ciliated epithelium may exist as a simple layer, as in the uterus and Fallopian tubes, the finest ramifications of the bronchia, and the central canal of the spinal cord and ventricles of the brain; but in various other parts as the nose, pharynx, Eustachian tube, the trachea and its larger divisions-there is a layer of elongated and irregular cells beneath the superficial ciliated range, filling up the spaces between the pointed and forked extremities of the latter. These cells have been supposed to acquire cilia, and take the place of ciliated cells which are cast off; but this is doubtful, and they appear rather to be concerned with the secretion of mucus, since mucigen occurs within them in all stages of formation, and they become eventually distended by it into goblet-cells (see fig. 237, where the intermediate cells, m', m2, and m3 show three stages of formation of mucus).

When the ciliated epithelium is artificially removed from a portion of the inner surface of the rabbit's trachea, the denuded surface speedily becomes again covered with epithelium which grows over it from the edge, but the cells form at first a single layer of flattened epithelium. They next acquire cilia, and afterwards become columnar, the epithelium thus assuming the character which it has normally in that situation.

VOL. I.

P

There is no reason to believe that ciliated epithelium-cells are in connection either with nerve-fibres, or with the cells of the subjacent connective tissue. An anatomical connection with subjacent cells and fibres has been described in reference to the columnar ciliated epithelium of the central canal of the spinal cord and of the Sylvian aqueduct. But this is a most difficult point to determine exactly, and even if such a connection should be proved, the cells in the situations above mentioned are entirely different in many respects from ordinary ciliated cells. They are relatively slender, and their fixed non-ciliated ends pass into branching fibres, which lose themselves in a network which underlies the epithelium, and appears to be formed chiefly, if not entirely, by the interlacement of the ramified cellprocesses. These peculiar ciliated cells closely resemble those which constitute the structures known as nerve-epithelia in some of the lower invertebrata, and which in some of these animals represent the whole central nervous system.

The cilia themselves differ widely in size in different animals, nor are they of equal size in all parts of the same animal. In the human windpipe they measure 10th to th of an inch in length; but in many invertebrate animals they are much larger than this, and in the human epididymis are from eight to ten times longer than in the trachea.

In figure they have the aspect of slender conical, or slightly flattened filaments; broader at the base, and usually pointed or rounded at their free extremity. Their substance is transparent, soft and flexible. It is to all appearance homogeneous, and no fibres, granules, or other indications of definite internal structure, have been satisfactorily demonstrated in it.

The flagellum of Noctiluca, which bears a general resemblance to a large cilium and has a similar rhythmic lashing action, is transversely striated, and the cilium or tail of a spermatozoon also shows certain indications of structure, but nothing of the kind has been observed in ordinary cilia.

Nature of ciliary movement and influence of varying conditions and reagents.If the cilia be detached from the cell they cease to move, and on this account it is thought by some, that the movement is entirely a passive one, caused by movements in the cell-protoplasın acting upon the rootlets of the cilia. But the apparently independent motion of the tails of the spermatozoa, which are comparable to long single cilia, and that of the long cilia which are protruded from many of the lower animal and plant organisms, has led other authorities to believe that the movement is due to the contraction of the cilia themselves.

There is, however, a third mode of explanation of the movements which may be suggested and which would have the advantage of bringing them into close relationship with the amoeboid movements of cell-protoplasm, and, as we shall afterwards see, with the process of contraction and extension of muscle. The explanation is briefly as follows:-If we suppose that a cilium is a hollow curved extension of the cell, occupied by hyaloplasm, and invested by a delicate elastic membrane (perhaps an extension of the spongioplasm), then it must follow that if there be a rhythmic flowing of hyaloplasm from the body of the cell, into and out of the cilium, an alternate extension and flexion of that process would thereby be brought about. The same result would be got, supposing the cilium to be a straight and not a curved exten. sion of the cell, if the enveloping membrane were thicker (or otherwise less extensible) along one side than along the other. This last assumption would also enable one the better to account for the spiral direction of the movement of certain cilia; for this form of movement would be produced if the line of lessened extensibility in these cilia were to pass in a corkscrew fashion along the cilium in place of straight along one side, as assumed for ordinary cilia. The cilia vibrate with a frequency of not less than ten times in a second when moving actively, but the rate of movement may be much slower than this. The movement of cilia is incessant so long as the cells remain alive, but that of spermatozoa often exhibits intervals of rest alternating with periods of rhythmic movement.

The manner in which cilia move, is best seen when they are not acting very briskly. The motion of an individual cilium may be compared to that of a carter's whip, the cilium being rapidly flexed in one direction, that of the current which they produce, and returning more slowly in the other direction. The motion does not involve the whole ciliated surface at the same moment, but is performed by the cilia in regular succession, giving rise to the appearance of a series of waves travelling along the surface, like the waves caused by the wind in a field of corn. When they are in very rapid action the undulation is less obvious, and, as Henle remarks, their motion then conveys the idea of swiftly-running water. The undulating movement may be beautifully seen on the gills of a mussel. The undulations, with some exceptions, seem always to travel in the same direction on the same parts. The impulsion, also, which