Chapter 8 Respiratory system
The respiratory system consists of:
Structure and function
Nose

Fig. 8.2 Longitudinal section to illustrate the upper respiratory tract of the dog.
(With permission from Colville T, Bassett JM 2001 Clinical anatomy and physiology for veterinary technicians. Mosby, St Louis, MO, p 222.)
Paranasal sinuses
It is thought that the function of the paranasal sinuses is to lighten the weight of the skull, allowing the areas of the skull used for muscle attachment to be larger. This is evident in those species that have large, heavy skulls, such as the horse, which has numerous large paranasal sinuses. The paranasal sinuses also act as areas for heat exchange and as sites for mucus secretion.
Pharynx
From the nasal cavity, the inspired air passes into the pharynx, a region at the back of the mouth that is shared by the respiratory and digestive systems. The pharynx is divided into the dorsal nasopharynx and the ventral oropharynx by a musculomembranous partition called the soft palate (Fig. 8.2). The soft palate extends caudally from the hard palate and prevents food from entering the nasal chambers when an animal swallows (see Ch. 9). The oropharynx conducts food from the oral cavity to the oesophagus; the nasopharynx conducts inspired air from the nasal cavity to the larynx, but air can also reach the respiratory passages from the mouth, e.g. during ‘mouth breathing’.
Larynx
The larynx is suspended from the skull by the hyoid apparatus (see Ch. 3), which allows it to swing backwards and forwards (Fig. 8.3). The hyoid apparatus is a hollow, box-like structure consisting of a number of cartilages connected by muscle and connective tissue. The most rostral of these cartilages, the epiglottis, is composed of elastic cartilage and is responsible for sealing off the entrance to the larynx or glottis when an animal swallows. This prevents saliva or food from entering the respiratory tract, causing the animal to choke. When the larynx returns to its resting position after swallowing the epiglottis falls forward, opening the glottis and thus allowing the passage of air to resume.
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