The Respiratory System 151 



passages by means of small openings. During respiratory infections, 

 these openings frequently close due to the swelling of the mucous 

 membranes. 



The Pharynx. — The pharynx serves as a common passageway for 

 both air and food. The air passes through the nasopharynx and crosses 

 the pathway of the food to enter the glottis. In mammals the glottis 

 is protected against the entrance of food by a triangular flap of tissue, 

 the epiglottis, which is strengthened by a special cartilage. 



The Larynx. — Directly below the glottis is an enlargement of the 

 trachea, the larynx. Within this cartilaginous box are two ridges com- 

 posed of elastic tissue, the vocal cords. These stretch across the larynx 

 and can be set in vibration by air passing over them from the lungs. 



This voice box with its contained vocal cords is best developed 

 in the voluble mammals. Other vertebrates, however, also are capable 

 of vocalizing. The noisy frogs, for example, have many familiar calls 

 in early spring. Many of the reptiles make hissing and grunting noises 

 by forceably expelling air through the glottis. The singing of birds is 

 not due to the vocal cords at all ; in fact, their larynx is but poorly de- 

 veloped. Instead they have a special organ, the syrinx, at the base of 

 the trachea. This contains a vibratory structure, somewhat like a reed, 

 which produces the familiar bird songs as air rushes over. Usually 

 vocalization occurs only as air is expelled. The bray of the donkey is 

 an example of an exception to this. 



In mammals there are nine cartilages that form the walls of the 

 larynx. One of the ventral cartilages, the thyroid, is larger in men than 

 in women and forms the well-known Adam's apple. In males, the 

 larynx enlarges and the vocal cords lengthen at puberty, resulting in 

 a lowered pitch to the voice. 



The Trachea and Bronchi. — In those vertebrates with well-de- 

 veloped lungs, the trachea extends from the larynx to the point where 

 it bifurcates into the two bronchi. In those birds such as the swan with 

 long necks, the trachea may be extremely elongated ; in mammals it 

 is usually quite short. Its walls are supported by rings of cartilage 

 which prevent its collapsing. In most mammals, these cartilages are 

 C-shaped, with the open portion resting against the esophagus. These 

 cartilages give both flexibility and strength to the walls. 



The walls of the bronchi are likewise supported by cartilaginous 

 rings. As each bronchi divides and redivides into smaller and smaller 

 bronchioles, the walls become thinner and the cartilages disappear. 



