66 VERTEBRATE RESPIRATION 



at the surface and following submergence for three minutes show 

 that the mean pressure remains approximately the same but that 

 the pulse pressure increases during the dive. The rate of rise of the 

 pulse curve is more rapid but its decline is much slower than nor- 

 mal. All these features suggest some vaso-constriction in the cir- 

 culatory system. After submergence for ten minutes the systolic, 

 diastohc, and pulse pressures all fall quite markedly. Following 

 surfacing of the animal there are increases in the diastolic and 

 systolic pressures but an initial fall in the pulse pressures. It seems 

 clear that in these animals there is an economic usage of the oxy- 

 gen in the lung and measurements have shown that this falls 

 rapidly during the first 20 minutes of a dive but only gradually 

 during subsequent periods of an hour and a half during which the 

 animal is kept submerged. During normal diving conditions the 

 animal rises after 15 minutes. The changes in lactic acid present 

 in the blood are not exactly as found in the seal, and suggest that 

 the blood supply to the muscles is maintained during the dive. 

 Similar conclusions were reached from the work on the guille- 

 mot. During a dive, as well as during recovery, the blood lactic 

 acid increased rather slowly. Similar tendencies were found in the 

 lactic acid content of the muscles and in some cases there was 

 even a decrease in lactic acid content observed at the beginning 

 of the submergence period. The conclusion that the muscles 

 remain open to the blood circulation during a dive seemed in- 

 escapable from these and other observations. 



In connection with these observations on diving tetrapods it 

 is of interest to recall the response of fish when removed experi- 

 mentally from water. Inhibition of the respiratory movements 

 occurs in most cases and distinct bradycardia has been observed 

 for many species. For example, in the carp the heart rate falls 

 from 60 to 5 per minute. In the cod, not only was bradycardia 

 observed but also a slow accumulation of lactic acid in the blood 

 which increased suddenly when the fish was once more returned 

 to the water. By the same arguments as those discussed above, 

 we must suppose the presence of some vaso-constriction in the 

 circulation which prevents the flow of lactic acid into the general 

 circulation during the period of asphyxia. 



