» The Nasal Cavity and Paranasal Sinuses
The deep nasal cavities extend well behind the level of the orbits (Fig. 32.4). Despite the widening of the face, they remain narrow because they are separated from the lateral surface of the head by the thick muscles of facial expression and by fat, not by paranasal sinuses as in cattle and horses.
Two conchae divide each cavity into the usual system of meatuses. The dorsal meatus leads to the fundus, which lies dorsal to the nasopharynx and is largely occupied by the ethmoidal conchae, which are covered by olfactory mucosa. This is extensive in a species endowed with a sense of smell sufficiently acute to be exploited in the search for buried truffles.The dorsal concha is a thick plate projecting from the dorsolateral wall of the cavity. The ventral concha, though shorter, is more complicated and consists of upper and lower scrolls arising in common from a lateral plate. Familiarity with the conformation of these conchae is necessary if the deformity that develops in atrophic rhinitis, a common debilitating disease of young pigs, is to be recognized (Fig. 32.5).
The paranasal sinus system is complicated and comprises frontal, maxillary, lacrimal, sphenoidal, and conchal units, but not all of these merit attention (Fig. 32.4). The maxillary sinus, level with the orbit, extends into the base of the deep zygomatic arch. The frontal sinuses of the mature pig excavate the entire dorsal surface of the skull caudal to the nasal bones. They spread the outer and inner plates of the cranial roof so widely apart that all correspondence between the external form and the cranial cavity is lost (Fig. 32.4/7). The brain thus lies at a depth of about 5 cm below the skin, protected by two plates of bone. The consequence is that pigs cannot be reliably stunned by mechanical means (hammer or captive bolt), and humane slaughter requires the use of electrocution or carbon dioxide gas, which are the methods commonly employed today. When shooting is employed, the target site must be carefully chosen; for most pigs it is the intersection of the diagonal lines connecting the eyes with the bases of the opposite ears (Fig. 32.6). In particularly large pigs, it is more satisfactory to shoot through the occipital bone from behind.