RESPIRATION OF TETRAPODS 



47 



gaseous exchange at the skin varies at different times of the year. 

 (Table 6). Most of the COg loss is through the skin at all times 

 of the year but the uptake of oxygen by this route is most im- 

 portant during the winter months. Cutaneous respiration 

 accounts for two-thirds of the total oxygen uptake during the 

 winter but only a quarter in the summer. The actual volume of 



Table 6 

 Frog. Seasonal differences in gaseous exchange (after Krogh, 1941) 



oxygen absorbed through the skin remains constant through- 

 out the year, the change in proportion being due to a consider- 

 able increase in the oxygen entering via the lung and the carbon 

 dioxide leaving by that route. 



The surface area of the skin of a frog is much greater than that 

 of the lung, being about 12-5 sq. cm. /gram compared with 8-4 sq. 

 cm./grm. for the lung surface. (The latter figure is high relative 

 to other measurements on frogs, see Table 1.) 



(b) RESPIRATION OF REPTILES 



Reptiles are better adapted to a land habit than amphibians 

 and this is associated with more efficient ventilation. The lungs 

 have an increased internal surface and they communicate with 

 the pharyngeal cavity by a distinct tracheal tube supported by 

 circular rings of cartilage. Some of the less active forms, how- 

 ever, have simple bag-shaped lungs as, for instance, in Spheno- 

 don (the Tuatara of New Zealand), but in larger lizards (e.g., 

 Varanus) the surface is greatly increased and definite bronchial 

 tubes may be recognised in the lung (fig. 12). The wall of the lung 



