RESPIRATION IN AIR 



63 



made, and a fairly probable figure is 90 m 2 for a volume of 3 1 

 or 300 cm 2 per cubic centimeter. In the lung of the frog the 

 capillary area is about 2/3 of the total surface. In man it 

 would appear to represent even a larger proportion. 



Fig. 36. Diagram of Rana lung* with complex septa. (Renault.) 

 * In the species of Rana known to the writer the septa are more simple. 

 Fig. 37. Diagram of Testudo lung with simple bronchial air passages. 

 (Renault.) 



In many reptiles (Wolf, 1933), and especially in snakes, 

 considerable parts of the lungs have no respiratory epithelium 

 and only a poor blood supply which is, moreover, often ar- 

 terial. In the birds these "air-sacs" reach their highest de- 

 velopment and will be discussed in a separate section. The 

 functions of the air-sacs in reptiles are very imperfectly 

 understood. 



With the exception of the birds and a few mammals (ele- 

 phants), in which the lungs adhere firmly to the ribs, each lung 

 is freely suspended in a "pleural" cavity and connected with 

 the rest of the body only through the hilus, where the main 

 bronchus, the pulmonary artery and vein and the pulmonary 

 nerves enter its substance. The lungs are highly elastic, and 

 in most cases they will contract and drive out most or all of 

 the air contained when the glottis is open and air is admitted 



