Maxillary sinuses are the largest sinonasal cavities, bordered by four bony walls. On the anterior wall the distal opening of the infraorbital canal is found. This canal, through which the terminal branch of maxillary nerve courses (infraorbital nerve), is enclosed within the maxillary sinus roof. The infratemporal wall, which is the thinnest and most fragile, separates the maxillary sinus from the pterygopalatine fossa and masticator space, posteriorly.
The medial wall is more complex, it has a natural ostium (Figs. 2.2, 2.12) towards which mucus is actively transported by cilia, and some accessory ostia, normally bypassed by secretions, located either anteriorly and posteriorly to the maxillary ostium (anterior and posterior fontanellae) (Fig. 2.5). Because the horizontal portion (the fetal descending part) of the uncinate process inserts on the medial wall, the maxillary ostium does not directly communicate with the nasal fossa. Conversely, its ostium opens into the floor of a narrow space - i.e. the inferior aspect of the ethmoidal infundibulum (Fig. 2.12). To arrive into the middle meatus, secretions are transported upward along the ethmoidal infundibulum to reach the hiatus semilunaris, a horizontal 2D plane located between the anterior surface of the bulla ethmoidalis and the free margin of the unci-
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