RESPIRATION OF TETRAPODS 65 



by the non-respiratory surfaces i.e. bronchii, tracheal tubes etc., 

 is quite large (about 1/llth of the total lung volume). Thus at 

 11 atmospheres pressure the total volume of the lungs will be 

 reduced to that of the inert passages and hence the air will not 

 be in contact with any respiratory surface and the dangers of 

 nitrogen uptake will be negligible. Assisting in the transmission 

 of the external pressure changes from the water to the lungs is 

 the horizontal position of the diaphragm in whales. The re- 

 duction in volume to 1/1 1th occurs at approximately 100 metres 

 and hence once the animal has dived to this depth the danger of 

 nitrogen being forced into solution in the blood does not increase 

 further. 



Evidently a whale can stay below this depth so long as its 

 oxygen store and its ability to undergo anaerobic glycolysis can 

 persist. On coming towards the surface it may do so quite 

 rapidly until it reaches the 100-metre depth, but above this its 

 progress to the surface results in the gradual coming out of 

 solution of the nitrogen forced into the blood during the dive. 

 This volume will be slight relative to the blood volume and some 

 nitrogen may be dissolved in the fat. If the whale's progress to 

 the surface is slow there will be little danger of the bubbles of 

 nitrogen forming in any quantity. On surfacing whales have 

 many times been observed to 'spout' which results from the 

 rapid expulsion of air from the lungs and, being hotter than the 

 external air, results in rapid condensation of any moisture con- 

 tained in it. Between dives the whale rests at the surface, under- 

 going brief and shallow dives as it pays off the oxygen debt 

 incurred during its longer stay beneath the surface. 



Information regarding the physiological adaptations of other 

 vertebrate divers has been obtained recently and indicates that 

 several of the phenomena described for the seal are quite general. 

 Notably the bradycardia following submergence has been 

 observed in ducks, guillemots, birds, snakes, alligators, and also 

 man. Studies on the changes in blood pressure and the pulse 

 pressure curves during diving of the guillemot and alligator have 

 indicated that changes must occur in the resistance to blood flow 

 during a dive. The pulse pressure curves from an aUigator when 



