CHAPTER VII. MOUTH DIGESTION. 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. FIG. 60. The mucous membrane of the mouth is covered with papillæ 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 papillæ, circumvallate, surrounded by a fossa, of which there are but a limited number (about a dozen). Diagram taken from a small portion of sacculated gland from Cockroach, showing branching duct and saccules. 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 Section of the Submaxillary Gland of the Dog, showing the a. One of the alveoli, several being grouped round the duct- b. Basement membrane in section. d. Larger duct with columnar epithelium. another, and appear to penetrate the membrane in order to reach the inside of the acini. The cavity of the little sacs is almost completely filled with large polygonal gland cells, so that only a very narrow space the centre. (Fig. 61.) From this space there is free munication 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 ch 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 FIG. 62. A dissection of the side of the face, showing the Salivary Glands. a. Sublingual gland. separable into lobules, and hence b. Submaxillary glands with their ducts are called compound acinous glands. The mouth glands are divided into two sets, which produce 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. different kinds of se (1) Mucous glands, which secrete mucus, and (2) Salivary yun, 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. 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 frænum. The sublingual gland has several ducts, some of which open into that of the submaxillary, and others unite to enter 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. THE CHARACTERS OF MIXED SALIVA. The liquid in the mouth is a mixture of the secretion of the salivary glands with that of the small, purely mucous glands. It is a slightly turbid, tasteless fluid, of a distinctly alkaline reaction, of 1004-1008 specific gravity, and so tenacious that it can be drawn into threads. The amount secreted by an adult human being during 24 hours varies greatly according to cir cumstances, and has been variously estimated by different authors, by whom the wide limits of 200-2000 grms. (7-70 oz.) have been assigned as the daily amount. Saliva contains about .5 per cent. of solids. Of these the greater part are organic; namely, (1) Mucin, from the submaxillary, sublingual and small mucous glands, which can be precipitated by acetic acid. To this substance the viscidity of the saliva is due. (2) Traces of albumin, precipitable by concentrated nitric acid and boiling. (3) Traces of globulin, precipitated by carbonic acid. (4) Ptyalin, a peculiar ferment. The inorganic constituents are salts, among which an inconstant amount of potassium sulphocyanate is found, a substance which does not exist in the blood. FIG. 63. There are also many morphological elements; of these the majority are accidental, being the remains of food, etc.; others are more or less characteristic ; namely, (1) Salivary corpuscles, which are rounded protoplasmic masses containing nuclei and coarse granules which show Brownian movements. (2) Epithelial scales, from the surface of the mucous membrane of the mouth. (3) Various forms of bacteria, which propagate readily amid the decaying particles of m The form elements from mixed saliva from tip of tongue, showing (e) large, irregular, scaly epithelial cells, (c) round salivary corpuscles, several (b) bacteria and (m) micrococci. food in the mouth. No bacteria or other fungi exist in the ducts of the glands or saliva taken from the ducts with the necessary aseptic precautions. COLLECTION OF THE SECRETION. Ordinary mixed saliva may be easily collected by chewing some insoluble material, such as a bit of rubber tubing, and collecting the fluid which the motion causes to be poured into the mouth. The collection of the secretion of the different glands requires |