RESPIRA TIOX. 451 



tion now gives place to a gradual diminution, and with this a lessening of the 

 aspiratory action due to the sub-atmospheric intrathoracic pressure; the blood- 

 supply is further reduced because of the lessened amount of blood coming 

 through the inferior vena cava; the abdominal veins, instead of being com- 

 pressed and their contents forced chiefly toward the heart, are now being 

 filled; finally, during the shrinkage of the lungs the intrapulmonary vessels 

 become lessened in capacity, and thus temporarily force more blood into the 

 left side of the heart and cause the brief rise of arterial pressure observed at 

 the beginning of expiration. 



Another factor believed by some to be involved in the respiratory undula- 

 tions in blood-pressure is a rhythmical excitation of the vaso-constrictor centre 

 in the medulla oblongata, asserted to occur coincidently with the inspiratory 

 discharge from the respiratory centre. This has, however, been disproved. 

 Others have held that the blood-pressure changes are due to the pressure ex- 

 erted by the expanding lungs upon the heart; while others contend that 

 rhythmical alterations in the heart-beats are important. This latter factor is 

 of importance in man and in the dog, in which there is a distinct increase in 

 the rate of the heart-beat during inspiration, and co-operates in producing the 

 general rise of pressure during inspiration. 



The Effects on the Pulse. — During inspiration the pulse-rate is more 

 rapid than during expiration. If we cut the pneumogastric nerves, it will be 

 seen that, while the rate is increased as the result of the section, the difference 

 during inspiration and expiration is abolished ; on the other hand, if the thorax 

 be widely opened, but the pneumogastric nerves are left intact, the inspiratory 

 increase in the rate still occurs. This indicates that the cardio-inhibitory 

 centre is either less active during inspiration or more active during expiration, 

 and that there is an associated activity of the respiratory and cardio-inhibitory 

 centres. Why this sympathy should exist between the respiratory and cardio- 

 inhibitory centres we do not know, but it has been suggested that during expi- 

 ration the blood reaching the centres is less highly arterialized than during 

 the inspiratory phase, and that the cardiac centre is so sensitive to the difference 

 as to be affected, and thus its activity is somewhat increased during the expira- 

 tory phase, with the consequent decrease in the pulse-rate. 



During inspiration the pulse-rate is not only higher than during expiration, 

 but the form of the pulse-wave is affected. The systolic, dicrotic, and >ir- 

 ondary waves are smaller and the dicrotic notch is more pronounced, SO that 

 the dicrotic character of the curves is better marked. 



The Effects of Obstruction of the Air-passages and of the Respira- 

 tion of Rarefied and Compressed Air on the Circulation. — The blood- 

 pressure undulations produced during quiet breathing become marked in pro- 

 portion to the depth of the respiratory movements. Inspiration or expiration 

 against extraordinary resistance — as after closing the mouth and nostrils, or 

 respiring rarefied or compressed air — may materially modify the circulatory phe- 

 nomena. When we make the most forcible inspiratory effort, the air passages 

 being fully open, not only is there a full expansion of the lungs, but great 



