1911 Encyclopædia Britannica/Mouth and Salivary Glands
MOUTH AND SALIVARY GLANDS. The mouth (A.S. múð), in anatomy, is an oval cavity at the beginning of the alimentary canal in which the food is masticated. The opening is situated between the lips, and at rest its width reaches to the first premolar tooth on each side.
The lips (A.S. lippa) are fleshy folds, surrounding the opening of the mouth, and are formed, from without inward, by skin, superficial fascia, orbicularis oris muscle, submucous tissue, containing numerous labial glands about the size of a small pea, and mucous membrane. In the deeper part of each lip lies the coronary artery, while in the mid-line is a reflection of the mucous membrane on to the gum forming the fraenum labii.
The cheeks (A.S. céace) form the sides of the mouth and are continuous with the lips, with which their structure is almost identical save that the buccinator muscle replaces the orbicularis oris and the buccal glands the labial. In the subcutaneous fascia is a distinct mass of fat, specially large in the infant, which is known as the sucking pad. On the buccal surface of the cheek, opposite the second upper molar tooth, is the papilla which marks the opening of the parotid duct, while, just behind, are four or five molar glands, larger than the buccal, the ducts of which open opposite the last molar tooth. The mucous membrane of the cheek, like that of the rest of the mouth, is of the stratified squamous variety (see Epithelial Tissues) and is reflected on to the gums.
The gums (A.S. góma) consist of mucous membrane connected by thick fibrous tissue to the periosteum of the jaws. Round the base of the crown of each tooth the membrane rises up into a little collar.
The vestibule of the mouth is the space between the lips and cheeks superficially and the gums and teeth deeply. It communicates with the true cavity of the mouth by the clefts between the teeth and by the space behind the last molar teeth.
The roof of the mouth is concave transversely and antero-posteriorly, and is formed by the hard and soft palate. The hard palate consists of mucous membrane continuous with that of the gums and bound to the periosteum of the palatine processes of the maxillae and palate bones by firm fibrous tissue. In the mid-line is a slight ridge, the palatine raphe, which ends in front in a little eminence called the palatine papilla, marking the position of the anterior palatine canal. From the anterior part of the raphe five or six transverse ridges or rugae of the mucous membrane run outward. (For a description of the soft palate see Pharynx.)
The floor of the mouth can only be seen when the tongue is raised, then the reflection of the mucous membrane from the gums to it is exposed. In the mid-line is a prominent fold called the fraenum linguae, and on each side of this a sublingua papilla, on to the summit of which the duct of the submaxillary gland opens. Running outward and backward from this is a ridge called the plica sublingualis, which marks the upper edge of the sublingual gland, and on to which most of the ducts of that gland open. (For a description of the Tongue and the Teeth see special articles on those structures.)
The salivary glands are the parotid, submaxillary and sublingual, though the small scattered glands such as the labial, buccal, molar, lingual, &c., probably have a similar function.
The parotid gland (Gr. παρά beside, οὖς ear), is the largest of these glands, and is situated between the ear and the ramus of the mandible. In a transverse section through the head about the level of the mouth the gland looks more or less triangular, its outer wall or base being bounded by the parotid fascia, its anterior by the jaw, and its posterior by the mastoid process and sterno mastoid muscle. Where the anterior and posterior walls meet to form the apex is the styloid process. Above the gland reaches to the zygoma, and below to the level of the angle of the jaw, where a strong process of the deep cervical fascia, called the stylo-mandibular ligament, separates it from the submaxillary gland; indeed the parotid is often described as lying in a bag formed by deep cervical fascia. The outline of the gland is obscured by several processes, one of which, the facial lobe, runs forward, superficial to the masseter muscle, accompanying the duct. A separate part of this is called the socia parotidis; another, known as the pterygoid lobe, passes forward, deep to the ramus of the jaw, to the space between the two pterygoid muscles. A third wraps round the front of the styloid process and may be termed the pre-styloid lobe, while a fourth, the post-styloid, insinuates itself behind that process. The upper part of the prestyloid lobe sometimes reaches the back part of the glenoid cavity, and is then called the glenoid lobe. All these processes, however, are very variable, and depend a good deal on the position of the head and neck. The facial nerve, the temporo-maxillary vein, and the termination of the external carotid artery, among other structures, are embedded in the gland. The parotid duct (Stensen’s duct) crosses the upper part of the masseter and then pierces the buccinator on its way to the mouth; it is about 2 in. long. Its position is described in the article on Anatomy (Superficial and Artistic), and its opening in the earlier part of this article.
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| From A. Birmingham Cunningham’s Text Book of Anatomy. |
Fig. 1.—The Salivary Glands and Their Ducts. The greater portion of the body of the mandible has been removed to expose the sublingual and the deeper parts of the submaxillary glands. Four ducts of the sublingual gland are shown opening on the floor of the mouth over the gland, a fifth is shown opening into the anterior end of Wharton’s duct. The course of Wharton’s duct is shown by a dotted line. |
The submaxillary gland lies deep to the posterior half of the body of the lower jaw; it is about the size of a walnut, and has an external, internal and inferior surface. The external surface rests against the submaxillary fossa in the jaw, the internal is in contact with the mylohyoid and hyoglossus muscles, while the inferior is subcutaneous. The whole gland is enclosed in a sac of deep cervical fascia, while a process, from which the duct arises, passes deep to the mylohyoid. The facial artery is embedded in the upper part of the gland. The submaxillary duct (Wharton’s duct) runs forward to the sublingual papilla already mentioned.
The sublingual gland is placed further forward than the submaxillary; it is like an almond in shape though larger; its outer flattened surface rests against the sublingual fossa in the lower jaw, while the inner one is in contact with the genio-hyoglossus muscle, the submaxillary duct and the lingual nerve. Its upper edge forms the sublingual fold (plica sublingualis) in the mucous membrane of the mouth, and along this its ducts, which are small and numerous, open; these are sometimes called the ducts of Rivini, but the term “sublingual ducts” is simpler and more expressive. Occasionally an anterior sublingual duct (duct of Bartholin) opens with or into the submaxillary duct.
Embryology.
The fore-gut (see Alimentary Canal) at first ends blindly, ventral to the region of the hind brain, while in front of it is the overhanging fore-brain. When the heart develops, ventral to the fore-gut, it also projects forward toward the fore-brain, so that a transverse cleft, without any lateral boundaries and lined by ectoderm, is left between these two structures. This is the stomatodaeum or primitive mouth, the ectoderm of which rests against the entoderm of the fore-gut to form the bucco-pharyngeal membrane, and so separates the two chambers. The position of this membrane does not correspond to the fauces or hinder limits of the adult mouth, but is much more oblique, so that the front part of the roof of the pharynx is formed by stomatodaeum while the greater part of the floor of the permanent mouth is fore-gut. During the third week the membrane disappears, and it is probable that to its early atrophy is due the fact that no traces of it can be seen in the adult. Growing down from the region of the fore-brain is the fronto-nasal process, which forms the nose and the middle piece of the upper lip, while the lateral parts of the mouth are closed in by two processes, on each side of which the lower or mandibular process rapidly meets its fellow in the mid-line to form the lower jaw and lip, thus separating the heart from the mouth cavity. The upper or maxillary process grows inward more slowly, but at last joins with the fronto-nasal process, and in the adult the lines of union are seen on each side as ridges of skin which run down from the nostril to the margin of the lip, and enclose that slightly depressed vertical gutter to which the term philtrum is given. Besides forming the philtrum the fronto-nasal process is responsible for that part of the roof of the mouth which corresponds to the premaxillary bones, an area marked out by lines drawn on each side from between the lateral incisor and canine teeth to the palatine papilla. At first the cavities of the mouth and nose are one, but they are later divided by the palatal processes, which grow in like shelves from the maxillary processes and meet in the mid-line. The submaxillary and sublingual salivary glands develop as solid outgrowths of the buccal epithelium which are canalized later, while the parotid according to Hammar (Archiv. f. mikr. Anat. LXI., 1902) appears first as a groove. The parotid is ectodermal in origin, all the others entodermal.
For further details and literature see Quain's Anat. vol. i. (London, 1908); J. P. McMurrich Development of the Human Body (London, 1907); O. Hertwig, Handbuch der Entwickelungslehre Th. II., (Jena).
Comparative Anatomy.
In the acrania (amphioxus) the mouth is developed on the left side and gradually shifts to the mid-line; later an extra chamber, the oral hood, is formed in front of it, the external opening of which is provided with bristle-like cirrhi, so that in the adult the mouth is merely an aperture in the velum or membrane which separates the oral hood from the pharynx.
In the cyclostomata (lampreys and hags) the mouth is a suctorial organ, and resembles a funnel, the narrow end of which opens into the pharynx. It is always open and is provided with horny teeth and a tongue. At this low stage of the vertebrate scale no jaws have yet appeared, but in the larval lamprey (ammocoetes) an oral hood, resembling that of amphioxus, is present. In the fishes jaws are present and the mouth can be closed at will. In the elasmobranchs (sharks and rays) the opening is crescentic and situated well on the ventral surface of the head, but in other fishes it is at the anterior end of the body. Until the dipnoi (mud fish) are reached there is no communication between the mouth and the nose but in these fishes the internal or posterior nares open into the front part of the roof of the mouth, thus adapting them to air-breathing. In the Amphibia the mouth has usually an enormous gape, and the position of the posterior nares resembles that of the dipnoi. It will be noticed that at this stage of phylogeny the condition resembles that of the ontogeny of man before the palatal processes appear. The premaxillary part of the fronto-nasal process separates the nasal cavity from the mouth in front, but behind that the cavity is the rudiment of the mouth and nose which no palate has yet appeared to separate. In Reptiles the hard palate appears, and henceforward the digestive and respiratory tracts only form one passage in the pharynx. In mammals definite lips provided with muscles first appear, though the Monotremes have such specialized mouths that lips are not found in that order. Many monkeys have the vestibule enlarged to form the cheek pouches. (F. G. P.)
Surgery of the Mouth.
In surgical operations upon the interior of the mouth which are likely to be accompanied with much bleeding, it is much the custom now to have the patient lying upon his back, with the head hanging over the end of the table, so that the blood may sink into the dome of the pharynx and escape by the nostrils, instead of running the risk of finding its way into the windpipe and lungs. (See Cleft Palate.)
Mumps.—Inflammation of the parotid gland is apt to occur as an epidemic, children being chiefly attacked. The disease, which is highly infectious, is called mumps, and is associated with much swelling below and in front of the ear, or ears. There is stiffness of the jaw and there is a difficulty in swallowing. There is slight local tenderness, and the temperature may, perhaps, run up a degree or two. For the sake of others, the child should be kept away from school for three or four weeks.
Salivary Calculus.—Sometimes a deposit of phosphate and carbonate of lime slowly takes place from the saliva, and gives rise to the formation of a small concretion in the duct of one of the salivary glands. When the concretion blocks the duct, so that the saliva is unable to find its way into the mouth, a fluid swelling forms behind the blockage, giving rise to inconvenience and unsightliness. The swelling is at its greatest during a meal, when the secretion of the saliva is necessarily rapid; subsequently it disappears, recurring, however, at the next meal-time. In many cases the patient is conscious of the fact of there being a hard, movable “kernel,” the size, perhaps, of a barleycorn, a cherry-stone or even of a small almond, in the course of the duct. In the removal of the calculus every endeavour should be made to effect its escape into the mouth, as, if the skin were incised for its extraction, the wound might refuse to heal, a salivary fistula resulting. (E. O.*)