THE NASAL CAVITY

The nasal cavity extends from the nostrils to the level of the eyes. Its rostral part, the nasal vestibule, is roughly tubular; caudal to the level of the infraorbital foramen, it widens and gains in height (Figure 11-7).

The nasal cavity is divided into two halves by the nasal septum. In dogs, only the caudal and dorsal parts of the septum ossify; the rostral extremity projecting beyond the skull remains cartilaginous and accounts for the passive mobility of the tip of the nose. The middle section of the septum is membranous. A cat’s nose is not actively mobile, and its cartilages resemble short­ened canine nasal cartilages.

In dogs, the cavity is more tightly filled with nasal and ethmoidal conchae than in other species, and the intervening meatuses are narrow. The rostral half lodges the dorsal and ventral conchae. The dorsal one (Figure 11-7/3) is a simple plate where it arises from the nasal bone; it widens caudally where it attaches to the ethmoid. The ventral concha, which is thick but short, arises from the maxilla and breaks up to form many scrolls that greatly enlarge the area that is covered by a richly vas­cularized mucosa (Figure 11-7/2). It extends from the level of the first to the third premolar teeth and is attached to the conchal crest on the medial surface of the maxilla. This crest creates a linear shadow that is a very distinctive radiographic feature (Figure 11-7, B∕14'). The ventral concha is continued rostrally by the alar fold. The caudal half of the nasal cavity is almost filled by ethmoidal conchae covered with olfactory mucosa. These conchae also invade the lower part of the frontal sinus. The olfactory mucosa in the German Shepherd reportedly covers an area of 150 cm² and pos­sesses more than 20 million receptors. The appearance of the olfactory membrane differs little from the remain­der of the mucous membrane, although it may be slightly thicker and grayer.

The nasal cavity of cats resembles the one of brachy­cephalic dogs. However, the ventral nasal concha is smaller, compensated for by enlargement and develop­ment of the middle concha and its lamellae. The middle concha reaches to the level of the entrance of the maxil­lary recess that it covers.

In both species, the nasolacrimal duct (Figure 11-8) opens where the floor of the vestibule meets the alar fold and is visible when the nostril is spread. As often as not, there is a second, more caudal opening level with the canine tooth. The duct is described more extensively later. The duct of the lateral nasal gland opens at the rostral end of the dorsal nasal concha, but as it is only about 0.5 mm in diameter, it can be difficult to identify, even at dissection. The gland lies in the lateral nasal wall close to the entrance of the maxillary recess. Its secretion may have a social significance that accounts for the nose-to-nose sniffing common when dogs meet. In cats, the lateral nasal gland and its duct are not visible macroscopically; the secretion is mucous instead of serous.

A few much smaller nasal glands found on the rostral part of the septum open at the caudal limit of the ves­tibule and contribute marginally to the wetness of the nose.

Figure 11-7 Paramedian section of the canine head (A) and tomogram of the feline nasal cavity (B). 1, Right nostril; 2, ventral nasal concha; 3, dorsal nasal concha; 4, ethmoidal conchae; 4', cribriform plate; 5, frontal sinus; 5', frontal bone; 6, hard palate; 7, tongue; 8, oropharynx; 9, soft palate; 9', nasopharynx; 10, epiglottis; 11, basihyoid; 12, trachea; 13, esophagus; 14, nasal bone; 14', horizontal crest of nasal bone; 14", dorsal part of nasal cavity invaded by ethmoidal conchae; 15, optic canal; 15', hypophysial fossa.

The watery secretions of the lacrimal, lateral nasal, and scattered minor nasal glands moisten the nasal plate. As is well-known, a moist nose is generally regarded as a sign of health in dogs.

The nasal cavity has an extremely good blood supply from both the external and the internal carotid arteries; anastomoses occur between the internal carotid artery and the maxillary arteries (the main branch of the exter­nal carotid artery) of both sides. The maxillary artery is the major supply to the nasal cavity. Ligation of the external carotid artery in dogs (in cases of persistent nose bleeding) gives rise to collateral connections between corresponding vessels of both sides.