RESPIRATION I43 



It is also of considerable importance during fluctuations in pressuie 

 which take place on respiration at the surface. Man takes about four 

 seconds to breathe in and out normally, during which roughly half a litre 

 of air is displaced. But big Rorquals displace some 1,500 litres of air in 

 I -5-2 seconds, two-fifths of the time being used for exhalation and three- 

 fifths for inhalation. We have seen under what great pressure the air 

 escapes, and clearly a system of fairly rigid tubes is a great advantage to 

 the pumping action of the thorax. On the other hand, some elasticity is 

 clearly needed, if only for emergencies, and hence, as we have seen, the 

 cartilage support is not uniformly annular and allows for a certain amount 

 of expansion (Fig. 77). Also, the walls of the bronchi are provided with a 

 double system of predominantly longitudinal, elastic fibres, which add 

 extra resilience. 



Great quantities of such elastic fibres are also found in the lining 

 (pleura) of the lungs. This is immediately apparent when we look at this 

 organ aboard a whaler, for the lungs have a yellow and crinkly appear- 

 ance, the colour being due to the fibres, and the crinkles to their contrac- 

 tions. In addition to being found on the walls of the bronchi and the 

 pleura, elastic fibres occur throughout the tissue of the lung, where they 

 are very much more profuse than in terrestrial mammals. By increasing 

 the flexibility of the lungs, great pressure changes can be more easily 

 effected. 



The presence in Cetacean tracheas and bronchi of an extensive network 

 of small veins distended with blood is not yet clearly understood, though, 

 apart froin warming the air, they may also act as shock absorbers during 

 the violent respiratory movements of Cetaceans. The position of this moist 

 cushion itself seems to make such an assumption probable. 



Another phenomenon particularly associated with diving is the presence 

 of a peculiar system of sphincters (valves) in the respiratory bronchioles. 

 Such systems have been found in all dolphins so far investigated, including 

 Berardius. The mucous membrane of the bronchi involved is here provided 

 with a succession of annular folds which serve to restrict the diameter of the 

 bronchioles (Fig. 83). Each fold is provided with a layer of smooth muscle, 

 also annular in shape, which is joined by strands of elastic fibre running 

 radially to the cartilage support. It follows that, when the muscles are 

 relaxed, the air passage is kept wide open, and when they contract, the 

 passage narrows. The number of such successive valves varies from 8-12 

 in the porpoise, and from 25-40 in the Common Dolphin and the Bottle- 

 nose Dolphin. 



What part do these valves play during diving? As the water pressure 

 mounts, the air in the highly compressible alveoli would normally be 

 squeezed into the incompressible part of the bronchial system. Since it is 



