tinct layers of cells-a stratified epithelium. It is clear that all the cells of a stratified epithelium can not be equally well nourished by

1000

Fig. 40.-Schematic diagram of stratified pavement epithelium.

the blood-supply from the vessels in the highly vascular connective tissue beneath. The middle and outer layers of cells accordingly suffer. The deeper layers are much better nourished, and as a consequence their cells increase much more rapidly than those above; they push outward, replacing the superficial cells as fast as they die or are thrown off. The proliferation of cells in a stratified epithelium occurs, therefore, chiefly in its basal layers.

(a) Stratified Squamous Epithelium.-Stratified squamous epithelium with superficial flattened cells forms the epidermis with its continuations into the body, as, for instance, the walls of the oral cavity and the esophagus, the epithelium of the conjunctiva, the vagina, the external auditory canal, and the external sheath of the hair follicles.

The cells of the basal layer are here mostly cubic-cylindric. Then follow, according to the situation of the epithelium, one or more layers of polyhedral cells, which become gradually flattened toward the surface, the outer

most layers consisting of thin plate-like cells.

In stratified squamous epi

thelia, where the outer cells become horny (as in the skin), the stratification is still more specialized. Here layers are inserted in which the horny or chitinous substance is gradually formed, although the cells do not become chitinous until the superficial layers are reached.

♡

て

Fig. 41.-Cross-section of stratified squamous epithelium from the esophagus

Especially characteristic of stratified squamous epithelium is the arrangement of the connective tissue on which this epithelium rests. There are cone-like projections, known as papillæ, arising from the connective tissue beneath the epithelium, projecting into the latter in such a way that on cross-section the junction of the two tissues appears as a wave-like line. These papillæ not only serve to fasten the epithelium more firmly to the connective tissue below, but influence very favorably the nourishment of the former by allowing a greater number of its basal cells to approximate the under

of man.

lying blood-capillaries. The pyramidal extensions of the epithelium. between the papillæ are designated interpapillary epithelial processes. In regions where the stratified squamous epithelium consists of many layers, the prickle cells, intercellular bridges, and the intercellular spaces are especially well developed. These spaces facilitate the passage of the lymph-plasma to the more superficial layers of cells.

(b) Transitional Epithelium.-Transitional epithelium is a stratified epithelium occurring in the pelvis of the kidney, the ureters, bladder, and the posterior portion of the male urethra. It is composed of four to six layers of cells and rests on a connective tissue free from papillæ. In sections the cells of the deeper layers appear to be of irregularly columnar, cubic or triangular shape. The

cells forming the superficial layer are large, somewhat flattened cells, with convex free surfaces, often possessing two, sometimes three, nuclei. They cover a number of the cells of the layer just beneath them, their under surfaces being pitted to receive the upper ends of the deeper cells. In teased preparations the cells of the deeper layers appear very irregular, often showing ridges or variously shaped processes. (See Fig. 42.)

(c) Stratified Columnar Epithelium.-In this type the superficial layer consists of columnar cells, the basal ends of which are usually somewhat pointed, or may branch. The deeper cells, which may be arranged in one or more layers, are of irregular, triangular, polyhedral, or spindle shape. It is found in the larger gland ducts, olfactory mucous membrane, palpebral conjunctiva, portions of the

male urethra and the vas deferens, and in certain regions of the larynx.

The ciliated variety of this epithelium differs from the foregoing in that the superficial columnar cells are provided with cilia. Stratified ciliated columnar epithelium is found in the respiratory portion

of the nose, larynx, trachea, and larger bronchi, in the Eustachian tube, epididymis, and a portion of the vas deferens.

All epithelial cells are probably joined together by short processes forming intercellular bridges, the lymph supplying them with nourishment circulating in the intercellular spaces thus formed. Toward the surface, these intercellular spaces are roofed over, thus preventing the escape of the fluid. When seen from the surface, epithelia treated by certain methods (iron-hematoxylin) show the cells joined together by very minute, clearly defined and continuous

Goblet cell.

-Cilia.

Fig. 46.-Cross-section of stratified ciliated columnar epithelium from the

cement-lines.

trachea of a rabbit.

Bonnet has called them terminal ledges or bars (Schlussleisten). The function of this structure would seem to consist in its power to prevent the escape of lymph from the surface, and the penetration of micro-organisms (M. Heidenhain, 92; Bonnet, 95).

3. GLANDULAR EPITHELIUM.

(a) The Gland-cell.-Certain cells lying scattered among other epithelial cells produce substances that are extruded and utilized in the body economy. The protoplasm of such a cell elaborates in its interior a substance that takes the form of vacuoles or granules, which gradually distend the cell; the substance thus produced is finally given off as the secretion. All these phases of the activity of a gland-cell are included under the term secretion.

Isolated glandular cells are frequently met with in epithelia, and are known in general as unicellular glands. They occur especially in the intestinal and respiratory epithelium, where, owing to their shape, they are termed goblet cells. All the intestinal epithelial cells and many of the cells of respiratory epithelium, have the power of changing into goblet cells. These are distinguished from the neighboring cells by the fact that their free ends are clearer and more

Cilia.

Mucin.

Nucleus.

Basal process.

Fig. 47.-Goblet cells from the bronchus of a dog. The middle cell still possesses its cilia; that to the right has already emptied its mucous contents (collapsed goblet cell); X 600. Technic No. 128.

vesicular, while their basal portions, containing the nuclei, are narrow and pointed. The clear substance elaborated by the protoplasm of the cell, but not yet extruded, is mucin. On closer examination it is seen that this substance fills the interspaces of a very fine protoplasmic network continuous with the protoplasm surrounding the nucleus.

Thus we have, during the phases of secretion, two distinct substances in the cell-body: the one the original protoplasm of the cell -protoplasm (Kupffer); the other its product, in this case mucinparaplasm (Kupffer). When the secretion is extruded the goblet cell collapses and then appears as a thin cord between the neighboring cells. There is as yet some question as to whether a collapsed goblet cell dies after the expulsion of its contents, or whether it may again become stored with mucin. Should it be destroyed, its place is soon occupied by the closing in of contiguous cells.

Multicellular glands originate by the metamorphosis of a number of adjacent cells into glandular cells. This is usually accompanied by a more or less marked dipping down of the epithelial layer into the underlying connective tissue. The simplest form of such an invagination is a cylindrical tube lined entirely by glandular cells. A further differentiation may take place in that all the invaginated cells do not assume a secretory function, those at the upper portion of the tube forming the lining membrane of an excretory duct. The originally uniform tube is thus differentiated into an excretory and a secretory portion.

Multicellular glands may lie entirely within the epithelium, and are then known as intra-epithelial glands, in contrast to the extra-epithelial or ordinary type, the greater part of which lies imbedded in the under

Fig. 48.-Simple tubular glands. Lieberkühn's glands from the large intestine of man.

lying connective tissue. Glands of the former type have been studied in amphibian larvæ, and, according to Sigmund Mayer, occur also in the epididymis, conjunctiva, etc., of mammals.

General Consideration of the Structure and Classification of Glands.-Variations in glandular types affect principally the secretory portions of glands, while the excretory ducts are more or less uniform. Glands are classified, according to their shape, into tubular and saccular glands; each of these types is further divided into simple and compound tubular, and simple and compound saccular (racemose) glands.

Tubular Glands.-The simplest form is a tubule of uniform diameter, as in the simple tubular glands of the cardiac region of