48 



VERTEBRATE RESPIRATION 



is supported by elastic fibres and once again plain muscle fibres 

 enable the lung to make spontaneous rhythmic contractions. 

 In some forms, such as chamaeleons, the presence of extensions 

 of the lung into air sacs is suggestive of a condition much 

 elaborated in birds. The air sacs do not contain any respiratory 

 epithelium and only appear to be important in ventilation. 

 Many reptiles have secondarily acquired an aquatic habit which 

 in some instances is associated with a decrease in the respiratory 

 area (Table 7). In aquatic turtles, for example, the lung volume is 

 about 3-6 ccs./lOOgm. whereas in their terrestrial relatives it 

 may be 21 vols.%. 



Table 7 

 Lung size of turtles from different habitats (after Agassiz, 1857) 



As in amphibians, three mechanisms for obtaining oxygen 

 have been described ; bucco-pharyngeal, pulmonary, and cutan- 

 eous. The last of these is apparently not so important because 

 the horny layers of the skin not only prevent water loss but 

 also hinder any gaseous exchange. There is evidence for the 

 importance of cutaneous respiration in some aquatic reptiles. 

 Oscillations of the pharyngeal floor occur in many reptiles but 

 their function is doubtful. Any gaseous exchange which they 

 assist is very slight, as shown by determinations of the carbon 

 dioxide content of the expired air; their function appears to be 

 mainly olfactory. Pulmonary ventilation is of major importance, 



