32 The Medium 



apparently withstand the great pressures by allowing their lungs to 

 be completely flattened and the air that was in them to be forced 

 into the strong, boxlike larynx. How a mammal swimming vigorously 

 can get along for more than a few minutes without a renewal of its 

 oxygen supply has been a matter for speculation for generations. 

 Evidence obtained primarily from the study of seals indicates that 

 these diving mammals can store an increased amount of oxygen in 

 their tissues and that this oxygen is reserved for the brain, heart, and 

 other vital organs by cutting off the circulation to other parts of the 

 body. The muscles build up an oxygen debt that is paid off when 

 the animal surfaces again (Scholander, 1940). Too rapid ascent may 

 result in the formation of gas bubbles in the blood, but the fact that 

 the whale, unlike the human diver, has only one lungful of air during 

 the dive presumably reduces the danger from absorbed nitrogen. 



The depth and duration of the whale's dive have been hotly con- 

 tested by captains of whaling ships and others for many years. It 

 seems probable that whales dive to 200 or 400 m regularly. Seemingly 

 indisputable evidence for an even greater dive was furnished by the 

 discovery of a sperm whale that had become entangled at a depth of 

 about 1000 m in a submarine cable running between two of the Carib- 

 bean Islands. At this depth the pressure is about 100 atmospheres 

 ( or over 100 kg per sq cm ) , and evidently the mechanism of the whale 

 is adapted to withstand pressures as great as this. Whales ordinarily 

 stay submerged for twenty minutes or so, but when harpooned they 

 may disappear from the surface for one to two hours. Such dives, 

 accompanied as they are by violent swimming, are a striking demon- 

 stration of the ability of these animals to remain active for long pe- 

 riods without renewal of oxygen. 



SUPPORT AND RESISTANCE TO MOTION 



The difference in density of the two elemental media, air and water, 

 also influences the degree to which they provide support and re- 

 sistance to motion. Since water has nearly the same density as proto- 

 plasm, whereas air is very much less dense, water furnishes much 

 more buoyancy than air. 



Effects on Structure and Size 



Since terrestrial organisms are very much heavier than their sur- 

 rounding medium, they would tend to collapse from their own weight 

 if it were not for special supporting structures. On land only very 



