The nasal cavity constitutes the first of the respiratory passages and is an irregularly shaped space which extends from the roof of the mouth upwards to the base of the skull. It is subdivided into right and left halves by a septum (fig. 331), which is approximately median in position. In the dried skull the septum is deficient anteriorly, and as a result a single anterior nasal aperture is present on the norma frontalis. The septum, however, reaches to the posterior limit of the cavity, which communicates with the nasal part of the pharynx through two posterior nasal apertures, placed immediately above the posterior border of the bony palate. The cavity is wider below than above and is widest and deepest in its central part. It communicates with the air-spaces of the frontal, ethmoidal, maxillary and sphenoidal sinuses.
The roof (figs. 332, 333) is horizontal in its central part but slopes downwards in front and behind. The sloping anterior part is formed by the frontal and nasal bones and contributes to the formation of the nose. The horizontal central part is formed by the cribriform plate of the ethmoid bone and separates the nasal cavity from the median part of the floor of the anterior cranial fossa. It presents numerous small openings for the passage of the olfactory nerves. The sloping posterior part is formed by the body of the sphenoid and is interrupted by the rounded orifice of the sphenoidal sinus.
The floor is smooth, gently concave from side to side, and slopes upwards a little as it passes backwards from the anterior to the posterior aperture. It is formed by the upper surface of the bony palate and therefore intervenes between the nasal and oral cavities. Portions of four bones contribute to its formation. Anteriorly the palatine processes of the two maxilla meet in the median plane, and behind them the horizontal plates of the palatine bones articulate with each other in the median plane and with the palatine processes of the maxilla in front. In its anterior part the floor is pierced close to the septum by a small funnel-shaped opening, which leads into the incisive canal.
The medial wall is formed by the bony septum (fig. 331) which extends between the roof and the floor. It is a thin sheet of bone and presents a wide deficiency in front, occupied in the recent state by the septal cartilage. It is formed almost entirely by the vomer and the perpendicular plate of the ethmoid. The vomer extends from the under surface of the body of the sphenoid to the bony palate and forms the lower and posterior part of the septum, including its free, posterior border. It is marked by small furrows for vessels and nerves. The perpendicular plate of the ethmoid forms the upper and anterior part of the septum (fig. 331) and is continuous above with the cribriform plate. The septum is often deflected to one or other side, and the deviation occurs most commonly on the line of the vomoro-ethmoidal suture.
The inferior concha is a curved lamina of thin bone and is an independent entity. It articulates with the nasal surface of the maxilla and the perpendicular plate of the palatine bone, and possesses a free lower border, which is gently curved. The inferior meatus lies under cover of the inferior concha and extends downwards to the floor of the nasal cavity. It is the largest of the three meatuses and covers almost the entire length of the lateral wall of the nose. At the junction of its anterior and middle thirds the inferior meatus is deepest, and in this situation it presents the lower orifice of the nasolacrimal duct.
The middle and superior conchae are projections from the medial surface of the ethmoidal labyrinth. The middle concha is much the larger and extends backwards to articulate with the perpendicular plate of the palatine bone. The middle meatus, which is directed backwards and downwards, is placed between the middle and inferior conchae. Its lateral wall displays several important features, which can be examined only after the removal of the middle concha (fig. 333). Its upper part is occupied by a rounded elevation, termed the ethmoidal bulla, which contains the middle ethmoidal sinuses. Below and in front of the bulla a curved fissure passes downwards and backwards across the lateral wall of the meatus. It is named the hiatus semilunaris, and its upper or anterior end is continuous with a curved canal, termed the infundibulum. In its posterior part the floor of the hiatus semilunaris receives the opening of the maxillary sinus, which is directed upwards and backwards by the edge of the fissure. The anterior ethmoidal sinuses open into the infundibulum, which, in fifty per cent of skulls, opens above into the frontal sinus. It may, however, be closed above by the fusion of the bulla ethmoidalis with the edge of the hiatus semilunaris and, in that event, the frontal sinus opens independently into the anterior part of the middle meatus. The middle ethmoidal sinuses open above, or near, the bulla. The lateral wail of the meatus below the hiatus semilunaris may be partially deficient and the gap or gaps lead into the maxillary sinus. In the recent state they are closed by the nasal mucosa.
The posterior nasal apertures (choanae) are separated from each other by the posterior border of the vomer. They are bounded below by the posterior border of the horizontal plate of the palatine bone, above by the base of the skull ; and laterally, on each side, by the medial pterygoid plate.
Particular features -The sloping anterior part of the roof (fig. 332) is formed by the nasal bone and the nasal spine of the frontal bone. In addition to the numerous small foramina for the transmission of the olfactory nerves the horizontal part of the roof is pierced by a separate foramen, situated anteriorly, which gives passage to the anterior ethmoidal nerve and vessels. The sloping posterior part of the roof is formed above by the anterior aspect of the body of the sphenoid, with which the sphenoidal concha is fused and below by the ala of the vomer and the sphenoidal process of the palatine bone.
At the upper and lower borders of the medial wall (fig. 331) other bones, in addition to the vomer and the perpendicular plate of the ethmoid, make minor contributions to the septum. Above and in front the nasal bones and the frontal spine, above and behind the rostrum of the sphenoid, and below the nasal crests of the maxillae and palatine bones all take small parts in its constitution. The vomer is grooved by the long sphenopalatine (nasopalatine) nerve, as it runs downwards and forwards to reach the incisive canal.
The lateral wall (figs. 332, 333) is formed anteriorly and above by the nasal bone and the frontal process of the maxilla. Behind the last-named, and articulating with its posterior border, the lacrimal bone lies on the lateral wall of the middle meatus and articulates below with the lacrimal process of the inferior concha. These two bones form the medial wall of the nasolacrimal canal (fig. 332), which conducts the nasolaorimal duct to the inferior meatus. Posteriorly the lacrimal bone articulates with the ethmoidal labyrinth and helps to close some of the ethmoidal sinuses. The uncinate process of the ethmoid springs from this part of the labyrinth and curves downwards and backwards in the lateral wall of the middle meatus. It is a very thin and fragile process, about 3mm wide, which curves across the maxillary hiatus and articulates near its extremity with the ethmoidal process of the inferior concha. The concave posterior border of the process is free and forms the medial edge of the hiatus semilunaris ; the convex anterior border is free in its upper part only. Owing to its position relative to the maxillary hiatus the uncinate process helps to form the medial wall of the maxillary sinus. The maxillary hiatus, which forms such a conspicuous opening on the nasal surface of the maxilla (fig. 385) is reduced in size very considerably by the neighboring bones. Its lower part is covered by the inferior concha and its maxillary process ; above the inferior concha the uncinate process of the ethmoid, as already stated, encroaches on the gap. Posteriorly the anterior hart of the perpendicular plate of the palatine bone closes it in still furtber, and above and in front small portions of the ethmoidal labyrinth and the lacrimal bone overlap its margins (fig. 386). As a result, the maxillary hiatus is reduced sometimes to . single orifice in the floor of the posterior part of the hiatus semilunaris, although as a rule additional openings exist behind the uncinate process; and between its lower border and the upper border of the inferior concha. The bulla ethmoidalis is very variable in its size and shape and, as already mentioned may be fused with the upper part of the uncinate process. In that event the duct of the frontal sinus opens into the upper part of the middle meatus medial to the blind end of the infundibulum. A third concha is usually present on the medial aspect of the ethmoidal labyrinth above and behind the posterior end of the superior concha ; it is little more than a slight ridge, separated from the superior concha by a shallow depression. The sphenopalatine foramen (fig. 332) lies at the posterior limit of the, superior meatus. It transmits the sphenopalatine artery and the long and short sphenopalatine nerves (nasopalatine and posterior superior nasal nerves) from the pterygopalatine fossa. The foramen is bounded above by the body of the sphenoid and the sphenoidal concha ; below by the notched upper border of the perpendicular plate of the palatine boric; and in front and behind by its orbital and sphenoidal processes respectively.
Previous | Next