Mouth and Salivary Glands

The submaxillary gland lies deep to the posterior half of the body of the lower jaw ; it is about the size of a walnut. The facial artery is embedded in the upper part of the gland. The submaxil lary duct (Wharton's duct) runs forward to the sublingual papilla already mentioned.

The sublingual gland is placed farther forward than the sub maxillary ; it is like an almond in shape though larger ; its outer flattened surface rests against the lower jaw ; its ducts are small and numerous.

Embryology.

The fore-gut (see ALIMENTARY CANAL) at first ends blindly, ventral to the hind brain ; in front of it is the over hanging fore-brain. When the heart develops, ventral to the f ore gut, it also projects forward toward the fore-brain, so that a trans verse 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 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 proc ess 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 de pressed vertical gutter to which the term philtrum is given. Be sides forming the philtrum the fronto-nasal process is responsible for that part of the roof of the mouth bounded 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. 1902) appears first as a groove. The parotid is ectodermal in origin, all the others entodermal.

For further details and literature see Quain, Anat. vol. i. (Lon

don, 1908) ; J. P. McMurrich Development of the Human Body (London, 1923) ; 0. Hertwig, Handbuch der Entwickelungslehre Th. II. (Jena). See also VERTEBRATE EMBRYOLOGY. (W. S. L.-B.) Comparative Anatomy.—In the Acrania 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 suc torial 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 appear, 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 con dition 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.) MOUTH AND SALIVARY GLANDS, DISEASES OF. Most of the conditions affecting these parts have been described under separate headings (see TONGUE, DISEASES OF; DENTISTRY; CALCULI; DIPHTHERIA; PHARYNGITIS; TONSILLITIS; MUMPS; ACTINOMYCOSIS) while as constituting the commencement of the respiratory and alimentary systems the mouth shares in some de gree in the affections of these systems, as well as in general diseases (see SCARLET FEVER; MEASLES; SCURVY; VENEREAL DISEASES; TUBERCULOSIS). A few other aspects of the subject remain to be considered.