Nose

The nose is the part of the respiratory tract superior to the hard palate and contains the peripheral organ of smell. It includes the external nose and nasal cavity, which is divided into right and left cavities by the nasal septum. The functions of the nose include olfaction (smelling), respiration (breathing), filtration of dust, humidification of inspired air, and reception and elimination of secretions from the paranasal sinuses and nasolacrimal ducts.

External Nose

The external nose is the visible portion that projects from the face; its skeleton is mainly cartilaginous. Noses vary considerably in size and shape, mainly because of differences in these cartilages. The dorsum of the nose extends from the root of the nose to the apex (tip) of the nose. The inferior surface of the nose is pierced by two piriform (Latin pear-shaped) openings, the nares (nostrils, anterior nasal apertures), which are bound laterally by the alae (wings) of the nose. The superior bony part of the nose, including its root, is covered by thin skin.

The skin over the cartilages of the nose is covered with thicker skin, which contains many sebaceous glands. The skin extends into the vestibule of the nose, where it has a variable number of stiff hairs (vibrissae). Because they are usually moist, these hairs filter dust particles from air entering the nasal cavity. The junction of the skin and mucous membrane is beyond the hair-bearing area.

Skeleton of External Nose

The supporting skeleton of the nose is composed of bone and hyaline cartilage. The bony part of the nose consists of the nasal bones, frontal processes of the maxillae, the nasal part of the frontal bone and its nasal spine, and the bony parts of the nasal septum. The cartilaginous part of the nose consists of five main cartilages: two lateral cartilages, two alar cartilages, and one septal cartilage. The U-shaped alar cartilages are free and movable; they dilate or constrict the nares when the muscles acting on the nose contract.

Nasal Septum

The nasal septum divides the chamber of the nose into two nasal cavities. The septum has a bony part and a soft mobile cartilaginous part. The main components of the nasal septum are the perpendicular plate of the ethmoid, the vomer, and the septal cartilage. The thin perpendicular plate of the ethmoid bone, forming the superior part of the nasal septum, descends from the cribriform plate and is continued superior to this plate as the crista galli. The vomer, a thin flat bone, forms the postero-inferior part of the nasal septum, with some contribution from the nasal crests of the maxillary and palatine bones. The septal cartilage has a tongue-and-groove articulation with the edges of the bony septum.

Nasal Cavities

The term nasal cavity refers to either the entire cavity or to the right or left half, depending on the context. The nasal cavity is entered anteriorly through the nares (nostrils). It opens posteriorly into the nasopharynx through the choanae. Mucosa lines the nasal cavity, except for the nasal vestibule, which is lined with skin.

The nasal mucosa is firmly bound to the periosteum and perichondrium of the supporting bones and cartilages of the nose. The mucosa is continuous with the lining of all the chambers with which the nasal cavities communicate: the nasopharynx posteriorly, the paranasal sinuses superiorly and laterally, and the lacrimal sac and conjunctiva superiorly. The inferior two thirds of the nasal mucosa is the respiratory area, and the superior one third is the olfactory area. Air passing over the respiratory area is warmed and moistened before it passes through the rest of the upper respiratory tract to the lungs. The olfactory area contains the peripheral organ of smell; sniffing draws air to the area.

Boundaries of Nasal Cavities

The nasal cavities have a roof, floor, and medial and lateral walls.

• The roof of the nasal cavities is curved and narrow, except at its posterior end, where the hollow body of the sphenoid forms the roof. It is divided into three parts (frontonasal, ethmoidal, and sphenoidal) named from the bones forming each part.

• The floor of the nasal cavities is wider than the roof and is formed by the palatine processes of the maxilla and the horizontal plates of the palatine bone.

• The medial wall of the nasal cavities is formed by the nasal septum.

• The lateral walls of the nasal cavities are irregular owing to three bony plates, the nasal conchae, which project inferiorly, somewhat like louvers.

Features of Nasal Cavities

The nasal conchae (superior, middle, and inferior) curve inferomedially, hanging like louvers or short curtains from the lateral wall. The conchae (Latin shells) or turbinates of many mammals (especially running mammals and those existing in extreme environments) are highly convoluted, scroll-like structures that offer a vast surface area for heat exchange. In both humans with simple plate-like nasal conchae and animals with complex turbinates, a recess or nasal meatus (singular and plural; passage(s) in the nasal cavity) underlies each of the bony formations. The nasal cavity is thus divided into five passages: a posterosuperiorly placed spheno-ethmoidal recess, three laterally located nasal meatus (superior, middle, and inferior), and a medially placed common nasal meatus into which the four lateral passages open. The inferior concha is the longest and broadest of the conchae and is formed by an independent bone (of the same name, inferior concha) covered by a mucous membrane that contains large vascular spaces that can enlarge to control the caliber of the nasal cavity. The middle and superior conchae are medial processes of the ethmoid bone. When infected or irritated, the mucosa covering the conchae may swell rapidly, blocking the nasal passage(s) on that side.

The spheno-ethmoidal recess, lying superoposterior to the superior concha, receives the opening of the sphenoidal sinus, an air-filled cavity in the body of the sphenoid. The superior nasal meatus is a narrow passage between the superior and the middle nasal conchae into which the posterior ethmoidal sinuses open by one or more orifices. The middle nasal meatus is longer and deeper than the superior one. The anterosuperior part of this passage leads into a funnel-shaped opening, the ethmoidal infundibulum, through which it communicates with the frontal sinus. The passage that leads inferiorly from each frontal sinus to the infundibulum is the frontonasal duct. The semilunar hiatus (Latin hiatus semilunaris) is a semicircular groove into which the frontal sinus opens. The ethmoidal bulla (Latin bubble), a rounded elevation located superior to the semilunar hiatus, is visible when the middle concha is removed. The bulla is formed by middle ethmoidal cells that form the ethmoidal sinuses.

The inferior nasal meatus is a horizontal passage inferolateral to the inferior nasal concha. The nasolacrimal duct, which drains tears from the lacrimal sac, opens into the anterior part of this meatus. The common nasal meatus is the medial part of the nasal cavity between the conchae and the nasal septum, into which the lateral recesses and meatus open.

Vasculature and Innervation of Nose

The arterial supply of the medial and lateral walls of the nasal cavity is from five sources:

1. Anterior ethmoidal artery (from the ophthalmic artery).

2. Posterior ethmoidal artery (from the ophthalmic artery).

3. Sphenopalatine artery (from the maxillary artery).

4. Greater palatine artery (from the maxillary artery).

5. Septal branch of the superior labial artery (from the facial artery).

The first three arteries divide into lateral and medial (septal) branches. The greater palatine artery reaches the septum via the incisive canal through the anterior hard palate. The anterior part of the nasal septum is the site of an anastomotic arterial plexus involving all five arteries supplying the septum (Kiesselbach area). The external nose also receives blood from first and fifth arteries listed, plus nasal branches of the infraorbital artery and the lateral nasal branches of the facial artery.

A rich submucosal venous plexus, deep to the nasal mucosa, provides venous drainage of the nose via the sphenopalatine, facial, and ophthalmic veins. The plexus is an important part of the body's thermoregulatory system, exchanging heat and warming air before it enters the lungs. Venous blood from the external nose drains mostly into the facial vein via the angular and lateral nasal veins. However, recall that it lies within the "danger area" of the face because of communications with the cavernous (dural venous) sinus.

Regarding its nerve supply of the nose, the nasal mucosa can be divided into postero-inferior and anterosuperior portions by an oblique line passing approximately through the anterior nasal spine and the spheno-ethmoidal recess. The nerve supply of the postero-inferior portion of the nasal mucosa is chiefly from the maxillary nerve, by way of the nasopalatine nerve to the nasal septum, and posterior superior lateral nasal and inferior lateral nasal branches of the greater palatine nerve to the lateral wall. The nerve supply of the anterosuperior portion is from the ophthalmic nerve (CN V₁) by way of the anterior and posterior ethmoidal nerves, branches of the nasociliary nerve. Most of the external nose (dorsum and apex) is also supplied by CN V₁ (via the infratrochlear nerve and the external nasal branch of the anterior ethmoidal nerve), but the alae of the nose are supplied by the nasal branches of the infra-orbital nerve (CN V₂). The olfactory nerves, concerned with smell, arise from cells in the olfactory epithelium in the superior part of the lateral and septal walls of the nasal cavity. The central processes of these cells (forming the olfactory nerve) pass through the cribriform plate and end in the olfactory bulb, the rostral expansion of the olfactory tract.