1

produced by the wind when it passes over a field of corn. When, however, their motion is very rapid, and their partial recovery from the preceding wave cannot be observed, their motion possesses the appearance of swiftly running water. The length of the cilia varies from about oth tooth of an inch. The cause of the movement of the cells is unknown, but it seems to have for its object the propulsion of secreted fluids along the surface towards the orifice of the part they cover, as is the case in the trachea and sinuses of the nose.

Where Found.

I. In the uterus from the middle of the neck to the fimbriated extremities of the Fallopian tubes.

2. In the testicle from the commencement of the vasa efferentia to the beginning of the vas deferens.

3. Scattered throughout the central canal of the spinal cord, and in the ventricles of the brain.

4. In the respiratory tract and its prolongations. The ciliated epithelium begins a little distance inside the nostril, and then

covering the whole of the membrane of the nose, except that part which is especially devoted to the sense of smell; it is prolonged into the antrum and into the ethmoidal and sphenoidal sinuses, and through the lachrymal duct into the lachrymal sac. From the upper and back part of the nose it passes back over the upper part of the soft palate and pharynx, and is prolonged up the Eustachian tube into the tympanum, the floor of which it lines. The rest of the pharynx is covered by squamous epithelium; but the ciliated epithelium begins again a little above the epiglottis, and lines the larynx, trachea, and the bronchi as far as the pulmonary vesicles. The pulmonary vesicles are lined by a single layer of non-ciliated flattened epithelial cells. The ciliated epithelial cells in the ventricles of the brain and in the tympanum, in the human subject, are of the spheroidal variety.

GENERAL USES OF EPITHELIUM.

One of the chief functions of epithelium is that of protection: it is protective in the skin where it covers the delicate nerve endings, or, as has been pointed out, in some cases actu

H

ally receiving the terminations of the nerve filaments into the interior of its cells. Besides affording mere mechanical protection it appears to possess the function of converting what otherwise would be a sense of pain into tactile and temperature sensations. In the horny layer of the epidermis it is protective in another sense, as here, owing to its nonabsorbent power, it prevents the skin absorbing, so long as the epidermis is intact, poisonous or noxious materials. It is also protective in the ciliated form where it propels particles towards the orifices of the body. Epithelium lines also the sensorial surface of the eye, ear, nose, and mouth, and forms the medium by which the various stimuli which affect these organs reach the delicate nerve endings and are conveyed to the brain. Then again, epithelium possesses the function of secretion, that is, of chemically transforming certain materials of the blood. In some cases the substance of the epithelial cells is differentiated into the secretion of the

gland they occupy. The cell substance of the epithelial cells of the intestine is, for instance, discharged, by the rupture of their

capsules, as mucus. As stated above, the epithelial cells, while constantly being renewed by the division of the deeper cells, are at the same time being cast off from the free surface.

CHAPTER IX.

BLOOD.

General Characters of the Blood-Its Microscopical Appearances-The Parts of the Blood-Histology of the white and red Corpuscles Properties and Composition of the Blood-Function of the Blood.

IN the preceding description, the tissues have been considered as belonging either to the active or the passive tissue group. This tissue fills a totally distinct position, and its chief features possess rather a physiological than a histological interest. Blood is, histologically, the simplest form of tissue, consisting of cells embedded in a fluid matrix.

Microscopical Appearances (P. VIII. Fig. 32). When examined under the microscope, in the living vessels, blood is seen to consist of a transparent, colourless fluid known as the liquor sanguinis, and a number of minute solid particles suspended in it called the corpuscles. These corpuscles are of two kinds, the coloured