(2.) Their quality of staining in solutions of carmine, hæmatoxylin, &c. Nuclei are most commonly oval or round, and do not generally conform to the diverse shapes of the cells; they are altogether less variable elements than cells, even in regard to size, of which fact one may see a good example in the uniformity of the nuclei in cells so multiform as those of epithelium. But sometimes nuclei appear to occupy the whole of the cell, as is the case in the lymph corpuscles of lymphatic glands, and in some small nerve cells.

Their position in the cell is very variable. In many cells, especially where active growth is progressing, two or more nuclei are present.

Minute structure of cells.-The protoplasm which forms the body as well as that which forms the nuclei of cells has been shown in many varieties of cells, e.g., the colourless blood-corpuscles, epithelial cells, connective-tissue corpuscles, nerve-cells, to be made up of a network of very fine fibrils, the meshes of which are occupied by a hyaline interstitial substance (Heitzmann's network) (fig. 12). At the nodes, where the fibrils cross, are little swellings, and these are the objects described as granules by the older observers but in the body of some cells, e.g., colourless blood-corpuscles, there are real granules, which appear to be quite free and unconnected with the intra-cellular network.

Modes of connection.-Cells are connected together to form tissues in various ways.

(1) By means of a cementing intercellular substance. This is probably always present as a transparent, colourless, viscid, albuminous substance, even between the closely apposed cells of epithelium, while in the case of cartilage it forms the main bulk of the tissue, and the cells only appear as imbedded in, not as cemented by, the intercellular substance. This intercellular substance may be either homogeneous or fibrillated. In many cases (e.g. the cornea) it can be shown to contain a number of irregular branched cavities, which communicate with each other, and in which branched cells lie through these branching spaces nutritive fluids can find their way into the very remotest parts of a non-vascular tissue.

As a special variety of intercellular substance must be mentioned the basement membrane (membrana propria) which is found at the base of the epithelial cells in most mucous membranes, and especially as an investing tunic of gland follicles which determines

their shape, and which may persist as a hyaline saccule after the gland-cells have all been discharged.

(2) By anastomosis of their processes. This is the usual way in which stellate cells, e.g. of the cornea, are united : the individuality of each cell is thus to a great extent lost by its connection with its neighbours to form a reticulum: as an example of a network so produced we may cite the stroma of lymphatic glands.

Sometimes the branched processes breaking up into a maze of minute fibrils, adjoining cells are connected by an intermediate reticulum this is the case in the nerve-cells of the spinal cord.

Derived tissue-elements. -Besides the Cell, which may be termed the primary tissue-element, there are materials which may be termed secondary or derived tissue-elements. Such are Intercellular substance, Fibres and Tubules.

a. Intercellular substance is probably in all cases directly derived from the cells themselves. In some cases (e.g. cartilage), by the use of re-agents the cementing intercellular substance is, as it were, analysed into various masses, each arranged in concentric layers around a cell or group of cells, from which it was probably derived (fig. 46).

B. Fibres. In the case of the crystalline lens, and of muscle both striated and non-striated, each fibre is simply a metamorphosed cell in the case of the striped fibre the elongation being accompanied by a multiplication of the nuclei.

:

The various fibres and fibrillæ of connective tissue result from a gradual transformation of an originally homogeneous intercellular substance. Fibres thus formed may undergo great chemical as well as physical transformation: this is notably the case with yellow elastic tissue, in which the sharply defined elastic fibres, possessing great power of resistance to re-agents, contrast strikingly with the homogeneous matter from which they are derived.

7. Tubules, such as the capillary blood-vessels, which were originally supposed to consist of a structureless membrane, have now been proved to be composed of flat, thin cells, cohering along their edges.

With these simple materials the various parts of the body are built up; the more elementary tissues being, so to speak, first compounded of them; while these tissues are variously mixed and interwoven to form more intricate combinations.

Thus are constructed epithelium and its modifications, the con

nective tissues, the fibres of muscle and nerve, &c.; and these, again, with the more simple structures before mentioned, are used as materials wherewith to form arteries, veins, and lymphatics, secreting and vascular glands, lungs, heart, liver, and other parts of the body.

In this chapter the leading characters and chief modifications of the first two of the great groups of tissues-the Epithelial and Connective-will be described; while the others will be appropriately considered in the chapters treating of their physiology.

Epithelium.

The term epithelium is applied to the cells covering the skin, the mucous and serous membranes, and to those forming a lining to other parts of the body as well as entering into the formation of glands. For example:

Epithelium clothes (1) the whole exterior surface of the body, forming the epidermis with its appendages-nails and hairs; becoming continuous at the chief orifices of the body-nose, mouth, anus, and urethra—with the (2) epithelium which lines the whole length of the (3) respiratory, alimentary and genito-urinary tracts, together with the ducts of their various glands. Epithelium also lines the cavities of (4) the brain, and the central canal of the spinal cord, (5) the serous and synovial membranes, and (6) the interior of all blood-vessels and lymphatics.

Epithelial cells possess an intracellular and an intranuclear network (p. 19). They are held together by a clear, albuminous, cement substance. The viscid semi-fluid consistency both of cells and intercellular substance permits such changes of shape and arrangement in the individual cells as are necessary if the epithelium is to maintain its integrity in organs the area of whose free surface is so constantly changing, as the stomach, lungs, &c. Thus, if there be but a single layer of cells, as in the epithelium lining the air vesicles of the lungs, the stretching of this membrane causes such a thinning out of the cells that they change their shape from spheroidal or short columnar, to squamous, and vice versa, when the membrane shrinks.

Epithelial tissues are non-vascular, but in some varieties minute channels exist between the cells of certain layers through which they may be supplied with nourishment from the subjacent blood-vessels. Nerve fibres are supplied to the cells of many epithelia.

Epithelial tissue is classified according as the cells composing it. are arranged in a single layer when it is simple, or in several layers when it is called stratified or laminated, or in two or three layers occupying a position between the other two forms, when it is termed transitional. Of each form when there are several varieties they are named according to the shape of the cells composing it.

A. Simple. (1.) Squamous, scaly, pavement or tesselated; (2.) Spheroidal or glandular;

(3.) Columnar, cylindrical, conical or gobletshaped;

(4.) Ciliated.

B. Transitional.

C. Stratified.

A. Simple.-Squamous Epithelium (fig. 13).-Arranged as a

single layer, this form of epithelium is found as (a) the pigmentary layer of the retina, and forms the lining of (6) the interior of the serous and synovial sacs, (c) the alveoli of the lungs, and (d) of the heart, blood- and lymph-vessels. It consists of cells, which are flattened and scaly, with a more or less irregular outline.

In the pigment cells of the retina, there is a deposit of pigment in the cell-substance. This pigment consists of minute molecules of melanin, imbedded in the cell-substance and almost concealing the nucleus, which is itself transparent (fig. 14).

In white rabbits and other albino animals, in which the pigment of the eye is absent, this layer is found to consist of colourless pavement epithelial cells.

The squamous epithelium which is found as a single layer lining the alveoli of the lungs, the serous membranes, and the interior of blood- and lymphatic-vessels, is generally called by a distinct name-Endothelium.

The presence of endothelium may be demonstrated by staining the part lined by it with silver nitrate.

When a small portion of a perfectly fresh serous membrane for

Fig. 15.-Part of the omentum of a cat, stained in silver nitrate, x 100. The tissue forms a "fenestrated membrane," that is to say, one which is studded with holes or windows. In the figure these are of various shapes and sizes, leaving trabeculæ, the basis of which is fibrous tissue. The trabeculae are of various sizes, and are covered with endothelial cells, the nuclei of which have been made evident by staining with haematoxylin after the silver nitrate has outlined the cells by staining the intercellular substance. (V. D. Harris.)

example, as the mesentery or omentum (fig. 15), is immersed for a few minutes in a quarter per cent. solution of silver nitrate, washed with distilled water and exposed to the action of light, the silver oxide is precipitated in the intercellular cement substance and the endothelial cells are thus mapped out by fine dark and generally sinuous lines of extreme delicacy. The cells vary in size and shape, and are as a rule irregular in outline; those lining the interior of blood-vessels and lymphatics being spindle-shape with a very wavy outline. They enclose a clear, oval nucleus, which, when the cell is viewed in profile, is seen to project from its surface. The nuclei are not however evident unless the tissue which has been