RESPIRATION OF TETRAPODS 63 



(ii) the heart beat rose to normal; 



(iii) there was an increase in the arterial oxygen tension and a 

 fall in the CO 2 tension. 



(iv) The most marked effect, however, was an enormous in- 

 crease in the lactic acid content of the blood. 



This work was the first of its kind and, together with subse- 

 quent observations, has suggested the presence of several im- 

 portant physiological adaptations : 



(a) The respiratory centre must be relatively insensitive to the 

 accumulation of carbon dioxide in the blood, for breathing 

 may be inhibited for as long as 15 minutes. 



(b) Anaerobic glycolysis evidently occurs in the musculature, 

 building up an oxygen debt with accumulation of lactic 

 acid. 



(c) Changes in the cardio-vascular system prevent this lactic 

 acid from affecting the whole body. The rate of heart beat 

 is lowered, but in spite of this the mean blood pressure is 

 maintained although the pulse pressure curves change their 

 wave form. These changes in the blood pressure curves 

 indicate the presence of shunting mechanisms in the vas- 

 cular system whereby the supply of blood to the brain and 

 head is maintained although its flow to the musculature 

 is reduced. The rete mirabile found in many parts of 

 diving mammals were at one time thought to represent 

 auxiliary oxygen stores but many of them are now re- 

 garded as elaborate arterio-venous anastomoses through 

 which blood can circulate without entering the tissues 

 themselves. A further adaptation in seals is the presence 

 of a sphincter in the inferior vena cava which, if shut dur- 

 ing a dive, will decrease very considerably the volume of 

 blood returning to the heart from the viscera. 



These mechanisms, then, ensure an economic usage of the 

 oxygen stores of the animal and maintain a sufficient supply of 

 oxygen to the brain. The cutting-oflf of parts of the peripheral 

 circulation enables the accumulation of lactic acid to proceed 



