140 WHALES 



Piked Whale has just one. In Odontocetes tlie number fluctuates between 

 4 and 8 (Fig. 81). 



How all these phenomena have arisen from the fact that, in Cetaceans, 

 the thoracic cavity is expanded primarily from the diaphragm and the 

 dorsal side, still needs to be investigated more closely. What is clear, 

 however, is that the mobility of the thoracic wall has been greatly 

 enhanced by the increase in floating and one-headed ribs. This increase 

 may thus be a form of adaptation to the strong fluctuations in pressure 

 which occur during diving and surfacing and possibly during respiration. 



Let us now take another look at Fig. 78, which may already have 

 impressed upon us the great diflference between the lungs of Cetaceans 

 and other mammals. The left lungs of most terrestrial mammals have two 

 fairly deep clefts which divide the organ into three lobes, while the right 

 lung is often divided into four. The actual figures vary from mammal to 

 mammal, the greatest number of lobes being found in porcupines which 

 have 10-12 lobes in each lung. While it is not absolutely clear why there 

 are these differences, it is believed that they are connected with the 

 changes of form which the lungs undergo during inflation and collapse, 

 i.e. during respiration. The greater the changes, the greater the number 

 of lobes, and the smoother the action. Now from the very fact that the 

 lungs of Cetaceans have no such clefts, we may deduce that they expand 

 and contract far more evenly and change shape less than those of terrestrial 

 species. (The same kind of undivided lung is also found in sea-cows and 

 seals.) 



While the dorsal position and shape of the lungs are thus not so much 

 connected with respiratory processes during diving, they are clearly 

 associated with achieving stability in the water. 



Most of us remember from our elementary biology lessons that the 

 lungs not only supply the blood with oxygen, but also remove carbon 

 dioxide from it. To effect this type of gaseous exchange quickly and 

 efficiently, the blood must be brought into close contact with the air, i.e. 

 blood and air must be separated by only the thinnest of membranes. In 

 essence, our lungs are nothing but an enormous ramification of the 

 trachea and the bronchi. Within the lungs each bronchus divides repeatedly 

 into successively thinner tubes, the thinnest of which are no longer sup- 

 ported by cartilage and lack internal ciliated epithelium. Ultimately, the 

 finest tubes pouch out into alveolar sacs with pulmonary alveoli resembling 

 so many bunches of grapes (Fig. 82). 



The alveoli are so squashed up against one another that the dividing 

 wall (septum) between any two consists of only two rows of flattened 

 epithelial cells between which there is room for only one thin layer of 

 capillaries (Fig. 84). Hence every capillary obtains its oxygen from two 



