The cavity of the mouth is lined by a bright red mucous membrane, which is continuous with the skin at the lips. It varies in structure in different parts of the buccal cavity, and in its general construction more resembles the outer covering of the body than the mucous membrane lining the alimentary tract. It consists of (1) a superficial part, composed of thick stratified epithelium, the upper cells of which are flat, scaly and tough, and are placed horizontally, while in the deeper layers the cells are soft, rounded or elongated, having their long axis perpendicular to the surface; and (2) a deeper part, composed of fibro-elastic tissue, which, over the alveoli of the teeth, is amalgamated with the periosteum and forms the dense, tough gums.

The mucous membrane of the mouth is covered with papillae which, on the dorsum of the tongue, attain great magnitude and variety of shape and epithelial covering. In man, three kinds are described: (1) Narrow pointed, filiform. (2) Blunt and clubbed at the apex, fungiform. (3) Broad complex papillae, circumvallate, surrounded by a fossa, of which there are but a limited number (about a dozen).

The special secreting organs or glands, which pour their juices into the mouth, have all the same general type of structure, though they vary much in the detail as to the variety and character of their cells. They are known as the acinous or sacculated glands, from their being made up of numerous acini, or minute elongated sacs or tubules, arranged at the end of a repeatedly branching duct, like grapes on the terminals of the successive little branches growing from the central stalk to form a bunch. In the glands the saccules are packed together closely around the ducts, and by mutual pressure are made to assume various shapes. The wall of the saccule is formed of a very delicate, clear, transparent membrane, on the outside of which are numerous flattened, branching stellate cells, the branches of which anastomose one with another, and appear to penetrate the membrane in order to reach the inside of the acini.

Diagram taken from a small portion of sacculated gland from Cockroach, showing branching duct and saccules.

Fig. 60. Diagram taken from a small portion of sacculated gland from Cockroach, showing branching duct and saccules.

Section of the Submaxillary Gland of the Dog, showing the commencement of a duct in the alveoli.

Fig. 61. Section of the Submaxillary Gland of the Dog, showing the commencement of a duct in the alveoli. X 425. (Schafer).

a. One of the alveoli, several being grouped round the ductlet {d'). .

b. Basement membrane in section.

d. Larger duct with columnar epithelium, s. Half-moon group of cells.

The cavity of the little sacs is almost completely filled with large polygonal gland cells, so that only a very narrow space exists in the centre. (Fig. 61.) From this space there is free communication to the main duct of the gland by means of the proper ductlet of each saccule. In the saccules of a few glands, viz., some of the so-called mucous salivary glands, another kind of cell element is seen between the gland cells just described and the wall of the sac, their outer side following accurately the concave boundary of the saccule, their inner side impinging upon the gland cells. They thus acquire a more or less half-moon shape. These demi-lune cells will be again referred to (page 143).

Between the saccules are numerous blood vessels which branch and form a network of capillaries on the outside of each little sac. Numerous nerves are also found, which, according to some observers, have ganglionic cell connections in the gland substance, and send terminals into the gland cells direct.

Although this account of the nerve terminations in the secreting cells of other glands has met with doubt, it is certain that in the lower animals nerve terminals have been traced into gland cells, and upon physiological grounds, as will presently appear, we are forced to believe that a similar connection must exist in mammalia.

The ducts are lined with short cylindrical epithelium which does not appear to have any secreting function. All the glands are made up of numerous packets of lobules bound together in one mass and- united by their ducts. Each of these lobules is itself a perfect gland. The smaller mouth glands are also separable into lobules, and hence are called compound acinous glands.

The mouth glands are divided into two sets, which produce different kinds of secretion: (i) Mucous glands, which secrete mucus, and (2) Salivary glands, which produce watery saliva. The functional distinction is seldom absolute, for most salivary glands have a mixed secretion, and various gradations of transition from purely salivary to purely mucous glands are met with.

A dissection of the side of the face, showing the Salivary Glands.

Fig. 62. A dissection of the side of the face, showing the Salivary Glands.

a. Sublingual gland.

b. Submaxillary glands with their ducts opening on the floor of the mouth beneath the tongue at (d).

c. Parotid gland and its duct, which opens on the inner side of the cheek.

The proper mucous glands are small, varying in size from a pin's head to a pea. They are found in groups under the mucous membrane in various parts of the mouth, and from their positions are called labial, buccal, etc. Their cells contain a clear mucilaginous substance.

The great Salivary glands are the three large glands which are known as the parotid, submaxillary and sublingual. On account of their great size they form striking anatomical objects, being large masses of irregularly arranged glandular packets, which might be spoken of as lobes, to distinguish them from the smaller packets or lobules. Their ducts are of considerable size, and have strong walls made of dense fibrous tissue, containing many elastic fibres, and in one of them, the submaxillary, smooth muscle tissue has been demonstrated.

The parotid duct (Steno's) opens into the mouth about the middle of the cheek just opposite the second molar tooth. The submaxillary has also a single duct (Wharton's), which opens beneath the tongue beside the fraenum. The sublingual gland has several ducts, some of which open into that of the submaxillary, and others unite to enter into the mouth beside Wharton's duct.

In different animals and in different glands of the same animal a variable amount of mucus is secreted by these glands, which are all called salivary, though the parotid alone deserves the name in the strictest sense of the term, owing to the freedom of its secretion from mucus.