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th, ventral surface of the tongue, and soft palate fall into this category. Generally, however, the oral mucosa has a definite stratum corneum, although the depth is considerably less than that of skin. It has been shown that stimulation of oral mucosa results in increased keratinization. Where the stratum corneum is thin, the stratum granulosum is indistinct and may appear to be absent. As the depth of stratum corneum increases, the stratum granulosum becomes larger and more obvious.
     The rete pegs of fingerlike processes that extend into connective tissue are more obvious and deeper than in skin. Between the epithelial rete pegs are connective tissue extensions or dermal papillae. The epithelium and connective tissue are separated by a so-called basement membrane, which is really part of the connective tissue ground substance and stains deeply for acid mucopolysaccharides. This may merely represent a zone of condensed ground substance beneath the epithelium. A true basement membrane-like structure has been described in electron microscope studies of oral mucosa, but its width is 600 Angstrom units and would be invisible with light microscopy.
     The connective tissue is composed of dense bundles of collagen fibers and numerous small blood vessels and nerves. The characteristic cell of connective tissue is the fibroblast, a large spindleshaped cell. Normally, these cells are scattered sparsely throughout the connective tissue. The deeper layers of the connective tissue are often referred to as submucosa, and their function is to bind mucosa to underlying muscle or bone. Larger blood vessels and numerous mucous glands appear in the deeper layers of the connective tissue. Mucous glands are an integral part of the normal mucous membrane structure; they secrete mucus, which is carried to the surface by fine ducts. The mucous glands are normally few in number, but large numbers may occur in certain areas of the oral mucosa, such as the hard palate and occasionally the lower lip. Sebaceous glands normally are not present in the connective tissue of oral mucosa, but they may appear as a developmental anomaly.
     In general terms, the oral mucosa is composed of three main types. A functioning mucosa such as that of the gingiva and hard palate is well keratinized and has a dense connective tissue. This type of mucosa is firmly attached to underlying bone, and the marginal gingiva is attached to the tooth surface by a specialized form of adaptation or organic union with enamel or cementum close to the enamel-cemental border. If the teeth are lost, the gingiva becomes edentulous alveolar mucosa, and this again is a firm, functioning mucosa.
     A nonfunctioning or mere lining mucosa is the type that occurs on the lip, floor of the mouth, ventral surface of the tongue, and buccal region. The mucous membrane of this area is freely movable and not attached to a firm osseous structure. The attachment involves musculature, and the mucosa is not greatly affected by masticatory stimulation or traumatic influences because it is easily pushed aside by pressure. The mucosa in these areas is relatively slightly keratinized, and the connective tissue is somewhat looser in texture.
     The third type of oral mucosa, the dorsum of the tongue, is highly specific and has a specialized function. The surface is composed of papillary structur