RESPIRATION. 559 



become constricted, and thus offer greater resistance to the flow from the right 

 side of the heart through the lungs to the left side of the heart, and subse- 

 quently into the general circulation. 



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 sec- 

 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 

 diastolic distention of the heart and dilatation of the intrapulmonary and intra- 

 thoracic vessels ; yet, notwithstanding that this powerful aspiratory action en- 

 courages the flow of an extraordinarily large amount of blood into the thoracic 

 vessels, the heart-beats may be very small, because intrathoracic negative 

 pressure is so great that the thin-walled auricles meet with great resistance 

 while contracting ; in consequence, then, of this forced inspiratory effort little 



