which finally terminate in the alveoli or acini, irregular and somewhat tortuous tubular structures with a lumen and possessing an epithelium characteristic of the particular variety of the gland (see below). The epithelium lining the different portions of the large excretory ducts varies somewhat. For a short distance from their oral end they are lined by a stratified columnar epithelium consisting of two layers of cells (Wharton's ducts are now and then lined for a short distance by a stratified pavement epithelium continuous with that lining the mouth). Beyond this stratified columnar epithelium, which extends for a variable distance, the large excretory ducts, the interlobar and interlobular ducts are lined by a pseudostratified columnar epithelium, possessing two rows of nuclei (Steiner). Outside of the epithelial lining there is found a firm fibro-elastic covering, forming the wall of the ducts. The intralobular ducts are lined by a single layer of columnar cells, the basal half of each cell showing a distinct striation. The intermediate portions of the ducts are lined by a low, cubic, or flattened epithelium.

Between the membrana propria and the secreting epithelium of the tube, and more especially in the acini, are branched cells which anastomose with each other, the so-called basket cells. Their processes penetrate between the glandular cells and form a supporting structure for them. The homogeneous membrana propria surrounding the entire glandular tube is in close relationship to these cells.

We shall now consider more in detail the structure of the alveoli or acini of the salivary glands.

SALIVARY GLANDS.

The Parotid Gland (Serous Gland).-The epithelial cells lining the acini of this gland are short, irregularly columnar or cubic cells, their structure changing according to their physiologic condition. When at rest the secreting cells are only slightly granulated and contain a large quantity of clear secretion (paraplasm), while the nuclei are irregular and indented. As soon as the protoplasm of the cells commences the formation of secretion, the cells become smaller, more granular and opaque, and their nuclei assume a spheric shape; when, however, the cells throw off a portion of the granular material, an immediate increase in their protoplasm is noticed. After a long period of secretion the cells become still smaller and their contents still more turbid. They now contain very little protoplasm. These phenomena can only be regarded as due to the fact that the granular paraplasm is formed at the expense of the protoplasm of the cell during the period of rest.

The Sublingual Gland (Mucous Gland). In the acini of mucous glands there are found two varieties of cells: (1) True mucous cells, which, when filled with secretion, are large and

clear, with their nuclei always at the periphery. During the expulsion of the secretion the mucous cells decrease in size and become cloudy, while the nuclei leave the periphery and increase in size. (2) Cells rich in protoplasm, situated in close apposition to the membrana propria. These cells resemble in structure serous cells, and are found either singly or in groups of crescentic shape. They are known as the crescents of Gianuzzi or the demilunes of Heidenhain. The margins of the individual cells composing the crescents are often so faintly outlined that the whole structure has the appearance of a large polynuclear giant cell.

Fig. 192.-Section through salivary gland of rabbit, with injected blood-vessels; X 70.

The demilunar cells have been variously interpreted by different observers. They have been regarded as permanent cells with a special secretion, as transitional structures, and again as cells destined to replace the degenerated mucous cells. Stöhr (87) believes that the cells of the acini are never destroyed in the process of mucous secretion, and that the crescents of Gianuzzi are therefore merely a complex of cells containing no secretion, which have been crowded to the wall by the adjacent enlarged and distended cells. Solger (96), on the other hand, does not regard the demilunes as transitional structures whose function is to replace the

destroyed cells, but considers them to be permanent secreting cells -an opinion which he bases on the results of special methods of investigation. According to him, then, the mucous salivary glands are mixed glands, in that the demilunes consist of cells of a serous type, while the remaining elements are mucous in character. The destruction of mucous cells during secretion is not admitted by him

(compare also R. Krause). This latter view seems more in accord with recent observations.

The Submaxillary Gland (Mixed Gland).-With regard to the mixed glands it is sufficient to say that there is a simultaneous secretion of serous and mucous fluids, and that these two substances are produced in separate but adjacent acini, of which the

one type possesses a structure identical with that found in the parotid and the other with that in the sublingual.

By means of various methods the existence of a network of tubules surrounding the glandular cells may be demonstrated both in the serous and mucous glands. The same arrangement may be

The

observed in the case of the cells forming the demilunes. course of these tubules may be followed to their junction with the lumen of the secreting portion of the gland tubule, and the whole. structure would seem to indicate that the entire surface of the cells is concerned in the act of secretion (Erik Müller, 95; Stöhr, 96, II).

As to the part that the intermediate tubules and the intralobular tubes play in the process of secretion, Merkel's (83) theory is of interest. He believes that the

former yield a part of the water in the saliva, while the salts are furnished by the rod - shaped epithelium of the intralobular tubes. These views of Merkel have been questioned, as it has been shown by chemic analysis that the relative quantity of water and salts in the secretion of the salivary glands is not at all proportionate to the number of the intermediate tubules and intralobular tubes. For example, Werther finds that although a great many intermediate tubules are present in the parotid gland of the rabbit and none at all in the submaxillary gland of the dog, nevertheless the secretions of these glands contain equal quantities of water. Furthermore, the secretions of the parotid of the rabbit and of the sublingual of the dog show equal quantities of salts, in spite of the fact that in the former there are large numbers of intralobular tubes with rod-shaped epithelium and in the latter none at all.

tubules.

THE SMALL GLANDS OF THE MOUTH.

Besides the larger glands, there are in the oral cavity numerous small lobular, tubulo-acinous and simple branched tubulo-acinous glands. They are mostly of the mixed type, and are called, according to their location, glandulæ labiales, palatinæ and linguales. Serous glands, known as v. Ebner's glands, occur in the tongue, their ducts opening into the depressions of the circumvallate and foliate papillæ. The absence of intralobular tubes and well-defined intermediate tubules is characteristic of all the smaller glands of the oral cavity. As a consequence the secreting tubules are composed almost entirely of those parts corresponding to the acini of the larger glands. It appears that the smaller mucous glands, except those of the lips (J. Nadler), do not, as a rule, contain typical demilunes.

The salivary glands and smaller glands of the mouth have a

rich blood supply. In the salivary glands the arteries follow the ducts through their repeated branching, ultimately ending in capillaries which form a network inclosing the acini and the terminal portions of the system of ducts.

The lymphatics begin in clefts in the connective tissue surrounding and separating the acini. Larger lymph-vessels are found in the connective tissue separating the lobules and lobes of the gland. The nerve supply of the salivary glands, may, owing to the importance of these structures, receive somewhat fuller consideration. Their nerve supply is from several sources. That of the sublingual and submaxillary glands will be considered first. Sensory nerve-fibers (no doubt the dendrites of sensory neurones, the cellbodies of which are situated in the geniculate ganglion) terminate in free sensory endings in the large excretory ducts and their branches. These medullated fibers accompany the ducts in the form of small bundles. From place to place one or several fibers leave these bundles and, after dividing a number of times, lose their medullary sheaths. After further division the nonmedullated branches form plexuses under the epithelial lining of the ducts. From the fibers of these plexuses terminal fibrils are given off, which enter the epithelium, to end, often near the free surface, on the epithelial cells (Arnstein, 95; Huber, 96). The secretory cells of the acini receive their innervation from sympathetic neurones. The cell-bodies of

those supplying the sublingual glands are grouped in a number of small, sympathetic ganglia situated in a small triangle formed by the lingual nerve, the chorda tympani and Wharton's duct, the chordalingual triangle. These ganglia may be known as the sublingual ganglia (Langley). The cell-bodies of the sympathetic neurones supplying the secretory cells of the submaxillary glands are grouped in small ganglia situated on Wharton's duct just before it enters the gland, in the hilum of the gland, and on the interlobar and interlobular ducts; they may be spoken of collectively as the submaxillary ganglia. In the glands under discussion, the neuraxes of the sympathetic neurones are grouped to form small bundles, which divide repeatedly, the resulting divisions joining to form plexuses situated in the outer portion of the walls of the ducts, and as such may be followed along the ducts, the bundles of nerve-fibers becoming smaller and the division of the bundles of fibers and the individual fibers occurring oftener as the smaller divisions of the system of ducts are reached. On reaching the acini, the terminal. branches of the nerve-fibers form a plexus outside of the basement membrane, epilamellar plexus; from this branches are given off which penetrate the basement membrane, some forming a hypolamellar plexus, others ending on the gland-cells in small granules or clusters of granules (Arnstein). Throughout their entire course the neuraxes of the sympathetic neurones are varicose, nonmedullated nerve-fibers. The nerve-fibers of the chorda tympani end in terminal end-baskets, inclosing the cell-bodies of the sympathetic