Larynx
It is a short passage connecting the pharynx with the trachea. The walls are lined with 5 individual pieces of cartilage and contains the thyroid cartilage (Adam’s Apple). It is held in position by the hyoid apparatus, suspended from the base of the skull, between the 2 mandibles. This short tubular structure sits cranial to the trachea and provides the muscular attachment for the epiglottis. It is also covered in mucus membranes. The larynx is considered the voice box, housing the vocal cords . As air passes through the larynx, causing the vocal folds to vibrate creating sound. The epiglottis is a large leaf-shaped piece of elastic cartilage. The epiglottis changes position along with the soft palate and arytenoid cartilages depending on if the horse is swallowing food or breathing. During breathing the epiglottis lies on top of the soft palate and allows air into the larynx. During swallowing the epiglottis covers the larynx and the soft palate rises to allow food into the esophagus from the oral cavity.
Trachea
This is the windpipe located ventral to the esophagus and extending from the larynx it divides into right and left primary bronchi. It is 75-100 cm long, 5 cm in diameter with 50-60 incomplete horizontal rings of hyaline cartilage resembling the letter ‘C’. They are stacked on one another providing a semi rigid support, so the trachea does not collapse inward during inhalation. The trachea is lined with pseudostratified epithelium and goblet cells to produce mucus and cilia to catch any remaining particles before they reach the lungs. The trachea contains smooth muscle and therefore is capable of constriction & dilation. The trachea bifurcates into left & right primary bronchi at the level of T5, just dorsal to the heart.
Bronchi
The trachea divides into right and left Primary (chief) Bronchi . The right primary bronchus is more vertical, shorter, and wider than the left and is the more likely place for aspiration of a foreign object. Primary bronchi enter the lungs and divide to form Secondary (lobar) Bronchi, one for each lobe of the lung (3 right, 2 left).
Secondary bronchi branch to form Tertiary (segmental) Bronchi that divide into Bronchioles .
Bronchioles branch into Terminal (Respiratory) Bronchioles which then become the alveoli.
The bronchi are held open by cartilaginous rings as well as being ciliated. Bronchioles are also lined by ciliated mucus membranes but are not supported by rings.
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