The nasal cavity is bounded anteriorly by the external nares and posteriorly by the nasal choana. It is composed of the left and right nares which are separated by the nasal septum. The septum is both osseous and cartilaginous and is composed of the vomer (inferior and posterior), the ethmoid bone (superior), and septal cartilage (anterior). Laterally the “scroll-shaped” superior, middle, and inferior turbinates or conchae are present. Openings to the paranasal sinuses are present laterally and include the openings to the sphenoid sinuses (located behind the superior turbinate), the frontal sinus ostia (located in the anterior region of middle turbinate), the primary (and accessory) maxillary ostia (located in the hiatus semilunaris), and openings to the anterior, middle, and posterior ethmoid air cells (located in the middle turbinate and behind the superior turbinate). The superior portion of the nasal cavity is bounded by the cribriform plate.

The vast majority of the nasal cavity is lined by ectodermally derived tissue.¹ The most anterior mucosa, that of the vestibule, is lined by a keratinized, stratified squamous epithelium. This epithelium is continuous with that of the external nose and the mucosa includes adnexal structures such as hair follicles and sebaceous and sweat glands (Fig. 1.1, eFig. 1.1). As one progresses posteriorly, this epithelium is replaced by nonkeratinizing squamous cells and ciliated columnar cells with occasional mucous or
goblet cells and intermediate cells (schneiderian mucosa) (Figs. 1.2, 1.3, 1.4, eFigs. 1.2-1.4). Seromucinous glands are present beneath the epithelium within a loose stroma. Occasional melanocytes may be present within the surface epithelium or underlying glands, and may even be seen in the lamina propria overlying the septum and turbinates. At the lower, anterior septum, a small tubular sac lined by nonciliated columnar cells (the vestigial
vomeronasal organ of Jacobson) may be seen.¹ This organ is better developed in the more olfactory-inclined animals. Dense, muscular vascular tissue is present beneath the seromucinous glands and is sometimes noted to appear “erectile” (Fig. 1.5, eFig. 1.5). Lymphatic channels, small nerves, and occasional, scattered inflammatory cells can also be present.

The cribriform plate, the medial superior turbinate, and the superior one-third of the nasal septum all may be lined by olfactory mucosa (Fig. 1.6, eFig. 1.6). This specialized mucosa is composed of tall, eosinophilic sustentacular or supporting cells, elongated, ciliated olfactory neurons,
and small basally located cells. The sustentacular cells can sometimes contain lipofuscin which may give the area a yellow appearance, clinically.² Beneath the mucosa are serous glands (the glands of Bowman). Focally, olfactory mucosa may be intermixed and replaced by schneiderian mucosa.

The paranasal sinuses include the frontal, maxillary, sphenoid, and ethmoid sinuses. The ethmoid sinuses are composed of numerous (usually 3-18) ethmoid air sacs. The sinuses are each in communication with the nasal cavity and may be considered “diverticula” of the nasal cavity. They grow until adulthood and are usually somewhat asymmetrical in shape and size.

The paranasal sinuses are lined by ciliated columnar cells with occasional mucinous cells (Fig. 1.7, eFig. 1.7). These cells tend to be less tall than those of the nasal cavity and the mucosa is only half as thick as the nasal mucosa. Between the epithelium and bone is only a thin layer of fibrous tissue. Seromucinous glands are rare and are present primarily near the ostia to the nasal cavity.

The mouth begins at the vermillion border of the lips. It is encircled by the hard and soft palates (superior), cheeks (lateral), and anterior two-thirds of the tongue and floor of mouth (inferiorly). The posterior
junction with the oropharynx (the fauces) is located superiorly at the posterior edge of the soft palate and uvula, laterally at the tonsillar pillars, and inferiorly at the circumvallate papillae of the tongue (the sulcus terminalis).

Although there are some regional differences, the majority of the oral cavity is lined by a nonkeratinizing squamous epithelium (Fig. 1.8, eFig. 1.8). Varying degrees of keratinization may be present overlying the gingiva and hard palate as they are exposed to mastication. Melanocytes and Merkel cells may be present and, although they are difficult to identify, they are generally scattered throughout the oral mucosa. The tongue is lined by filiform, fungiform, foliate, and circumvallate papillae which may have taste buds along the sides of their invaginations (Fig. 1.9, eFig. 1.9). These are composed of gustatory, sustentacular, and basal cells akin to olfactory mucosa of the nasal cavity. Immediately beneath the epithelium throughout the mouth is a basal lamina which is composed of type IV collagen, heparan sulfate, laminin, and enactin.² Beneath the epithelium of the tongue, a small amount of loose connective tissue overlies skeletal muscle (Fig. 1.10, eFig. 1.10). The remainder of the oral cavity has prominent seromucinous glands which may be especially prominent overlying the hard palate (Fig. 1.11, eFig. 1.11). The proportion of mucous to serous glands varies throughout the mouth. Surrounding these glands is the dense and loose connective tissue lamina propria and

submucosa. Adipose tissue is sometimes seen within the submucosa. In the region of the retromolar trigone, nests of nonkeratinizing squamous cells (the organ of Chievitz) may be found within the stroma (Fig. 1.12, eFig. 1.12).² It is important that these not be confused with invasive malignancy.