When living ciliated epithelium, e.g., from the gill of a mussel, or oyster, or from the mouth of the frog, or from a scraping from a polypus from the human nose, is examined under the microscope, the cilia are seen to be in constant rapid motion; each cilium being fixed at one end, and swinging or lashing to and fro. The general impression given to the eye of the observer is very similar to that produced by waves in a field of corn, or swiftly running and rippling water, and the result of their movement is to produce a continuous current in a definite direction, and this direction is invariably the same on the same surface, being always, in the case of a cavity, towards its external orifice.

Ciliary Motion.-Ciliary, which is closely allied to amoeboid and muscular motion, is alike independent of the will, of the direct influence of the nervous system, and of muscular contraction. It continues for several hours after death or removal from the body, provided the portion of tissue under examination be kept moist. Its independence of the nervous system is shown also in its occurrence in the lowest invertebrate animals apparently unprovided with anything analogous to a nervous system, in its persistence in animals killed by prussic acid, by narcotic or other poisons, and after the direct application of narcotics, such as morphia, opium, and belladonna, to the ciliary surface, or of electricity through it. The vapour of chloroform arrests the motion; but it is renewed on the discontinuance of the application (Lister). The movement ceases when the cilia are deprived of oxygen, but is revived on the admission of this gas. Carbonic acid stops the movement. The contact of various substances, e.g., bile, strong acids, and alkalies, will stop the motion altogether; but this seems to depend chiefly on destruction of the delicate substance of which the cilia are composed. Temperatures above 45° C., and below 0° C., stop the movement, but moderate heat and dilute alkalies are favourable to the action and revive the movement after temporary cessation.

As a special sub-division of ciliary action may be mentioned the motion of spermatozoa, which may be regarded as cells with a single cilium.

B. Transitional Epithelium.-This term has been applied to cells, which are neither arranged in a single layer, as is the case with simple epithelium, nor yet in many superimposed strata as in laminated; in other words, it is employed when epithelial cells are found in two, three, or four superimposed layers.

The upper layer may be either columnar, ciliated, or squamous.

When the upper layer is columnar or ciliated, the second layer consists of smaller cells fitted into the inequalities of the cells above them, as in the trachea (fig. 24, b).

The epithelium which is met with lining the urinary bladder and ureters is, however, the transitional par excellence. In this variety there are two or three layers of cells, the upper being

more or less flattened according to the full or collapsed condition of the organ, their under surface being marked with one or more depressions, into which the heads of the next layer of clubshaped cells fit. Between the lower and narrower parts of the second row of cells, are fixed the irregular cells which constitute the third row, and in like manner sometimes a fourth row (fig. 24). It can be easily understood, therefore, that if a scraping of the mucous membrane of the bladder be teased, and examined under the microscope, cells of a great variety of forms may be made out (fig. 25). Each cell contains a large nucleus, and the larger and superficial cells often possess two.

C. Stratified Epithelium. This term is employed when the cells forming the epithelium are arranged in a considerable number of superimposed layers. The shape and size of the cells of the different layers, as well as the number of the layers, vary in different situations. Thus the superficial cells are as a rule of the squamous, or scaly variety, and the deepest of the columnar form.

The cells of the intermediate layers are of different shapes, but those of the middle layers are more or less rounded. The superficial cells overlap by their edges (fig. 26); they are broad (fig. 13). Their chemical composition is different from that of the underlying cells, as they contain keratin, and are therefore horny in character.

The nucleus is often not apparent. The really cellular nature of even the dry and shrivelled scales cast off from the surface of the epidermis, can be proved by the application of caustic potash, which causes them rapidly to swell and assume their original form.

Fig. 26.-Vertical section of the stratified epithelium of the Rabbit's cornea. a. Anterior epithelium, showing the different shapes of the cells at various depths from the free surface. b. Portion of the substance of cornea. (Klein.)

The squamous cells exist in the greatest number of layers in the epidermis or superficial part of the skin; and the most superficial of these are being continually removed by friction, and new cells from below supply the place of those cast off.

The intermediate cells approach more to the flat variety the nearer they are to the surface, and to the columnar as they approach the lowest layer. There may be considerable intercellular intervals; and in many

of the deeper layers of epithelium in the mouth and skin, the outline of the cells is very irregular, in consequence of processes passing from cell to cell across these intervals.

Fig. 27.-Jagged cells of the middle layers of pavement epithelium, from a vertical section of the gum of a newborn infant. (Klein.)

Such cells (fig. 27) are termed "ridge and furrow," "cogged " or "prickle" cells. These "prickles" are prolongations of the intra-cellular network which run across from cell to cell, thus joining them together (Martyn), the interstices being filled by the transparent intercellular cement substance. When this increases in quantity in inflammation, the cells are pushed further apart, and the connecting fibrils or "prickles" elongated, and therefore more clearly visible.

The columnar cells of the deepest layer are distinctly nucleated; they multiply rapidly by division; and as new cells are formed beneath, they press the older cells forwards to be in turn pressed forwards themselves towards the surface, gradually altering in shape and chemical composition until they are cast off from the surface.

Stratified epithelium is found in the following situations:(1.) Forming the epidermis, covering the whole of the external surface of the body; (2.) Covering the mucous membrane of the tongue, mouth, pharynx, and oesophagus; (3.) As the conjunctival epithelium, covering the cornea; (4.) Lining the vaginal part of the cervix uteri,

Functions of Epithelium. -According to function, epithelial cells may be classified as :—(1.) Protective, e.g., in the skin, mouth, blood-vessels, &c. (2.) Protective and moving-ciliated epithelium. (3.) Secreting-glandular epithelium; or, Secreting formed elements-epithelium of testicle secreting spermatozoa. (4.) Protective and secreting, e.g., epithelium of intestine, (5.) Sensorial, e.g., olfactory cells, rods and cones of retina, organ of Corti.

Epithelium forms a continuous smooth investment over the whole body, being thickened into a hard, horny tissue at the points most exposed to pressure, and developing various appendages, such as hairs and nails, whose structure and functions will be considered in a future chapter. Epithelium lines also the sensorial surfaces of the eye, ear, nose, and mouth, and thus serves as the medium through which all impressions from the external world— touch, smell, taste, sight, hearing-reach the delicate nerve-endings, whence they are conveyed to the brain.

The ciliated epithelium which lines the air-passages serves not only as a protective investment, but also by the movements of its cilia promotes currents of the air in the bronchi and bronchia, and is enabled to propel fluids and minute particles of solid matter so as to aid their expulsion from the body. In the case of the Fallopian tube, this agency assists the progress of the ovum towards the cavity of the uterus. Of the purposes served by cilia in the ventricles of the brain nothing is known.

The epithelium of the various glands, and of the whole intestinal tract, has the power of secretion, i.e., of chemically transforming certain materials of the blood; in the case of mucus and saliva this has been proved to involve the transformation of the epithelial cells themselves; the cell-substance of the epithelial cells of the intestine being discharged by the rupture of their envelopes, as mucus.

Epithelium is likewise concerned in the processes of transudation, diffusion, and absorption.

It is constantly being shed at the free surface, and reproduced in the deeper layers. The various stages of its growth and development can be well seen in a section of any laminated epithelium such as the epidermis.

The Connective Tissues.

This group of tissues forms the Skeleton with its various connections-bones, cartilages, and ligaments-and also affords a supporting framework and investment to the various organs composed of nervous, muscular, and glandular tissue. Its chief function is the mechanical one of support, and for this purpose it

Fig. 28.-Horizontal preparation of cornea of frog, stained in gold chloride; showing the network of branched cornea corpuscles. The ground substance is completely colourless. X 400. (Klein.)

is so intimately interwoven with nearly all the textures of the body, that if all other tissues could be removed, and the connective tissues left, we should have a wonderfully exact model of almost every organ and tissue in the body, correct even to the smallest minutiæ of structure.

Classification of Connective Tissues.-The chief varieties of connective tissues may be thus classified :

I. The Fibrous Connective Tissues.

A.-Chief Forms.

a. White fibrous.

b. Elastic.

c. Areolar.

B.-Special Varieties.

a. Gelatinous.

b. Adenoid or Retiform.

c. Neuroglia.

d. Adipose.

II. Cartilage.

III. Bone.

D