The superior laryngeal nerves originate from the inferior vagal ganglia high in the neck (Fig. 8.228). On each side, the nerve descends medial to the internal carotid artery and divides into internal and external branches just above the level of the superior horn of the hyoid bone:
The external branch (external laryngeal nerve) descends along the lateral wall of the pharynx to supply and penetrate the inferior constrictor of the pharynx and ends by supplying the cricothyroid muscle.
The internal branch (internal laryngeal nerve) passes anteroinferiorly to penetrate the thyrohyoid membrane—it is mainly sensory and supplies the laryngeal cavity down to the level of the vocal folds.
The recurrent laryngeal nerves are (Fig. 8.228): sensory to the laryngeal cavity below the level of the vocal folds, and motor to all intrinsic muscles of the larynx except for the cricothyroid.
The left recurrent laryngeal nerve originates in the thorax, whereas the right recurrent laryngeal nerve originates in the root of the neck. Both nerves generally ascend in the neck in the groove between the esophagus and trachea and enter the larynx deep to the margin of the inferior constrictor. They may pass medial to, lateral to, or through the lateral ligament of the thyroid gland, which attaches the thyroid gland to the trachea and lower part of the cricoid cartilage on each side.
The two nasal cavities are the uppermost parts of the respiratory tract and contain the olfactory receptors. They are elongated wedge-shaped spaces with a large inferior base and a narrow superior apex (Figs. 8.229 and 8.230) and are held open by a skeletal framework consisting mainly of bone and cartilage.
The smaller anterior regions of the cavities are enclosed by the external nose, whereas the larger posterior regions are more central within the skull. The anterior apertures of the nasal cavities are the nares, which open onto the inferior surface of the nose. The posterior apertures are the choanae, which open into the nasopharynx.
The nasal cavities are separated: from each other by a midline nasal septum, from the oral cavity below by the hard palate, and from the cranial cavity above by parts of the frontal, ethmoid, and sphenoid bones.
Lateral to the nasal cavities are the orbits.
Each nasal cavity has a floor, roof, medial wall, and lateral wall (Fig. 8.230A).
The lateral wall is characterized by three curved shelves of bone (conchae), which are one above the other and project medially and inferiorly across the nasal cavity (Fig. 8.230B). The medial, anterior, and posterior margins of the conchae are free.
The conchae divide each nasal cavity into four air channels (Fig. 8.230C,D): an inferior nasal meatus between the inferior concha and the nasal floor, a middle nasal meatus between the inferior and middle concha, a superior nasal meatus between the middle and superior concha, and a spheno-ethmoidal recess between the superior concha and the nasal roof.
These conchae increase the surface area of contact between tissues of the lateral wall and the respired air.
The openings of the paranasal sinuses, which are extensions of the nasal cavity that erode into the surrounding bones during childhood and early adulthood, are on the lateral wall and roof of the nasal cavities (Fig. 8.231). In addition, the lateral wall also contains the opening of the nasolacrimal duct, which drains tears from the eye into the nasal cavity.
Each nasal cavity consists of three general regions—the nasal vestibule, the respiratory region, and the olfactory region (Fig. 8.232):
The nasal vestibule is a small dilated space just internal to the naris that is lined by skin and contains hair follicles.