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lial or intraepithelial. The latter type has a thin covering since only part of the epithelium forms the vesicular membrane. The fluid may be clear or slightly red, depending upon the severity of the inflammatory reaction at the base of the vesicle. As the vesicular membrane ruptures, an ulcer forms. Vesicalar eruptions may be seen in such diverse conditions as viral infections, drug reactions, burns, and vesicular dermatoses.
     Bulla. A bulla is a large, vesicular-type lesion ranging in size from 5 mm to several centimeters. The outer covering of the bulla or bleb may be thin or relatively thick, depending upon the subepithelial or intraepithelial character of the lesion. The covering membrane of the bulla may be tense or lax, depending upon the amount of fluid. A bullous lesion may develop directly or may form from the union of several vesicles erupting close together. Bullae are characteristically seen in such diseases as pemphigus and pemphigoid. Bullae may also appear in such characteristically vesicular eruptions as erythema multiforme and herpetic stomatitis. During eruptions usually involve both vesicles and bullae.
     Pustule. A pustule is a vesicular type of lesion containing purulent material instead of clear fluid. Pustules on oral mucosa are unusual but .appear in such conditions as pyostomatitis.
     Ulcer. An ulcer is a defect or break in continuity of the epithelial component of punched-out area exists. The floor of the ulcer is composed of connective tissue and fibrin with polymorphonuclear leukocyte infiltration. The loss of epithelium results in an acute inflammatory lesion. An ulcer may be small (2 mm) or quite large (several centimeters). The edges of an ulcer may be ragged or sharp, may be undermined, as in tuberculosis, or raised and indurated, as in malignant tumors. Usually, ulcers are round, but elongated ulcerated areas may occur, particularly as a response to mechanical or chemical trauma. The floor of the ulcer may appear yellow and purulent, reddened, or gray-white. Ulcers may result from the rupture of vesicles or bullae. Varying amounts of erythema may appear at the periphery of the ulcer. Ulcers tend to be painful.
     Erosion: An erosion is a shallow defect in mucosa representing loss of epithelium down to but not including the stratum germinativum. Erosions of the mucosa are usually produced by mechanical trauma; they may have a linear outline with indistinct margins. They appear red because, with the loss of a major portion of epithelium, the vascularity of the underlying connective tissue becomes more visible. Erosions may be painful.
     Nodule. A nodule is a solid elevated lesion of the mucosa, varying in size from 5 mm to 2 cm. A nodule is formed by a mass of connective tissue forcing the overlying epithelium outward to produce a surface lesion. Fibroma and nevus are typical examples of nodules.
     Tumor. A tumor is a solid growth of varying size that arises from mucosa and projects outward. These lesions include true neoplasms with specific histopathologic features and tumorlike lesions such as granuloma pyogenicum and angiogranuloma. True tumors may be benign or malignant. Gross features are entirely insufficient for the differentiation of benign and malignant lesions, and a biopsy specimen for histologic study is mandatory. Certain clinical feature