f)15» Comparative Morphology oi' Chordates 



with the size of the animal and the corresponding size of the lungs; 

 also, the size of the alveolus varies in different animals. The total 

 respiratory area of the lungs has been estimated. According to F. E. 

 Schulze (1906), the cat's lungs contain about 400,000,000 alveoli and 

 the total respiratory surface is about 20 square meters. In the human 

 lungs there are about 150,000,000 alveoli, but a single human alveolus 

 has about four times as much surface as an alveolus of the cat. There- 

 fore the total respiratory area in man is about 30 square meters. These 

 estimates cannot be accurate, but they have at least comparative 

 significance because the same method was applied to several animals. 



Lungs of Cetaceans. The almost incredible ability of whales to 

 endure long submergence presents problems. Accurate observations 

 are difficult to obtain, but what seem to be conservative statements 

 credit the blue whale and the cachelot (a sperm whale) with sub- 

 mergences lasting an hour or longer. A man cannot easily "hold his 

 breath" for one minute. The following factors are probably involved 

 in the whale's performance. In contrast to the fact that human lungs 

 are rarely filled to capacity and never completely emptied, it is prob- 

 able that, when the whale submerges, the lungs are filled to the limit. 

 In view of the fact that elastic tissue is very much more strongly 

 developed in whales' lungs than in lungs of land mammals, it may be 

 inferred that there is comparatively little residual air in the whale's 

 lungs after expiration. It has been reported, too, that the capillary 

 net related to the air-spaces in the whale's lung is much denser than 

 that in the lung of a land mammal, so that relatively more blood is 

 exposed to the air, and that the red corpuscles of whale's blood are 

 richer in the oxygen-carrier, hemoglobin. F. E. Schulze, cited above, 

 estimated that a porpoise whose weight was about that of an average 

 man had 137,000,000 respiratory alveoli, with a total surface of 43 

 square meters, in contrast to man's 150,000,000 alveoli and 30 square 

 meters of surface. 



When a whale returns to the surface after long submergence, it 

 thoroughly cleans out the lungs by a succession of vigorous exhalations 

 and inspirations — the characteristic "spouting." The nostrils are 

 situated dorsally and far- back from the anterior end of the head. The 

 dorsal "blow hole" of the sperm whale is the common single nostril 

 through which the two nasal cavities open externally. 



COMPARISON OF LUNGS OF AMNIOTES 



The lungs of reptiles, birds, and mammals are similar to the extent 

 that they all possess an internal structure which serves to provide more 

 or less expansion of respiratory surface, and in that this expansion 

 results from elaboration of the posterior end of the bifurcated (forming 



