342 COMPARATIVE ZOOLOGY OF CHORDATES 



efficiency of this mechanism lies in the fact that there is a 

 through- draught right through the lungs. All the air can be 

 renewed, whereas in other forms, the lungs are blind sacs and 

 there is always a certain amount of stale residual air at the 

 bottom of them which cannot be renewed. The efficiency 

 of the respiratory system has played a large part in the evolution 

 of the birds, which require a high rate of metabolism in order 

 to perform the very arduous muscular exertion of maintaining 

 the body in the air during flight. 



Attention may now be turned to two modifications which 

 may occur in connexion with the respiratory system. The 

 first concerns the formation of the false palate. This structure 

 is a secondary roof to the mouth, closing over the original 

 internal nostrils, and enclosing the nasal passage as far back 

 as the secondary choana. The secondary choana is opposite 

 the glottis (the opening through which the pharynx com- 

 municates with the larynx and trachea, and so with the 

 lungs), and the whole structure is an adaptation enabling 

 the animal to breathe and yet have its mouth full of food or 

 water at the same time. It is especially developed in aquatic 

 forms such as the crocodile and the whale, but it is character- 

 istic of the higher Theromorph reptiles and mammals in 

 general. In the whales the glottis can be pushed right up into 

 the secondary choana, thus making a closed communication 

 between the external nostrils (above the surface of the water) 

 and the lungs, without running the risk of water entering 

 the latter from the mouth. In the higher vertebrates, and 

 especially those which frequent deep waters, the windpipe or 

 trachea is prevented from collapsing by rings of cartilage or 

 bone. 



The fact that respiration in terrestrial vertebrates involves 

 the pumping of air in and out of the body, has been made use 

 of in connexion with the production of sound. Bands of 

 connective tissue stretch across the cavity of the larynx, and 

 can be thrown into vibration by the passage of the air. These 

 bands are the vocal cords. In the male frog there are vocal 

 sacs at the corners of the mouth, and these become distended 

 with air when the animal " croaks " and act as resonators. 



