The Nasal Cavity

Respiratory System of the Horse

The nasal cavity (Cavum nasi), the first segment of the respiratory tract, is a long, somewhat cylindrical passage, inclosed by all the facial bones except the mandible and hyoid. It is separated from the mouth ventrally by the palate. It opens externally at the nostrils, and communicates posteriorly with the pharynx through the posterior nares.

The nostrils or anterior nares (Nares) are somewhat oval in outline, and are placed obliquely, so that they are closer together below than above. They are bounded by two alæ or wings (Alæ nasi), which meet above and below, forming the commissures. The outer ala is concave; the inner one is convex above, concave below. The upper commissure is narrow, the lower one wide and rounded. If the finger is passed into the nostril at the upper commissure, it enters the so-called false nostril, which leads backward to a blind cutaneous pouch. This cul-de-sac, the diverticulum nasi, extends backward to the angle of junction of the nasal bone and the nasal process of the premaxilla. The posterior part of the diverticulum is blind, but the false nostril communicates below and internally with the vestibule of the nasal cavity. The external opening of the naso-lacrimal duct is seen when the nostril is dilated; it is situated on the floor of the vestibule, perforating the skin close to its junction with the mucous membrane, about two inches (ca. 5 cm.) from the lower commissure. (It is not rare to find one or two accessory orifices further back.)

Structure.—The skin around the nostrils presents long tactile hairs as well as the ordinary ones. It is continued around the alæ and lines the vestibule. In the posterior part of the diverticulum the skin is thin and usually black, and is covered with exceedingly fine hairs; it is provided with numerous sebaceous glands. The inner wing is supported by the alar cartilage (Cartilago alaris), which is shaped somewhat like a comma, the convex margin being direct medially. The cartilages are attached by fibrous tissue to the extremity of the septal cartilage. Each consists of an upper, quadrilateral curved plate, the lamina, and a narrow cornu which curves downward and outward, supporting the inner wing and the lower commissure, but not entering into the formation of the outer wing. The lamina causes the projection of the upper part of the internal wing, from which the thick alar fold (Plica alaris) passes backward along the external wall of the nasal cavity to cover the cartilaginous prolongation of the inferior turbinal bone. This fold separates the true from the false nostril. The extremity of the cornu usually causes a slight projection of the skin a short distance behind and below the lower commissure.

The muscles of the nostrils have been described.

Blood-supply.—Palato-labial, superior labial, and lateral nasal arteries.

Nerve-supply.—Trigeminal and facial nerves.

The nasal cavity is divided into two symmetrical halves by the median septum nasi. The osseous part of the septum (Septum osseum) is formed behind by the perpendicular plate of the ethmoid and below by the vomer. A few ridges on the former correspond to the ethmoidal meatuses. The major part of it, however, is formed by the septal cartilage (Cartilago septi nasi). The surfaces of the cartilage are marked by faint grooves for the vessels and nerves which course over it. The dorsal border is attached along the frontal and nasal sutures, and extends beyond the apices of the nasal bones about two inches (ca. 5 cm.). From this border a thin, narrow plate, the parietal cartilage (Cartilago parietalis) curves outward for a short distance on either side. Near the nostrils these plates are somewhat wider, partially making good the defect in the bony roof of the cavity in this situation. The ventral border is thick and rounded; it lies chiefly in the groove of the vomer, but anteriorly it occupies the space between the premaxillary bones. The posterior border is continuous with the perpendicular plate of the ethmoid bone. The alar cartilages are attached to the anterior extremity by fibrous tissue in such a manner as to allow very free movement—in fact, an actual joint may be found. The ventral part of the cartilage is about half an inch (ca. 1 cm.), the middle part about one-tenth of an inch (ca. 2.5 mm.), and the dorsal part about a quarter to a third of an inch (ca. 6 to 8 mm.) in thickness.

The two turbinal bones (Conchæ nasales) project from the lateral wall, and divide the outer part of the cavity into three meatuses—superior, middle, and inferior.

The superior meatus (Meatus nasi dorsalis) is a narrow passage, bounded dorsally by the roof of the cavity, and ventrally by the superior turbinal bone; its posterior end is closed by the junction of the inner plate of the frontal bone with the cribriform plate and lateral mass of the ethmoid. It transmits air to the upper part of the olfactory region.

The middle meatus (Meatus nasi medius) is between the two turbinal bones. It is somewhat larger than the superior meatus, and does not extend back so far; it ends near the great ethmoturbinal and the ethmoidal meatuses. In its posterior part is found the extremely narrow, slit-like interval by which the maxillary sinus communicates with the nasal cavity—the naso-maxillary fissure (Aditus naso-maxillaris). The fissure is not visible from the nasal side, being concealed by the overhanging superior turbinal. A fine flexible probe, passed outward and somewhat backward between the turbinal bones, enters the maxillary sinus; if introduced a little further in the same direction, it usually passes through the orifice of communication between the maxillary and the frontal sinus and enters the latter. A small part of the fissure usually brings the anterior division of the maxillary sinus into communication with the nasal cavity. The spaces inclosed by the coiled portions of the turbinals also open into the middle meatus. This passage may be characterized as the sinus-meatus, but it also conducts air to the olfactory region.

The inferior meatus (Meatus nasi ventralis) is situated between the inferior turbinal and the floor of the cavity. It is much larger than the other two, and is the direct passage between the nostrils and the pharynx.

The common meatus (Meatus nasi communis) is situated between the septum and the turbinals, and is continuous externally with the other meatuses. It is very narrow dorsally, but widens ventrally.

The lateral masses of the ethmoid bone project forward into the posterior part (fundus) of the nasal cavity (Fig. ). Between the ethmoturbinals of which each mass is composed there are three principal and numerous small passages, the ethmoidal meatuses (Meatus ethmoidales).

The posterior nares (Choanæ) are two elliptical orifices by which the nasal cavity and pharynx communicate. They are in the same plane as the floor of the nasal cavity, and are separated from each other by the vomer. They are, taken together, about two inches (ca. 5 cm.) wide and three to four inches (ca. 8 to 10 cm.) long.

The nasal mucous membrane (Membrana mucosa nasi) is thick, highly vascular, and is, in general, firmly attached to the underlying periosteum and perichondrium. It is continuous in front with the skin which lines the nostrils, and behind with the mucous membrane of the pharynx. It is also continuous at the naso-maxillary fissure with the very thin and only slightly vascular mucous membrane which lines the sinuses. In the anterior part of the cavity it forms prominent thick folds on the lateral wall, which extend from the turbinal bones to the nostril. There are usually two superior turbinal folds which unite anteriorly. The upper one incloses a thin plate of cartilage which is continuous with the superior turbinal bone. The inferior turbinal fold is curved, and incloses an ᔕ-shaped cartilaginous plate which prolongs the inferior turbinal bone; this fold is continuous with the alar fold of the nostril, and forms with it the upper margin of the entrance from the true nostril to the nasal cavity. Below this there is a rounded ridge produced by the nasal process of the premaxilla. The mucous membrane of the greater part of the cavity (Regio respiratoria) is red in color, and is covered with a stratified ciliated epithelium. It contains numerous acinous nasal glands (Glandulæ nasales). The submucosa contains rich venous plexuses which form in certain situations a sort of cavernous tissue (Plexus cavernosus), composed of several strata of freely anastomosing veins, between which there are unstriped muscle-fibers. This arrangement is most marked in the turbinal folds, on the lower part of the inferior turbinal bone, and the lower part of the septum. On the posterior part of the lateral masses of the ethmoid and the adjacent part of the superior turbinal and the septum (Regio olfactoria) the mucous membrane is brownish-yellow in color and thicker; it contains the olfactory nerve-endings in a special non-ciliated epithelium. In it are numerous olfactory glands (Glandulæ olfactoriæ); these are long, tubular, and often branched.

The vomero-nasal organ of Jacobson (Organon vomeronasale) is situated on the floor of the nasal cavity, on either side of the ventral border of the septum (Fig. .) It consists of a tubular cartilage lined with mucous membrane, to which fibers of the olfactory nerve may be traced. Its anterior part communicates with the nasal cavity by a slit-like orifice.

The paranasal sinuses are described in the Osteology.