Respiratory System

The respiratory system (apparatus respiratorius) uses the nose, nasal cavity, pharynx, larynx, trachea, bronchi, and smaller passageways to bring air to the alveoli or sites of gaseous exchange within the lungs. Various structures associated with these passageways modify or regulate the flow of air, serve as olfactory receptors, facilitate water and heat exchange, and make phonation possible.

The nasal cavity and the conchae (also called turbinates) warm and moisten the air and remove foreign material. The pharynx serves as a passageway for both the respiratory and the digestive systems. The larynx guards the entrance to the trachea, functions in vocalization, and regulates both the inspiration and the expiration of air. The trachea is a cartilage-reinforced tube lined by ciliated epithelium. It divides into principal bronchi and continues into the two lungs as lobar bronchi, segmental bronchi, bronchioles, alveolar ducts, alveolar sacs, and alveoli. The terminal divisions are located in the elastic, well-vascularized lungs, which passively expand and collapse in response to changes in intrathoracic pressure, created by the action of the muscles of the diaphragm and thoracic wall.

The nose (nasus), in a broad sense, refers to the external nose (nasus externus) and its associated nasal cartilages (cartilagines nasi) as well as to the internal nose, or nasal cavity (cavum nasi) with its scrolls of chonchae. The facial portion of the respiratory system and the rostral portion of the digestive system collectively constitute what is called the muzzle. In dolichocephalic breeds (Greyhound) the muzzle is long and may account for half of the total length of the skull. In brachy-cephalic breeds (Bulldog), the shortened muzzle often is the cause of respiratory difculties because of the crowded conchae that restrict the air passageways within the nasal cavity.

When referring to nasal structures or diseases the root rhin-, from the Greek rhinos, for nose, is frequently employed.

External Nose

The external nose is the part of the face rostral to the frontal region and dorsal to the infraorbital, buccal, and oral regions. It consists of a fxed bony case and a cartilaginous framework. The cartilaginous portion is movable by virtue of several skeletal muscles associated with the external nose (). The short hair on the skin of the nose is directed caudally on the middorsal surface and gradually slopes in a caudoventral direction laterally, where it is continued on the lips. The apical portion of the nose (apex nasi) is fattened and devoid of hair. It is called the nasal plane (planum nasale) and includes the nostrils (nares), which are separated from each other by a groove, or philtrum (). The integument of the nasal plane is hairless and nonglandular. It presents epithelial elevations (areae) or papillary ridges that result in patterns characteristic for each individual. For this reason nose prints may be used as a means of identification in the dog, similar to the way fngerprints are used in humans ().

The lateral walls of the bony portion of the nose are formed by the incisive bones and maxillae, whereas the roof is formed by the paired nasal bones. The concave rostral ends of the nasal bones, dorsally, and the incisive bones, laterally and ventrally, bound the largest opening into the skull. This opening, called the bony nasal aperture (apertura nasi ossea), formerly called the piriform aperture, is wider ventrally than dorsally and lies in an oblique plane. In life this opening is bounded rostrally by the nasal cartilages. The aggregate of these cartilages, with their ligaments and covering skin, composes the movable portion of the nose (pars mobilis nasi). The movable part of the nose ends in a truncated apex (apex nasi).

Cartilages of the Nose

The mobile part of the external nose has a framework composed entirely of the nasal cartilages (). These include the unpaired septal cartilage, the paired dorsal lateral and ventral lateral nasal cartilages, and the paired lateral accessory cartilages. Related to the ventral part of the septal cartilage is the vomeronasal cartilage, enclosing the organ.

The septal cartilage of the nose (cartilago septi nasi) is a perpendicular median plate that separates most of the nasal airway into right and left nasal cavities. It is a rostral continuation of the perpendicular plate of the ethmoid bone that does not ossify. In the region of the aperture of the osseous skull, the septum is lacking in cartilage over a distance of approximately 1 cm so that the nasal septum in this region is formed of the membranous nasal septum (pars membranacea septi nasi), which connects the cartilaginous immovable caudal part with the mobile rostral part.

The caudal part of the cartilaginous nasal septum is thicker ventrally, where it lies in the septal groove of the vomer, than it is dorsally, where it blends with the thin, conjoined ventral processes of the nasal bones. It presents a prominent caudal process on the median plane that occupies the space between the osseous perpendicular lamina of the ethmoid dorsally and the groove in the vomer ventrally.

The rostral part of the cartilaginous nasal septum continues a median course rostrally from the membranous portion of the nasal septum (). It lies between the right and the left nasal vestibule. The rostral border of this portion of the septum is divided into right and left laminae. The cleft between the two leaves is deeper and much wider ventrally than it is dorsally. It forms a depressed triangular area ventrally. The dorsal portion of each lamina is rolled laterally to form the dorsolateral nasal cartilage. Arising from the ventral portion of the rostral part of the septal cartilage is the small ventrolateral nasal cartilage, which turns dorsally and medially toward the dorsolateral nasal cartilage (). The accessory cartilage is united by collagenous tissue to the ventrolateral nasal cartilage.

The dorsolateral nasal cartilage (cartilago nasi lateralis dorsalis) is the most expansive of the cartilages in the mobile part of the external nose. On each side it is a continuation of half of the dorsal portion of the septal cartilage. From this dorsal origin it is rolled into a tube by curving laterally, ventrally, and medially. Its widest portion is its rostral half. Caudally, it joins the dorsal part of the bony aperture, to which it is attached by fibrous tissue along the concave border of the nasal bone. The free rolled-in border of the dorsolateral nasal cartilage is greatly thickened rostrally and contains a plexus of blood vessels that form a meshwork in the collagenous tissue directly caudal to the nostril. It becomes much thinner caudally. At a transverse plane through the bony aperture it blends with the rostral extremity of the ventral nasal concha (formerly maxilloturbinate). The free medial border of the dorsolateral cartilage curves ventral to the thicker free lateral border of the ventro-lateral cartilage to a transverse plane through the caudal angle of the midlateral slit of the nostril.

The ventrolateral nasal cartilage (cartilago nasi lateralis ventralis) () is a continuation of the rostral portion of the lateral half of the septal cartilage. Caudally, its origin moves obliquely dorsad on the lateral surface of the ventral part of the septal cartilage. It is slightly shorter and approximately one-fourth as wide as the dorsolateral cartilage. As it rolls dorsally, it is neither of uniform thickness nor of uniform curvature. Rostrally, it runs into the apex of the nose. It ends caudal to the lateral leaf of the septal cartilage adjacent to the articulation of this cartilage with the accessory cartilage. Caudally, it assumes a sigmoid shape in cross-section, being bent in such a way that its free border is added to the free border of the dorsolateral cartilage in forming the cartilaginous basis of the alar fold (plica alaris) that continues the ventral nasal concha to the vestibule.

The accessory cartilage (cartilago accessoria) () is a laterally convex leaf that articulates with the ventrolateral angle of the wide ventrally divided portion of the septal cartilage and extends dorsocaudally to the lateral surface of the expanded portion of the dorsolateral nasal cartilage. For a considerable portion of its length, it lies directly deep to the integument that covers the ventral surface of the midlateral slit in the nostril.

A second small accessory cartilage is occasionally located directly dorsal to the septal cartilage in the groove formed by the origins of the right and left dorsolateral cartilages. Its position is only a few millimeters rostral to the internasal suture.

When sniffing for olfactory purposes the mobile part of the nose is moved and the shape of the nostrils is altered. This is accomplished by the action of intrinsic muscles and of the nasal part of the levator labii superioris and of the levator nasolabialis muscles, which are inserted on these cartilages.

Vomeronasal Organ

The paired vomeronasal organ (organum vomeronasale) (), long known as Jacobson’s organ, is located in the rostral base of the nasal septum as a tubular pocket of olfactory epithelium partially enclosed by a scroll of cartilage (cartilago vomeronasalis).

Each vomeronasal organ opens rostrally into an incisive duct that connects the nasal and oral cavities (). Adams and Wiekamp () investigated the surface and transectional anatomy of the vomeronasal organ in the dog and described its epithelium in detail. They observed both receptor and nonreceptor areas. The receptors in the vomeronasal organ of the dog were unusual in that the cilia did not appear to be of the motile type (no dynein arms or radial spokes) and did not resemble the cilia of the cat or rabbit. Dog receptor cells did have basal bodies and apical mitochondria. The lateral wall of the vomeronasal organ has an extensive venous plexus. The neural pathways used to transmit stimuli from the vomeronasal mucosa to the brain are distinct from those of normal olfactory mucosa ().

The olfactory nature of this organ in mammals was long suspected, and now we know that it plays a role in sexual behavior and kin recognition via pheromones, some of which have been identifed chemically. The vomeronasal organ and its innervation was investigated by Read (1908) and Ramser (1935) in the cat and dog, McCotter (1912) in the opossum and other mammals, Mann (1961) in the bat, Kratzing (1971) in the sheep, Estes (1972) in several ungulates, and Salazar et al. (1984) in the dog. The role of olfaction in social communication in mammals was been reviewed by Eisenberg and Kleiman (1972). The efects of olfaction on the reproduction of mammals were considered by several authors in Doty (1976).

Associated with the sexual role of the vomeronasal organ is the behavioral “olfactory refex” or “lip-curl” seen in many mammals and described by Schneider (1930) as fehmen. This curling of the superior lip, which exposes the gums and teeth and allows air to be drawn in, clears the passageway to the vomeronasal organ and helps aspirate pheromones so they may contact the mucus in the vicinity of the incisive duct and be drawn into the organ to reach receptor sites. Goodwin et al. (1979) made extracts from the vaginal fluid of dogs in estrus and found that the major component of the sexual attractant was methyl p-hydroxybenzoate.

The incisive duct (ductus incisivus), formerly called the nasopalatine duct or Stensen canal (), passes through the palatine fissure and connects the nasal and oral cavities. The oral orifice of each duct lies lateral to the incisive papilla, caudal to the superior central incisor teeth. Dogs have been observed to make rapid licking movements with the apex of the tongue, across the incisive papilla, while holding their lips slightly curled (fehmen) and their incisor teeth slightly apart. (A film of this behavior has been made by Flora Lindsey of the Glasgow Veterinary College.) The paired incisive ducts pass dorsocaudally to open into each nasal cavity, and the vomeronasal organ exits from it before it reaches the cavity. The incisive duct is not enclosed in a bony canal, nor does it contact the margin of the palatine fissure through which it passes.

Ligaments of the Nose

Tree ligaments, composed of one pair and one unpaired, attach the mobile part of the nose to the dorsal portion of the osseous nose (). The dorsal nasal ligament (lig. nasale dorsale) is a single band of collagenous tissue that runs from the caudodorsal end of dorsolateral nasal cartilage to the dorsum of the nasal bones. The lateral nasal ligament (lig. nasale laterale), one on either side, is a collagenous band that runs from the midlateral surface of the caudal aspect of the dorsolateral nasal cartilage to the border of the bony nasal aperture directly dorsal to the end of the nasomaxillary suture. The ligaments of the nose are best developed in old dogs of the working breeds.

Nasal Cavity



The trachea () runs from a transverse plane through the middle of the body of the axis to a similar plane through the fibrocartilage between the fourth and fifth thoracic vertebrae. It is a relatively noncollapsable tube that extends from the cricoid cartilage of the larynx to its bifurcation (bifurcatio tracheae) dorsal to the cranial part of the base of the heart. Done (1978) has reported on collapse of the trachea in dogs. The crest of the partition at the site where the trachea divides into the two principal bronchi is called the tracheal carina (carina tracheae). Approximately 35 C-shaped hyaline tracheal cartilages (cartilagines tracheales) form the skeleton of the trachea. The diameter and thickness of the tracheal rings is smallest at the level of the thoracic inlet (). The space left by failure of these cartilages to meet dorsally is bridged by fibers of the smooth, transversely running tracheal muscle (musculus trachealis) and connective tissue. The rings so formed are united in a longitudinal direction by bands of fibroelastic tissue, called the annular ligaments of the trachea (ligg. anularia [trachealia]). They are approximately 1 mm wide, compared with the 4-mm width of each tracheal cartilage. The annular ligaments allow considerable intrinsic movement of the trachea without breakage or collapse of the tube. Macklin (1922) reconsidered the network of longitudinal elastic fibers in the tunica propria of the trachea and bronchial tree. This elastic membrane beneath the epithelium is thick in the trachea and larger bronchi, but thin in the terminal respiratory passageways. It provides a recoil mechanism for the lung.


Thoracic Cavity and Pleurae

To obtain a clear understanding of the lungs and how they function passively in the mechanical act of breathing, it is necessary first to understand the morphologic characteristics of the thoracic cavity and its lining membrane, the pleurae.