50 VERTEBRATE RESPIRATION 



as is the large inspiratory movement which succeeds it. But the 

 final expiration is passive and results from movements of the 

 abdominal viscera which are not separated from the thoracic 

 region by a diaphragm. Ventilation of the lungs themselves is 

 certainly diphasic in character, for the glottis is open at the 

 beginning of the initial expiratory phase and closes before the 

 final passive expiration. 



The glottis and nostrils are guarded by sphincters in most 

 lizards. The bucco-pharyngeal movements are frequently found 

 during the long pauses between successive pulmonary move- 

 ments of many reptiles. The nostril valves play little part in 

 normal ventilation of the lungs but their importance in aquatic 

 forms is much greater. Thus in some turtles which are wholly 

 marine, it has been observed that the nostrils may be filled with 

 highly vascular tissue which plugs them and so prevents the 

 entry of water. When other aquatic turtles come to the surface, 

 very marked peristaltic movements of the throat can be observed 

 and there seems little doubt that the animal is actively venti- 

 lating the bucco-pharynx although there are no visible move- 

 ments suggestive of pulmonary ventilation. In the chamaeleon, 

 the active participation of a buccal force-pump enables the 

 animal to inflate its lungs and air sacs to a large size. Some 

 turtles are said actively to ventilate the pharyngeal cavity when 

 they are submerged and oxygen is absorbed by means of fine 

 filaments which project down from the roof of the mouth. 

 Oxygen consumption of these forms is very low during sub- 

 mergence (7ccs/kg per hour) ; 30 per cent seems to be absorbed 

 in the bucco-pharynx and 70 per cent through the soft skin. It 

 has recently been shown that some freshwater turtles (Pseud- 

 emys) can exist on anaerobic metabolism during prolonged 

 diving. 



(c) BIRDS 



As might be expected from their constant body temperature 

 and great activity, birds have a high metabolic rate (Table 1). 

 This demands an efficient system for gaseous exchange but 

 perhaps unexpectedly the bird lung is a small compact structure 



