METABOLISM AND TRANSPORT IN ANIMALS 



535 



The lungs of reptiles are more complex 

 than those of amphibians. Certain aquatic 

 turtles possess cloacal sacs that act as auxil- 

 iary respiratory organs; these are compara- 

 ble to the tracheal gills of dragonfly naiads, 

 and the gill trees of sea cucumbers. In birds, 

 the lungs are assisted by air sacs; and where 

 the trachea divides into two bronchi, an en- 

 largement known as a syrinx serves as a vocal 

 organ. 



Mammals are provided with respiratory 

 systems similar to those of human beings. In 

 man, air passes through the nose or mouth 

 into the lungs by way of the larynx, trachea, 

 bronchi, and bronchioles (Fig. 389). Air is 

 warmed, moistened, and filtered in the nose. 

 The larynx is a triangular organ; the Adam's 

 apple, in man has 9 pieces of cartilage in 

 its walls to prevent it from collapsing. The 



slitlike opening in the pharynx is the glottis, 

 which is protected by a leaf-shaped lid, the 

 epiglottis. Within the larynx are the vocal 

 cords. Air forced out of the lungs may vi- 

 brate these cords, and the column of air 

 above them gives rise to sounds. This may 

 be compared to an organ pipe in which a 

 reed and a column of air are set into vibra- 

 tion by a blast of air. Certain notes can be 

 played on the organ which are remarkably 

 similar to the human voice. The cavities of 

 the pharynx, mouth, and nose act as res- 

 onators; the amplitude of the vibrations and 

 the volume and force of the air current de- 

 termine the loudness and intensity; and the 

 length, tightness, and frequency of vibra- 

 tion of the cords determine the pitch of the 

 voice. In women and children the vocal 

 cords are usually short, and the voice is 



Air enters 



Lung 

 collapsed 



Lung 

 expanded 



Bell jar 



Push up on diaphragm 



Pull down on diaphragm 



Expiration 



Inspiration 



Figure 388. An apparatus to show the role of the diaphragm in breathing — rubber balloons 

 in bell jars. The bottom of each jar is covered with a flexible rubber membrane. When the 

 rubber membrane is pulled down, the space inside the jar is increased and pressure of air 

 through the tube causes the balloons to expand. Similarly, when the diaphragm is lowered, the 

 chest cavity becomes larger and air rushes into the lungs. In expiration, the diaphragm moves 

 upward. An adult normally breathes on the average of about 16 times per minute. 



