Page:EB1911 - Volume 18.djvu/977

 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 (Superficial and Artistic), and its opening in the earlier part of this article.

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 ) 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.

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