THE RESPIRATORY APPARATUS 1 95 



which in the production of sound are caused to vibrate. These folds 

 are half -moon shaped elastic structures, located in the bony, arrow-' 

 like way, intero-inf eriorly . These structures are called the membfuna 

 tympana interna. In the duck this inferior larynx is represented by 

 a drum-like cartilaginous and bony structure, called the bulla tym- 

 paniformis. This bulla is a resonant apparatus which serves to 

 strengthen the voice. 



In song birds there is a double glottis, usually produced by a 

 bony bar, called the pessulus, or os transversale, which traverses the 

 lower end of the trachea from front to rear. It supports a thin 

 membrane which ascends into the tracheal area, and, terminating 

 there by a free concave margin, is called the membrana semi-lunaris. 

 This is most developed in singing birds, and being vibratile, forms 

 an important part of their trilling vocal apparatus. The air passes 

 on each side of the membrana semi-lunaris and its sustaining bone to 

 and from the bronchi and lungs. 



The last ring of the trachea usually expands as it descends, with 

 its fore and posterior parts produced, and the lower lateral borders 

 concave; the extremities of the pessulus, butts against the angle 

 thus formed and expands to be attached, also with the fore and pos- 

 terior terminations of the first half ring of the bronchus, strength- 

 ening and clamping together the upper part of the vocal framework. 

 The second bronchial half ring is flattened and curved with the con- 

 vexity outward, like the first, but is more movable. The third 

 half ring is less curved and further separated from the second, to 

 the extremities of which its own are connected by a ligament, and, 

 for the intervening extent, by a membrane; its inner surface supports 

 the fibrous cord, or fold, which forms the outer lip of the glottis 

 of that side; it is capable of rotary movements on its axis, aiicjl is 

 an important agent in the modulation of the voice. All these 

 parts just described are bony. 



The Bronchi and the Lungs. The bronchi, two in number, 

 are provided with only incomplete cartilaginous rings. They enter 

 the inferior face of the lungs, toward their anterior and middle 

 thirds and break up into primary bronchi, which give off at right 

 angles, secondary bronchi, and these latter in turn give off tertiary 

 branches. 



The lungs occupy only about one-seventh of the thoracic space. 

 They are long, flattened, and oval, extending along each side of the 

 spine from the second dorsal vertebra to the anterior end of the kid- 



